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?.
Two different designs of the 82120A rechargeable battery pack
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