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Storing a lead-acid battery at a very low charge state can cause permanent crystal formation (sulfation) that reduces capacity. Lithium-ion batteries at low charge can develop copper structures that short the battery, making it dangerous to use. The exact recharge instructions depend on the battery design. Follow these guidelines if you do not have access to manufacturer instructions: Lead AcidRecharge to full whenever voltage drops below 2.07 volts / cell (12.42V for a 12V battery).One charge every six months is typical. Lithium Ion (Li-ion)Recharge to 3050% capacity whenever voltage drops below 2.5 V/cell. Do not recharge if voltage drops to 1.5V/cell.One charge every few months is typical. If your rechargeable batteries drop to low charge levels for more than a few days, they will likely require special treatment before you can use them again: Lead AcidThe battery will usually recharge, but with permanently reduced capacity. If a small lead acid battery fails to recharge, apply a very low amount of current at a high voltage (~5V) for two hours.Anti-sulfation devices are not recommended without an experienced operator. Lithium Ion (Li-ion)The battery may enter "sleep mode" and fail to recharge. Use a charger with a "boost" feature, taking care to apply the voltage with the correct polarity.Never boost a battery that falls below 1.5V/cell for a week or more, as it is permanently damaged and dangerous to use. Nickel-based (NiMH, NiZn, NiCd)No major issues. Some types may require charging and fully discharging a couple times to return to full capacity.For large scale use, consider a battery analyzer that can "recondition" the battery.

Summary:
Recharge lead acid and lithium-ion batteries periodically. Restore discharged batteries.