HP92266 N cell charger

[Dieter]

(03-22-2015 05:37 PM)TASP Wrote:  As received, the cells had a voltage ~ 1.25 volts. I'm not sure what % charge that translates to.

It translates to nothing. The voltage of NiCd and NiMH batteries is very similar from near-full to almost-empty. It also depends on ambient temperature and other factors. That's why you cannot reliably determine the charge status of a NiCd/NiMH battery. Well, at least not by determining the voltage.

(03-22-2015 05:37 PM)TASP Wrote:  Assuming, an average of 15 mA for a full charging time, the math for a 500 mAH capacity comes out to 33 hours.

First of all, 15 mA was the charging current of the standard HP82120A NiCd-pack when connected to the dedicated AC adapter (82066A, 82059A etc.). This was fine for the low capacity NiCd cells of the day, but it leads to very long charging times if you want to fully charge a contemporary NiMH cell. Actually you have to multiply the 500/15 by a factor of approx. 1,25 (the charging process is not 100% efficient - there is a loss of approx. 20% which leads to a factor 1/0,8 or 1,25). This yields about 40 hours. Or a bit more since low charging currents are a bit less efficient than higher currents.

(03-22-2015 05:37 PM)TASP Wrote:  Meh, that ain't bad. Back in the 80s, I just left a set of NiCADs in it all the time. I've checked around, and NiMH cells in emergency lights, and other standby applications seem to survive trickle charging OK, so if I did leave a set in the charger for a year, it probably isn't a disaster.

If NiCd/NiMH batteries may be used at all with such excessive charging times depends on their construction. There are special battery types that can handle this kind of charging better than others that can be easily destroyed. In the NiCd days the usual limit for trickle charge was about 0,03 CA, which in this case translates to 0,03 x 500 mA = 15 mA. But I can't say anything about the batteries you use. Anyway, since 40 hours are required for a full charge, this charging time should not cause major problems, provided the battery was empty before.

(03-22-2015 05:37 PM)TASP Wrote:  As it is, I'll charge sets (I have 3, so far) for 48 hours, and then bag them and and put them in the kitchen battery drawer, ready for another stint in a 41.

NiCd batteries, and especially today's NiMHs, suffer from self discharge. After a month the capacity may be down to 70%, after a year you can consider a previously fully charged NiMH empty. So it's not a good idea to charge these batteries and then store them in a drawer for an undetermined time.

There is a solution to this problem: NiMHs with low self discharge, e.g. Sanyo/Panasonic Eneloop batteries. But AFAIK these are not available in small sizes like the N-type you need.

Get a decent charger with moderate charging current and processor control (e.g. the mentioned BC700) and charge your batteries as you need them. This can be done in 2 or 3 hours. And after just 30 minutes you already have charged the same capacity as the one you used to get with the HP battery pack in 1980. ;-)

Dieter