HP 82120A (Hurricane) - Rechargeable battery pack

The HP 82120A is a rechargeable battery pack for the HP-41 series calculators. This pack contains 4 small (1/2 N size) rechargeable batteries and an electronic circuit to charge these batteries. After years of using this module the cells no longer keep charge and start leaking. I opened the module to prevent it for additional damage caused by the leaking cells and to look for a possibility to replace the cells.

	The HP 82120A module. To open this module a sharp knife did the job.
	The interior of the battery module. The charger electronics in build on a small printed circuit board. The four rechargeable cells are visible on the bottom. The cells are model 1/2 N and have a diameter of 12.8 mm and a length of 14.1 mm. The four cells are connected in series.
	The backside of the small printed circuit board. The battery cells and the printed circuit board are in separated compartments. Leaking batteries will not directly damage the electronics.

 

The schematic of the charger part can be easily extracted from the PCB design. The same schematic is found in the Battery Refurbishment document of Helmut Wabnig, see link below.

The exact model for the diodes is hard to read on the components and the coding contains non standard model numbers. Diodes D1, D2 and D6 are the 1N4002. The diodes D3 and D4 are different, smaller types and are coded K 020 965 838 with the logo of Motorola. Why HP selected two different type of diodes to build this diode bridge is not clear for me. The coding on D5 is K 020 057 834, also with the Motorola logo. This D5 component is a zener diode with a zener voltage value of 6.2 Volts.

 

Box with battery pack and manual

 

Rebuild of a HP 82120A battery pack

On Ebay rebuild battery packs of the HP82120A are offered. See picture below. On the same auction site the "applied" batteries were found and I ordered these batteries to rebuild my battery packs.

Unfortunately these batteries didn't fit. The length was just a few millimeter too long. The cells used in this Ebay rebuild look very similar to the ones I bought. Same color and same imprint. The batteries can be inserted in the pack but no room is left for the original battery contacts. The battery contacts are also used as a spring. Very tight placement of these battery contacts will prevent its elasticity. My expectation is that this elasticity is needed to easily place and remove the battery pack. I was afraid that the absence of a proper elasticity could damage the battery contacts of the calculator. The only way to free up some extra space is to remove the partitioning.

 

HP 82120A related Links

• HP41 Rechargeable Battery Pack Refurbishment by Helmut Wabnig
• HP-41 Bat by ASkr
• HP82120A repair by WHS
• Wiederbelebung eines HP-41C Akkupacks (82120A) by Malte Weyer (German)