Recommanded NiMh battery cells for HP82162A

[Dieter]

(09-25-2017 09:50 AM)Sadsilence Wrote:  Details of charge curcuit within HP41 rechargeable battery pack are perfectly explained here.

Yes, I had already linked to Kees van der Sanden's site in post #9. This is the later version of the 82120A, earlier ones had a 78L05 regulator and an additional contact at the center pin of the charger connector. The newer design avoids some problems of the earlier version.

(09-25-2017 09:50 AM)Sadsilence Wrote:  There is a 1/3 AAA NiMH C=170 mAh datasheet from GP, that states an overcharge time of one year for 0.1C charging current (=17mA) does no harm to the cell.

...if "no conspicuous deformation and/or leakage" occurs. Yes, I also linked to that data sheet in post #17. ;-)

Many, if not most NiCds and NiMHs are designed to handle a permanent charging current of about 0,03...0,05 C, and most of them should even withstand 0,1 C without major harm. So a 0,1 C charge with full 12...14 hours every now and then (if you don't know if the battery is nearly empty or almost fully charged) should do no harm either.

(09-25-2017 09:50 AM)Sadsilence Wrote:  Looking at HP82120A curcuit actual charging current Ic depends on voltage of cells.
Formula is ( 12V - 0.7V - battery voltage )/ 365 Ohm.

Empty pack @ 4*1V = 4V -> Ic = 12V - 0.7V - 4V / 365 Ohm = 20 mA
Loaded pack @ 4*1.4V = 5.6V -> Ic = 12V - 0.7V - 5.6V / 365 = 15.6 mA

That perfectly matches 17 mA standard charge current of mentioned 170 mAh 1/3 AAA cells.

So that's 12 hours for an empty pack.

(09-25-2017 09:50 AM)Sadsilence Wrote:  To get a completely empty battery fully loaded you need around 16 hours (typical formula is 15-16h @ charge current C/10 as there are energy transforming losses).

The actual efficiency factor for NiCds and NiMHs and currents around 0,1...0,3 C usually is given as 0,83, leading to a factor of 1,2x for the charging time. Nevertheless, often a time of 14...16 hours is given for 0,1 C. This is because the actual capacity of new cells may be higher than the nominal value (e.g. for the GP17AAAH the manufacturer states 180 instead of 170 mAh) and at charging rates this low a bit of overcharge does not hurt. But if you look at the respective diagrams in the data sheets you will notice that the point of full charge indeed is reached at about 120% the capacity. You can also see this in the mentioned GP data sheet for higher rates: Here the factor for 0,5 C is about 1,2 and for 1 C even lower at 1,1.

Anyway, we are talking about low charging currents near 0,1 C here, so about 15...17 mA for 12 hours or maybe 14 should be fine for an empty 170 mAh battery pack. And it most probably won't do any harm if a fully charged pack is charged this long once in a while.

Let's not forget: the original 1/2N cells in the 82120A had around 60...80 mAh so the new cells will provide more than twice the operation time. And the correct charging time was around 6 hours back then while HP suggested much more, leading to overcharge. So the rebuilt 82120A now finally meets the original specification of a 12 hour charge. ;-)

Dieter