|Re: Rebuilding a HP41 battery pack|
Message #8 Posted by Dieter on 18 May 2011, 3:00 p.m.,
in response to message #7 by M. Joury
This topic - rebuilding a 82120A pack and related questions - has been discussed here not too long ago. You will find further information there.
The 82120A battery pack did not use 1/3 AAA size cells, but 1/2 N size batteries. This size is no longer available, but 1/3 AAA is a nice (slightly smaller) replacement. The original batteries had a capacity of about 70-80 mAh, so an empty pack was fully charged after 7 hours. Now think of thousands and millions of users who left their pack charging for 10 or 12 hours, or even longer...
The charging current indeed is about 15 mA. A friend of mine measured 15,6 mA in his pack, and this is confirmed by a quick look at the schematics. Please take a look at this site. The AC-adapter 82066 (European version) provides the usual nominal 8 V AC, but since there is virtually no load the effective voltage is more like 11 V AC. Behind the rectifier and the smoothing capacitor the voltage should be something like 15 V DC, just right for the following 12 V DC-regulator. The charging circuit finally is essentially a 365 Ohm resistor across which the voltage is 12 V minus the 6,2 V Zener diode minus roughly 0,6 V drop caused by D6. Thus the nominal charging current is (12 - 6,2 - 0,6) V / 365 Ohm = 14 mA. Voila.
There are slight differences between earlier and later 82120A packs, there were even packs with three pins. However, I assume the basic schematics are similar.
Using Li-Ion or Li-Po batteries is an option that comes to mind soon once you think about alternative solutions. But the voltage will be a problem. The BAT announciator comes on at 4 Volts, and two cells (around 8 Volts) are too much in my humble opinion.