Recommanded NiMh battery cells for HP82162A

[Dieter]

(09-25-2017 03:26 AM)Neve Wrote:  It was originally delivering 5.8v. Now it shows 4.94v. Even after 10h or so (I guess...) of charging it doesn’t go back up to 5.8v. Is this normal? I guess it would be since the original charger was not designed to charge Ni-Mh batteries. But I just want to make sure.

Both NiCd and NiMH batteries are charged with constant current. And that's exactly what the circuitry in the 82120A pack provides. It should deliver something like 13...16 mA. I am not sure about the capacity of the original 1/2N cells, but if we assume 70 mA this would mean a charging time of 70/15*(1,2...1,4) ~ 6 hours (assuming the calculator is off). In general, the charging time is the battery capacity divided by the charging current, times an efficiency factor which for NiCd and NiMH is about 1,2 (a bit less for high currents and a bit more for lower ones, here often a 1,4x factor is stated).

Re. the 5,8 V 4,94 V thing: this is the first time I hear of a NiMH battery where charging substantially reduces the voltage. That's hard to believe, I'd say there was something wrong with the 5,8 V measurement. But you're the only one how can know. #-)

(09-25-2017 03:26 AM)Neve Wrote:  What are the risks of damaging it if I overcharged? I’m not sure how long I left it charging as I just recently read that 10h was the max time.

Overcharge will heat up the batteries and finally damage them. However, the relative charging current with today's 170 mAh cells is merely 15/170 = 0,09 C so that a full charge would mean about 15 hours, and exceeding this time a bit should not harm the batteries.

Please note that the charging time refers to a completely empty battery pack. So for a halfway depleted battery it's only half that time!

(09-25-2017 03:26 AM)Neve Wrote:  On a side note, I also read that the charger is just a transformer. No electronics inside.

Right. There is not even a rectifier, the output is AC.

(09-25-2017 03:26 AM)Neve Wrote:  The electronics inside the pack is what regulates the charging cycle. Do I understand this right?

Yes. You should take a look at this site, also consider the links down the page. But note that there were different versions of the 82120A which may have different electronics inside. Early samples featured a 5 V regulator and other differences (e.g. here), so you have to check what you got!

(09-25-2017 03:26 AM)Neve Wrote:  If this is the case, can it be modified to charge faster and/or more efficiently? If so, how?

Since I am not an EE either I will not suggest any modifications. But AFAIK the 78L12 can handle up to 100 mA and the AC-adapter also has plenty of power (it was also used to charge the Sub-C cells in the printer's battery pack), so this should not be much of a problem. But there still is no charge control, so you should not choose a higher charging current than, say, 20 mA which – for your new 170 mAh cells – means a max. charging time of 12 hours. If you got the version with a 78L12 and 365 Ω resistor I'd say the latter may simply be replaced with 250 Ω – but again: I am not an EE so let's hear what the experts have to say.

Dieter