Luxury Coach Lifestyles - View Single Post

NewellCrazy · 09-20-2013, 01:11 AM

Temperature Effects on Batteries
Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass.
Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature.

Even though battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity.
One last note on temperatures - in some places that have extremely cold or hot conditions, batteries may be sold locally that are NOT standard electrolyte (acid) strengths. The electrolyte may be stronger (for cold) or weaker (for very hot) climates. In such cases, the specific gravity and the voltages may vary from what we show.
Cycles vs Lifespan
A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.
Back to top
Battery Voltages
All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge. This "leakage" or self discharge varies considerably with battery type, age, & temperature. It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem. However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months. A "float" trickle charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even most "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 18 to 30 months.
Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower. Typical self discharge rates for flooded are 5% to 15% per month.
[h=5]Myth: The old myth about not storing batteries on concrete floors is just that - a myth. This story has been around for 100 years, and originated back when battery cases were made up of wood and asphalt. The acid would leak from them, and form a slow-discharging circuit through the now acid-soaked and conductive floor.[/h] [h=4]State of Charge[/h] State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries.
[h=4]"False" Capacity[/h] A battery can meet the voltage tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. This same thing can occur in gelled cells if they are overcharged and gaps or bubbles occur in the gel. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load.
On the table below, you have to be careful that you are not just measuring the surface charge. To properly check the voltages, the battery should sit at rest for a few hours, or you should put a small load on it, such as a small automotive bulb, for a few minutes. The voltages below apply to ALL Lead-Acid batteries, except gelled. For gel cells, subtract .2 volts. Note that the voltages when actually charging will be quite different, so do not use these numbers for a battery that is under charge.
Amp-Hours - What Are They?
All deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps x hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The generally accepted AH rating time period for batteries used in solar electric and backup power systems (and for nearly all deep cycle batteries) is the "20 hour rate". (Some, such as the Concorde AGM, use the 24 hour rate, which is probably a better real-world rating). This means that it is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours it supplies is measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications. The 6-hour rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 hour rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup amp-hour requirements.
[h=4]Why amp-hours are specified at a particular rate:[/h] Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some manufacturers and vendors have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets:

[TABLE]
[TR]
[TD="width: 25%, align: center"]Battery Type[/TD]
[TD="width: 25%, align: center"]100 hour rate[/TD]
[TD="width: 25%, align: center"]20 hour rate[/TD]
[TD="width: 25%, align: center"]8[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]Trojan T-105[/TD]
[TD="width: 25%, align: center"]250 AH[/TD]
[TD="width: 25%, align: center"]225 AH[/TD]
[TD="width: 25%, align: center"]n/a[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]US Battery 2200[/TD]
[TD="width: 25%, align: center"]n/a[/TD]
[TD="width: 25%, align: center"]225 AH[/TD]
[TD="width: 25%, align: center"]181 AH[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]Concorde PVX-6220[/TD]
[TD="width: 25%, align: center"]255 AH[/TD]
[TD="width: 25%, align: center"]221 AH[/TD]
[TD="width: 25%, align: center"]183 AH[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]Surrette S-460 (L-16)[/TD]
[TD="width: 25%, align: center"]429 AH[/TD]
[TD="width: 25%, align: center"]344 AH[/TD]
[TD="width: 25%, align: center"]282 AH[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]Trojan L-16[/TD]
[TD="width: 25%, align: center"]400 AH[/TD]
[TD="width: 25%, align: center"]360 AH[/TD]
[TD="width: 25%, align: center"]n/a[/TD]
[/TR]
[TR]
[TD="width: 25%, align: center"]Surrette CS-25-PS[/TD]
[TD="width: 25%, align: center"]974 AH[/TD]
[TD="width: 25%, align: center"]779 AH[/TD]
[TD="width: 25%, align: center"]639 AH[/TD]
[/TR]
[/TABLE]

[h=3]State of Charge[/h] [h=4]Here are no-load typical voltages vs state of charge[/h] (figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.

[TABLE]
[TR]
[TH="width: 132, bgcolor: #FFFFFF, align: center"]State of Charge[/TH]
[TH="width: 132, bgcolor: #FFFFFF, align: center"]12 Volt battery[/TH]
[TH="width: 132, bgcolor: #FFFFFF, align: center"]Volts per Cell[/TH]
[/TR]
[TR]
[TD="width: 132, align: center"]100%[/TD]
[TD="width: 132, align: center"]12.7[/TD]
[TD="width: 132, align: center"]2.12[/TD]
[/TR]
[TR]
[TD="width: 132, align: center"]90%[/TD]
[TD="width: 132, align: center"]12.5[/TD]
[TD="width: 132, align: center"]2.08[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #00FF00, align: center"]80%[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]12.42[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]2.07[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #00FF00, align: center"]70%[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]12.32[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]2.05[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #00FF00, align: center"]60%[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]12.20[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]2.03[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #00FF00, align: center"]50%[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]12.06[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]2.01[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #00FF00, align: center"]40%[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]11.9[/TD]
[TD="width: 132, bgcolor: #00FF00, align: center"]1.98[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #FFFF00, align: center"]30%[/TD]
[TD="width: 132, bgcolor: #FFFF00, align: center"]11.75[/TD]
[TD="width: 132, bgcolor: #FFFF00, align: center"]1.96[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #FFFF00, align: center"]20%[/TD]
[TD="width: 132, bgcolor: #FFFF00, align: center"]11.58[/TD]
[TD="width: 132, bgcolor: #FFFF00, align: center"]1.93[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #FF0000, align: center"] 10%[/TD]
[TD="width: 132, bgcolor: #FF0000, align: center"] 11.31[/TD]
[TD="width: 132, bgcolor: #FF0000, align: center"] 1.89[/TD]
[/TR]
[TR]
[TD="width: 132, bgcolor: #FF0000, align: center"] 0[/TD]
[TD="width: 132, bgcolor: #FF0000, align: center"] 10.5[/TD]
[TD="width: 132, bgcolor: #FF0000, align: center"] 1.75[/TD]
[/TR]
[/TABLE]

Back to top
[h=3]Why 10.5 Volts?[/h] Throughout this FAQ, we have stated that a battery is considered dead at 10.5 volts. The answer is related to the internal chemistry of batteries - at around 10.5 volts, the specific gravity of the acid in the battery gets so low that there is very little left that can do. In a dead battery, the specific gravity can fall below 1.1. Some actual testing was done recently on a battery by one of our solar forum posters, and these are his results:

I just tested a 225 ahr deep cycle battery that is in good working order..
I put a load on it 30a for 4 hrs it dropped its voltage to 11.2
I then let it cool down for 2 hrs

then put the load back on again in 1hr 42 mins it dropped to 10.3v
35 mins under 30a load 9.1v (273w)
10 mins later max output current 11.6a 8.5v (98.6w)
5 mins later max output current 5.2 amps 7.9v (41w)
3 mins later 7.6v and 2.3a (17.5w)

This shows after it gets below 10.3 v you only have 35 mins of anything useful available from the battery.

battery is now dead and most likely will not fully recover

Battery Charging
[h=4]Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.[/h]

Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts. (The internal resistance gradually goes up because there is less and less to be converted back to normal full charge).

Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts.
Chargers: Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods. Among the regulated chargers, there are the voltage regulated ones, such as Iota Engineering, PowerMax, and others, which keep a constant regulated voltage on the batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called "taper charge" - as if that is a selling point. What taper charge really means is that as the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 20 amps may only be supplying 5 amps when the batteries are 80% charged. To get around this, Xantrex (and maybe others?) have come out with "smart", or multi-stage chargers. These use a variable voltage to keep the charging amps much more constant for faster charging.
We stock all of the Iota Engineering battery chargers.
Back to top
Charge Controllers

A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types. Some of the various brands and models that we use and recommend are listed below. Note that a couple of them are listed as "power trackers" - for a full explanation of this, see our page on "Why 120 watts does not equal 120 watts".

Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no "best" controller for all applications - some systems may need the bells and whistles of the more expensive controls, others may not.

[h=4]These are some of the charge controllers that we recommend, but almost any modern controller will work fine. Exact model will depend on application and system size, amperage and voltage.[/h]

Xantrex
Morningstar
Midnite Solar
Outback Power
Steca

[h=4]Using any of these will almost always give better battery life and charge than "on-off" or simple shunt type regulators[/h]

Back to top

[h=3]Battery Charging Voltages and Currents:[/h]

Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. While some battery manufacturers state a higher maximum charge rate, such as C/3, higher charge rates can result in high battery temperatures and/or excessive bubbling and loss of liquid. ("C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps.) Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde and some other AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle for a short period. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell.

Never add acid to a battery except to replace spilled liquid. Distilled or deionized water should be used to top off non-sealed batteries. Float and charging voltages for gelled batteries are usually about 2/10th volt less than for flooded to reduce water loss. Note that many shunt-type charge controllers sold for solar systems will NOT give you a full charge - check the specifications first. To get a full charge, you must continue to apply a current after the battery voltage reaches the cutoff point of most of these type of controllers. This is why we recommend the charge controls and battery chargers listed in the sections above. Not all shunt type controllers are 100% on or off, but most are.
Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes "stratified". You can have very strong solution at the bottom, and very weak at the top of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most - check the manufacturers recommendations, especially on gelled.
[h=3]Battery Aging[/h]

As batteries age, their maintenance requirements change. This means longer charging time and/or higher finish rate (higher amperage at the end of the charge). Usually older batteries need to be watered more often. And, their capacity decreases while the self-discharge rate increases.

Mini Factoids

Nearly all batteries will not reach full capacity until cycled 10-30 times. A brand new battery will have a capacity of about 5-10% less than the rated capacity.

Batteries should be watered after charging unless the plates are exposed, then add just enough water to cover the plates. After a full charge, the water level should be even in all cells and usually 1/4" to 1/2" below the bottom of the fill well in the cell (depends on battery size and type).

In situations where multiple batteries are connected in series, parallel or series/parallel, replacement batteries should be the same size, type and manufacturer (if possible). Age and usage level should be the same as the companion batteries. Do not put a new battery in a pack which is more than 6 months old or has more than 75 cycles. Either replace with all new or use a good used battery. For long life batteries, such as the Surrette and Crown, you can have up to a one year age difference.

The vent caps on flooded batteries should remain on the battery while charging. This prevents a lot of the water loss and splashing that may occur when they are bubbling.

When you first buy a new set of flooded (wet) batteries, you should fully charge and equalize them, and then take a hydrometer reading for future reference. Since not all batteries have exactly the same acid strength, this will give you a baseline for future readings.

When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast.

Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure.

Inactivity can be extremely harmful to a battery. It is a VERY poor idea to buy new batteries and "save" them for later. Either buy them when you need them, or keep them on a continual trickle charge. The best thing - if you buy them, use them.

Only clean water should be used for cleaning the outside of batteries. Solvents or spray cleaners should not be used.

Some Peukert Exponent values (not complete, just for info). We don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20.

09-20-2013, 01:11 AM	#5
NewellCrazy Senior Member Join Date: Sep 2006 Location: Sugarland, TX or Salida,CO Posts: 1,867	Temperature Effects on Batteries Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher. Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass. Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature. Even though battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity. One last note on temperatures - in some places that have extremely cold or hot conditions, batteries may be sold locally that are NOT standard electrolyte (acid) strengths. The electrolyte may be stronger (for cold) or weaker (for very hot) climates. In such cases, the specific gravity and the voltages may vary from what we show. Cycles vs Lifespan A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years. Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary. Back to top Battery Voltages All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge. This "leakage" or self discharge varies considerably with battery type, age, & temperature. It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem. However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months. A "float" trickle charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even most "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 18 to 30 months. Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower. Typical self discharge rates for flooded are 5% to 15% per month. [h=5]Myth: The old myth about not storing batteries on concrete floors is just that - a myth. This story has been around for 100 years, and originated back when battery cases were made up of wood and asphalt. The acid would leak from them, and form a slow-discharging circuit through the now acid-soaked and conductive floor.[/h] [h=4]State of Charge[/h] State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries. [h=4]"False" Capacity[/h] A battery can meet the voltage tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. This same thing can occur in gelled cells if they are overcharged and gaps or bubbles occur in the gel. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load. On the table below, you have to be careful that you are not just measuring the surface charge. To properly check the voltages, the battery should sit at rest for a few hours, or you should put a small load on it, such as a small automotive bulb, for a few minutes. The voltages below apply to ALL Lead-Acid batteries, except gelled. For gel cells, subtract .2 volts. Note that the voltages when actually charging will be quite different, so do not use these numbers for a battery that is under charge. Amp-Hours - What Are They? All deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps x hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The generally accepted AH rating time period for batteries used in solar electric and backup power systems (and for nearly all deep cycle batteries) is the "20 hour rate". (Some, such as the Concorde AGM, use the 24 hour rate, which is probably a better real-world rating). This means that it is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours it supplies is measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications. The 6-hour rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 hour rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup amp-hour requirements. [h=4]Why amp-hours are specified at a particular rate:[/h] Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some manufacturers and vendors have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets: [TABLE] [TR] [TD="width: 25%, align: center"]Battery Type[/TD] [TD="width: 25%, align: center"]100 hour rate[/TD] [TD="width: 25%, align: center"]20 hour rate[/TD] [TD="width: 25%, align: center"]8[/TD] [/TR] [TR] [TD="width: 25%, align: center"]Trojan T-105[/TD] [TD="width: 25%, align: center"]250 AH[/TD] [TD="width: 25%, align: center"]225 AH[/TD] [TD="width: 25%, align: center"]n/a[/TD] [/TR] [TR] [TD="width: 25%, align: center"]US Battery 2200[/TD] [TD="width: 25%, align: center"]n/a[/TD] [TD="width: 25%, align: center"]225 AH[/TD] [TD="width: 25%, align: center"]181 AH[/TD] [/TR] [TR] [TD="width: 25%, align: center"]Concorde PVX-6220[/TD] [TD="width: 25%, align: center"]255 AH[/TD] [TD="width: 25%, align: center"]221 AH[/TD] [TD="width: 25%, align: center"]183 AH[/TD] [/TR] [TR] [TD="width: 25%, align: center"]Surrette S-460 (L-16)[/TD] [TD="width: 25%, align: center"]429 AH[/TD] [TD="width: 25%, align: center"]344 AH[/TD] [TD="width: 25%, align: center"]282 AH[/TD] [/TR] [TR] [TD="width: 25%, align: center"]Trojan L-16[/TD] [TD="width: 25%, align: center"]400 AH[/TD] [TD="width: 25%, align: center"]360 AH[/TD] [TD="width: 25%, align: center"]n/a[/TD] [/TR] [TR] [TD="width: 25%, align: center"]Surrette CS-25-PS[/TD] [TD="width: 25%, align: center"]974 AH[/TD] [TD="width: 25%, align: center"]779 AH[/TD] [TD="width: 25%, align: center"]639 AH[/TD] [/TR] [/TABLE] [h=3]State of Charge[/h] [h=4]Here are no-load typical voltages vs state of charge[/h] (figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that *voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%. [TABLE] [TR] [TH="width: 132, bgcolor: #FFFFFF, align: center"]State of Charge[/TH] [TH="width: 132, bgcolor: #FFFFFF, align: center"]12 Volt battery[/TH] [TH="width: 132, bgcolor: #FFFFFF, align: center"]Volts per Cell[/TH] [/TR] [TR] [TD="width: 132, align: center"]100%[/TD] [TD="width: 132, align: center"]12.7[/TD] [TD="width: 132, align: center"]2.12[/TD] [/TR] [TR] [TD="width: 132, align: center"]90%[/TD] [TD="width: 132, align: center"]12.5[/TD] [TD="width: 132, align: center"]2.08[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #00FF00, align: center"]80%[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]12.42[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]2.07[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #00FF00, align: center"]70%[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]12.32[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]2.05[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #00FF00, align: center"]60%[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]12.20[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]2.03[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #00FF00, align: center"]50%[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]12.06[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]2.01[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #00FF00, align: center"]40%[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]11.9[/TD] [TD="width: 132, bgcolor: #00FF00, align: center"]1.98[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #FFFF00, align: center"]30%[/TD] [TD="width: 132, bgcolor: #FFFF00, align: center"]11.75[/TD] [TD="width: 132, bgcolor: #FFFF00, align: center"]1.96[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #FFFF00, align: center"]20%[/TD] [TD="width: 132, bgcolor: #FFFF00, align: center"]11.58[/TD] [TD="width: 132, bgcolor: #FFFF00, align: center"]1.93[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #FF0000, align: center"] 10%[/TD] [TD="width: 132, bgcolor: #FF0000, align: center"] 11.31[/TD] [TD="width: 132, bgcolor: #FF0000, align: center"] 1.89[/TD] [/TR] [TR] [TD="width: 132, bgcolor: #FF0000, align: center"] 0[/TD] [TD="width: 132, bgcolor: #FF0000, align: center"] 10.5[/TD] [TD="width: 132, bgcolor: #FF0000, align: center"] 1.75[/TD] [/TR] [/TABLE] Back to top [h=3]Why 10.5 Volts?[/h] Throughout this FAQ, we have stated that a battery is considered dead at 10.5 volts. The answer is related to the internal chemistry of batteries - at around 10.5 volts, the specific gravity of the acid in the battery gets so low that there is very little left that can do. In a dead battery, the specific gravity can fall below 1.1. Some actual testing was done recently on a battery by one of our solar forum posters, and these are his results: I just tested a 225 ahr deep cycle battery that is in good working order.. I put a load on it 30a for 4 hrs it dropped its voltage to 11.2 I then let it cool down for 2 hrs then put the load back on again in 1hr 42 mins it dropped to 10.3v 35 mins under 30a load 9.1v (273w) 10 mins later max output current 11.6a 8.5v (98.6w) 5 mins later max output current 5.2 amps 7.9v (41w) 3 mins later 7.6v and 2.3a (17.5w) This shows after it gets below 10.3 v you only have 35 mins of anything useful available from the battery. battery is now dead and most likely will not fully recover Battery Charging* [h=4]Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.[/h] Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take. Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts. (The internal resistance gradually goes up because there is less and less to be converted back to normal full charge). Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts. Chargers: Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods. Among the regulated chargers, there are the voltage regulated ones, such as Iota Engineering, PowerMax, and others, which keep a constant regulated voltage on the batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called "taper charge" - as if that is a selling point. What taper charge really means is that as the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 20 amps may only be supplying 5 amps when the batteries are 80% charged. To get around this, Xantrex (and maybe others?) have come out with "smart", or multi-stage chargers. These use a variable voltage to keep the charging amps much more constant for faster charging. We stock all of the Iota Engineering battery chargers. Back to top Charge Controllers A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types. Some of the various brands and models that we use and recommend are listed below. Note that a couple of them are listed as "power trackers" - for a full explanation of this, see our page on "Why 120 watts does not equal 120 watts". Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no "best" controller for all applications - some systems may need the bells and whistles of the more expensive controls, others may not. [h=4]These are some of the charge controllers that we recommend, but almost any modern controller will work fine. Exact model will depend on application and system size, amperage and voltage.[/h] Xantrex Morningstar Midnite Solar Outback Power Steca [h=4]Using any of these will almost always give better battery life and charge than "on-off" or simple shunt type regulators[/h] Back to top [h=3]Battery Charging Voltages and Currents:[/h] Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. While some battery manufacturers state a higher maximum charge rate, such as C/3, higher charge rates can result in high battery temperatures and/or excessive bubbling and loss of liquid. ("C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps.) Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde and some other AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle for a short period. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less. Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell. Never add acid to a battery except to replace spilled liquid. Distilled or deionized water should be used to top off non-sealed batteries. Float and charging voltages for gelled batteries are usually about 2/10th volt less than for flooded to reduce water loss. Note that many shunt-type charge controllers sold for solar systems will NOT give you a full charge - check the specifications first. To get a full charge, you must continue to apply a current after the battery voltage reaches the cutoff point of most of these type of controllers. This is why we recommend the charge controls and battery chargers listed in the sections above. Not all shunt type controllers are 100% on or off, but most are. Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes "stratified". You can have very strong solution at the bottom, and very weak at the top of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most - check the manufacturers recommendations, especially on gelled. [h=3]Battery Aging[/h] As batteries age, their maintenance requirements change. This means longer charging time and/or higher finish rate (higher amperage at the end of the charge). Usually older batteries need to be watered more often. And, their capacity decreases while the self-discharge rate increases. Mini Factoids Nearly all batteries will not reach full capacity until cycled 10-30 times. A brand new battery will have a capacity of about 5-10% less than the rated capacity. Batteries should be watered after charging unless the plates are exposed, then add just enough water to cover the plates. After a full charge, the water level should be even in all cells and usually 1/4" to 1/2" below the bottom of the fill well in the cell (depends on battery size and type). In situations where multiple batteries are connected in series, parallel or series/parallel, replacement batteries should be the same size, type and manufacturer (if possible). Age and usage level should be the same as the companion batteries. Do not put a new battery in a pack which is more than 6 months old or has more than 75 cycles. Either replace with all new or use a good used battery. For long life batteries, such as the Surrette and Crown, you can have up to a one year age difference. The vent caps on flooded batteries should remain on the battery while charging. This prevents a lot of the water loss and splashing that may occur when they are bubbling. When you first buy a new set of flooded (wet) batteries, you should fully charge and equalize them, and then take a hydrometer reading for future reference. Since not all batteries have exactly the same acid strength, this will give you a baseline for future readings. When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast. Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure. Inactivity can be extremely harmful to a battery. It is a VERY poor idea to buy new batteries and "save" them for later. Either buy them when you need them, or keep them on a continual trickle charge. The best thing - if you buy them, use them. Only clean water should be used for cleaning the outside of batteries. Solvents or spray cleaners should not be used. Some Peukert Exponent values (not complete, just for info). We don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20. __________________ Sean If Ain't a Newell, It Ain't Wurt Oonin!