Q. You have a fantastic magazine; your articles always seem to answer many of my model aircraft questions. But one question that seems to come up quite often in our flying group is the proper long-term storage of Li-poly batteries; everyone has a different idea on how we should store them over the winter. Is there a proper way to store LiPo batteries over a long period of time, like three to four months? — Dave Rogers
A. There are certain things you can do for long-term LiPo storage that will prolong the life of a battery pack. How and where the packs are stored is perhaps the biggest factor in prolonging their performance. Keep batteries in a cool dark environment and not in a place with temperature extremes such as a car, a trailer or an uninsulated storage shed. High temperatures will destroy a battery in short order, so always keep battery packs out of the sun and heat. The other extreme is allowing packs to freeze; this will also damage them beyond repair. A refrigerator that maintains a temperature of about 40 to 45 degrees is just about the perfect place to store packs (do not use the kitchen refrigerator that has food in it). Allow the packs to come to room temperature before using or charging.
LiPo batteries do self-discharge—granted, at a very slow rate; but over time, they will lose their charge. Packs that go completely dead, or fall below 2.5 volts per cell, can be damaged beyond repair and thus become useless. Never store a discharged battery for long periods of time. Also, don’t store a fully charged battery because the cells will drift and discharge at different rates and result in a pack in which the cells have become out of balance from one another. If left unbalanced, the cells in this battery pack will continue to drift farther apart after each charge and discharge cycle. The best thing to do is put the batteries away with a “storage charge” of about
3.85 volts. This gives each cell enough voltage to keep them stable for long-term storage. The cells will discharge at a similar rate and maintain a better balanced pack over time. When it’s time for the charge and discharge cycles to begin, the battery pack will start the cycle in balance and will perform better. The other advantage of the 3.85V charge is that it still provides plenty of storage time before the pack reaches the low-voltage minimum of 2.5 volts per cell.
I recommend that you label the battery with the date and type of charge (i.e., full or storage), as it’s easy to forget when you’ve last charged a battery pack. Also, when you charge the battery for the first time after storage, charge it at 1
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