'Maintenance free' or sealed batteries can lose water if they are overcharged for extended periods of time. Actually the term 'sealed' is a misnomer since they do have vents to allow gas to escape during charging.
I have switched to Concorde Lifeline AGM batteries for my house batteries and will likely switch to the same batteries for my starting batteries when they need replacement. They are completely sealed batteries and will not leak acid even if the case is broken open. They are expensive but I don't have to worry about them boiling out their water.
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