Thursday, July 1, 2010

Voltage polarity reversal ...

Q Tom Dunigan, N1VBL, asks, “My wife and I both own 2-meter H-Ts, both of which have failed within the same year. According to service technicians, as well as my own examinations, the failures appeared to be caused by a voltage polarity reversal. I don’t understand how this could have happened unless the entire NiCd battery pack reversed in some fashion. I’ve often powered the H-Ts from a cigarette lighter adapter in the car, but the failures occurred the rigs were running off NiCds. Have you heard of similar problems with other battery packs?”

A The Doctor has been working with NiCds for a number of years now and I’ve seen a lot of odd behaviors. However, while I have seen individual cells in a pack reverse during the discharging process (due to variations in the current capacity between cells), I have never seen a whole pack reverse (or even a majority of cells in a pack reverse). That said, it might be possible to make a NiCd pack reverse by connecting it to a charger in reverse polarity.

The evidence points to something out of the ordinary in the H-Ts. I’d inspect the battery packs again, with particular emphasis on how they attach to the radios. With some H-Ts it is surprisingly easy to install the batteries incorrectly if you are not careful.

From QST July 1999