Q-38: After discharging my batteries (either by flying or when simply discharge testing them), should I recharge them before storing or just leave them in a discharged state?
A-38: Believe it or not, you can store Ni-Cd or NiMH cells either way.
I recharge them when I return from a flying session and usually recharge them
every month or so until I fly again. If I perform a discharge capacity check
(also called cycling), I also recharge the battery when the test is concluded.
Since Ni-Cd and especially NiMH cells lose much of their charge in just a
month’s time, these cells are never exactly topped off at “full.” Keeping the
cells close to full charge while storing them has worked for me for many years.
On the other hand, my flying partner Tom Hunt has always stored his batteries
discharged and only recharges them just before a flying session (or let’s say
the night before). However, he does make sure that the pack has more than 1.0
volt per cell (at rest, with no load) before leaving it dormant for any length
of time. If the pack has too low a potential, it could damage the cells if the
voltage drops too low during self-discharge. He has had no problems using this
technique for many years.
So it seems that either way is okay and is apparently not that important. The
only thing that is important is that the batteries are at or close to full
charge potential just before flying. But regardless of the way you store your
batteries, it will take a few “cycles” to wake up the pack to full capacity
after it has been dormant for some period of time.
With respect to the new Li-Poly batteries, the jury is not in yet! Most
“experts” say that you should store them charged. Since the Li-Poly batteries
lose very little charge—only 1- or 2%—during a six-month period, it's easy to
store them at full or close to full charge for long periods of time. You could
conceivably store a charged Li-Poly battery pack and fly with it six months
later, with a perfectly normal motor run time.
By the way, you should not trickle-charge Li-Poly batteries!
An interesting thing has come up recently concerning Li-Poly battery charging.
It has been suggested to me that when you first connect your Li-Poly batteries
to the charger, you should initially set the charge current for a low value,
such as 100 mA. Hopefully your charger has variable charge current and an LCD
screen that lets you monitor current and voltage.
Observe the battery's voltage with that initial charge level. Allow it get to
approximately 3.5 volts per cell. That should happen quickly, but it will depend
on how discharged the battery was at the start.
Once it has reached 3.5 volts per cell, adjust the current upward to the normal
recommended current of “1C” (or one times the rated capacity of the battery).
Allow the charging to take place until it cuts off at close to full capacity and
at 4.2 volts per cell.
As I mentioned, this is a suggestion; the need for this charging technique has
not been proven. Would any Li-Poly battery experts out there like to comment on
this charging technique?