HP41 three prong Rechargable battery


I have several three prong battery packs for the HP41. They have the same 82120A number. Are these for different calculators or for different volts?


No these are an earlier model of the same device.

The middle connector is actually part of a switch. You'll notice another piece of metal pushing against it. This contact opens when you insert the charger.

Beware. This version of the charger can output a much higher voltage to the calculator if the nicads are baddly deteriorated (to the point of being open circuit)

This charger is a little smarter than its replacement, but I doubt whether anyone would notice the difference.

In essence, if the nicads are OK then they are totally interchangable.

Another difference you might notice is that the models with 3 prongs also have a yellow (well mine are) plastic film over the ends of the nicads. The connection to the calculator is on this.

On the more recent version the ends of the nicads are the terminals.


Can these be modified to the two prong?


Well the internal circuit is quite a lot different, so it would be a rather large effort.

But the easier solution is to place a 6.2V Zener across the output. A 6.2V 1W zener can be easily wired to the board, and then you have all the benefits of the old design with complete safety should the batteries become open circuit.

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