HP-31E Battery Pak


Dear Forum,

One of my sons, knowing of my interest in old HP calcs, gave back to me for Christmas the 31E that I had given him in 1981.

It is in excellent condition with Owners Manual, AC adaptor, but minus the battery pak. My son says it works fine with just the adaptor. But I remember reading in the Forum as to the 25C, never to use the calc on the adaptor without the pak in place.

Questions: Is this true for the 31E and can I somehow rig up a pak using two AA rechargable batteries?



Two AA ni-cad/ni-mh cells and a bit of foil opposite the connector to complete the connection will do in a pinch.

Long-term it's best to get two cells with a strap welded on to make the connection. Don't have the cells glued together, best to have them loose so they can make proper contact. It's the same arangement for Woodstock batteries.


There's next to nothing to do to make a 3x (Spice) series battery pack from scratch. Just order part number P253-F021 from Digikey http://www.digikey.com and put a piece of tape across the gap (between the cells) on one side of the pack to prevent you from inserting it into the calculator the wrong way. Take a look at these pictures to see the right orientation:




The woodstock machines need the batteries to not be glued together for proper contact. The spice machines have those spring contacts in the battery compartment that work best with cells that are glued together.


Dear Trent,
It also works without adapter.
Just use two 1.5V batteries instead of rechargeable 1.28V batteries.


Thank you all so much. It works!



Don't forget that you cannot use 1.5V batteries with charger!!!


Thank you Emmanuel.

I'm using 1.2V Nickel-Metal Hydride.



I've used loose NIMHs too and it works great. I've also used alkaline cells and since i was not sure if the calc wanted a full 1.5v i fed it slightly used batteries.

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