Message for David Smith re: batteries


The thread on some of the messages get quite long and I don't think you saw the one from Erik asking where you get your 150mAH NiMH batteries for the HP-41 rechargeable battery pack. I would also be interested in the answer to this. Thanks.


Several months ago I found a place that had around 50 of the 1/2N cells in stock. I bought them all and had them wired them up into groups of 4. Unfortunately I have used them all to rebuild packs. They tried to find me some more but were unsuccesful.

It looks like it's NiMH time from now on. I have been unable to find a source for the 150mAH NiMH cells that TGWORKS sold. TGWORKS told me that they were Sanyo cells, but my Sanyo supplier could not find them in his catalog.

#4 sells 1/3 AAA NiMH (100mah) cells for $2.00 and 1/3 AAANiCd (50mah) cells for $1.50. They'll add solder tabs for free and ship very quickly. I'm quite pleased with this company.


Thanks so much for the information.


Will 1/3 AAA NiMH (100mah) cells work properly with the 41C charger?



Yes, they should work just fine. I have found the 1/3AAA cells to be a bit too small for a good fit without a lot of shimming, etc. That's why I liked the 150mAH cells. They were only a little shorter, but their diameter was a good fit.

The HP battery pack dumps a pretty good slug of current into the batteries on charge (around 15mA). This amounts to a C/5 charge rate for the original NiCd cells, C/7 for the 100mA NiMH cells, C/10 for the 150mAH cells. I prefer not to leave NiMH cells on a constant charge of over C/15-C/20. The 15mA trickle charge rate does not seem to heat the cells up, so it is probably OK. To assure maximum battery life, you may not want to charge them for more than 10 hours or so. Also stop charging if they begin to get overly warm.


I'm curious about this statement:

"To assure maximum battery life, you may not want to charge them for more than 10 hours or so."

I have no experience with NiMH cells but I've noticed that for regular NiCads, standard charge is usually defined as C/10 for 15 hours. If I followed your post correctly, the 10 hour recommendation referred to 150 mAH cells at 15 mA. Are NiMH cells different from standard NiCads in this regard? If the extra 50% charging at the C/10 standard rate is good for full capacity and battery life, would charging for less than 15 hours constitute undercharging? If so, would it lead to the "memory effect" (which I understand has been de-bunked) or just reduced capacity per charge? In general, is the important thing for long life and full capacity to make sure you don't recharge till the cells are quite discharged (down to 1V or less), always making sure to avoid reverse charging in multi-cell strings?


Yes, you usually need to overcharge a battery from 20-50% for a full charge. I was trying to be safe with the NiMH cells for a while since I don't have a long history of using them in the HP41 pack. Best thing to do is keep an eye (and finger) on them and take them off charge if they start to get warm. This gives a good feel for how long they should be charged. The NiCD cells in the HP41 pack don't seem to heat up even with extended charge times. They have quite a bit of mass for 15mA to warm up, I think this is why HP dumps such a big slug of current into them without ill effects.

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