What do I do if I have to store my batteries outside for winter?
AGM and GEL VRLA DRY CELL batteries can easily be stored in sub-freezing temperatures as low as -30°F/-35°C or lower as long as they are fully charged before storage. The same goes for flooded batteries.
The self-discharge rate of fully-charged AGM and GEL batteries is very low in these conditions, and they will not require charging for many months. Flooded batteries have higher self-discharge rates, so that they may require top charging sooner.
If your AGM or GEL batteries do freeze, they most likely will not recover. To attempt recovery, bring them inside and let them sit until the temperature of the inside of the battery reaches the temperature of the room it is in (approx. 20°C/68°F). Then charge the batteries normally. DO NOT ATTEMPT TO CHARGE THEM UNTIL THE INTERNAL TEMPERATURE IS EQUAL TO ROOM TEMPERATURE. You may have to cycle the battery several times before deciding on its final condition.
If a flooded battery freezes – because it has excess water volume – it will probably freeze solid, expand, break the plates and even crack the case. A flooded battery that freezes is not typically recoverable.