HP25 Replacement Batteriy Type

[Maximilian Hohmann]

Here are some pictures for illustration:

This is an HP-25 with a "butchered" battery holder in place. You really can't tell from the outside:



This is what the battery holder looks from the other side. All you have to do is remove the central bar between the batteries with a sharp knife. The spring is required and must be preserved. These springs are made from non-corrosive material and survive battery leakage. If it is encrusted with oxidation products just pour vinegar over it and it will become like new. The picture shows a set of very cheap NiMh Batteries from supermarket "Aldi" or "Lidl" which have been powering this HP-25 for ten years or so.



If you are careful with your knife it is possible to cut the bar on one side only and still be able to remove and insert batteries. From a little distance one couldn't even tell that the battery holder has been "butchered" in the process. On this picture you can see normal disposable batteries in the holder which work equally fine. I do not buy disposable batteries myself (as I fully agree with Greta on these issues...), but many of the vintage calculators that I buy come with batteries inserted which are still good for many hours of operation.



And here, most important of everything! , mark the polarity of the batteries in your holder. The calculator is not protected against overvoltage (therefore don't use the charger) and it is not protected against reverse polarity either.



Enjoy your little calculator - these are among my all-time favorites!

Max