Re: Reverse Polarity NiCd
- From: Roy Lewallen <w7el@xxxxxxxxx>
- Date: Sat, 08 Jul 2006 12:15:10 -0700
MG wrote:
Hi
I have a NiCd battery from a Jet Ranger Helicopter containing 19 cell
units to give a 24V 13Ah rated unit. However, 85% seem to show
negative response when tested with a multi meter. Do I take it they
are in reverse polarity state? If so, anything to rectify them?
The full details of the battery are:
FB Type MA-103M
Manufactured by Furukawa battery co Japan (circa 1998)
P/nos.503936
Thanks for any help/advice in getting this batt usable again!
A cell can be reverse charged during battery discharge if it fully discharges before the other cells in the battery. The reverse charging is a result of the discharge current from the stronger cells flowing backward through the weaker discharged cells. That's why discharge of a battery should be terminated before any cell fully discharges.
Some manufacturers claim their NiCd cells will tolerate a small amount of reverse charge without damage. However, if the cell is reverse charged for a sufficient time or at a sufficient rate, pressure builds, the cell vents, and it loses capacity. This will cause it to fully discharge even earlier the next time the battery is discharged, and get more severely reverse charged. So the battery deteriorates further when this has been allowed to happen.
What you can do is to charge the battery with a constant current. The voltages of the reversed cells will eventually drop to zero, then increase again to a normal value in the right direction. Apply a full charge of 14 - 16 hours at C/10 (1.3 A in this case) after the last one turns around to the right polarity. Then test the battery by discharging it at a known rate, monitoring the cell voltages. Terminate the discharge when the weakest cell drops to 1.0 volt, and don't in any case allow any to get to zero and begin a reverse charge as they did before. You'll probably find that the cells which were reversed are the first to fully discharge. The capacity might improve after a few charge-discharge cycles, but I'll bet you'll find the capacity to be considerably less than its specified 13 Ah. Whether the battery is still usable will be your decision.
Roy Lewallen
.
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