Two different designs of the 82120A rechargeable battery pack

I have two different battery packs for the HP-41 calculators, both are part number 82120A but they are very different. I have repaired the oldest one with serial number 2203S. With new 1/3AAA batteries it works just fine. I have taken some notes of the charging circuit and I see it is very similar to the classic series chargers. It has one circuit to charge the batteries and another one to keep the calculator working. The plug has three pins and one separated connector. When connected to the 9V transformer the connector doesn´t make contact with the third pin and both circuits work separately. When not plugged the third pin makes contact and the batteries feed the calculator.
The newest one has original cells and works fine. Serial number is 3112S. But it has two pins in the plug. It seems that it works like spices or Woodstock calculators, so it can be dangerous for the calculator if it is being charged and any cell fails. It seems that HP modified it to reduce costs.
Can anybody confirm my impression?. Does anybody know if charging times are the same for both designs?.


Jon, good detective work. The 3-pin version is similar to the classics circuit. Perhaps after the failure lessons of the Woodstocks, they went back to what worked. But, I suspect the kapton film that acted as both an insulator and conductor had reliability, manufacturing or cost issues. So, out goes the safe circuit for a lower cost, easier to manufacture 2-pin unit.

But, unlike the Woodstocks, the 2-pin version 82120A has a 6.4 volt zener diode across the battery so that should be the maximum voltage the calculator will ever see. The only problem with the design is that there is no series current limiting device. When the batteries begin to fail and no longer present a low impedance to the source, the zener diode alone provides the load. Yes, it prevents the calculator from frying but leave it on charge long enough and the diode will fail due to overheating. Ask me how I know... At least they short when they fail and you have a dead battery pack but the calculator survives.

Charging times for both versions are the same.

Edited: 21 Nov 2003, 8:41 p.m.


Actually, the three pin version is not as good or safe as the later version. It can glitch the power when the plug is mated. Also it can overvoltage the calculator if the cells open up or go bad... the later version has a protection zener (which I install in all the old versions when I rebuild them). Both units seem to pump about 12-15 ma into the cells.


David, thank you, thats very good information. Have you ever had/heard of a situation where you end up with the dreaded MEMORY LOST message when plugging the charger in? I recently experienced this with a 3-pin pack and dismissed it to the pack having shifted and loosing connection with the calculator. Sounds like my assumption was incorrect?


Sounds like the switch glitch hitch. Usually the cap in the machine will keep the machine running through the transient, but you know how glitchy glitches can be...

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