Q-115: I wonder what the shelf life of the new Li-Poly batteries is as compared to the NiMH batteries? Also I have read that some of these new Li-Poly batteries might possibly explode and that has me concerned about my safety. What are your thoughts on this?
A-115: First, the easy answer. The shelf life of the new Li-Poly batteries is extraordinarily good! You might expect to lose approximately 1-2% of capacity over a six-month period. For that reason you should not consider leaving these cells on long-term trickle charging. In fact, Li-Poly batteries should never be trickled charged.
NiMH cells tend to have a poor shelf life and can lose a good portion of
their capacity in just a month’s time. Trickle charging these cells is
encouraged. You should make sure you charge a NiMH battery pack at the overnight
rate when you plan to fly the next day. That applies to your RC-system batteries
as well as your electric-power batteries. This goes for Ni-Cd cells as well, but
their shelf life appears slightly better than NiMH.
Now a few words about exploding Li-Poly batteries. Li-Poly batteries do not
explode. Their characteristic soft-pouch wrapper or container does not really
permit the buildup of internal gases when the battery is overcharged or deep
discharged. The wrapper may swell. In the worst case, there may be some
resulting flames, but not an explosion.
Since the introduction of Li-Poly batteries to our hobby, there have been stories of Li-Poly batteries "exploding," flaming, and such! Most of these incidents resulted from improper charging (wrong cell count selected or wrong charge current selected). But we are constantly seeing better and safer chargers appear on our hobby market. The Li-Poly cell quality has been constantly improving. More importantly, our modelers are learning the proper techniques for charging and using these new batteries.
I attended the big JR Indoor Electric Festival held in Columbus, Ohio (a report on this event was in the February 2005 MA). There were 117 registered pilots at this 2-1/2-day activity. On the Saturday alone more than 550 documented flights were made. Almost everyone who flew used Li-Poly battery packs. Probably more than 1,000 batteries were recharged during the weekend.
The event director provided sand pails and special fire extinguishers as a
safety precaution, but there was not a single incident of anything other than a
slightly warm battery pack during the entire event. So my answer to those who
are still apprehensive about this new technology is that the time has come to
reconsider and begin enjoying all of the advantages of Li-Poly-powered electric
flight!
A final note on this subject of Li-Poly batteries. Soon you will see even more
sophisticated chargers available. The next generation will have each cell within
a pack charged individually. The new Li-Poly battery packs will be wired with
special connectors that permit access to each cell. As each cell within the pack
reaches its maximum allowed voltage, the charging to that cell is cut off. The
other batteries in the same pack will continue to charge until the maximum
voltage is reached.
When all charging ceases, each cell within that pack should be at the same potential, meaning that they will be perfectly balanced. Because each cell is being charged separately, you will no longer have to worry about selecting the correct number of cells on your charger. Should you set too high a charge current, each charger output will cut off as a safety feature. This improvement in Li-Poly batteries and chargers should eliminate all previous objections to their use.