Calculator blog

Musings and comments about our common interest


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Tomcat battery prototype

image0-38We have received the first two halves of the Tomcat battery bay replacement. The initial tests with the Sub-C batteries show that it is a little tight in all directions for the batteries. We'll enlarge it . Both halves clipping together, on the other hand, works well as expected. Now we need to solder the terminals and it should be ready! anyone interested?

The prototype is in white but most likely the final units will be in black. Problem is, the parts are very expensive and I don't see it being below 50€ VAT included (20% cheaper if outside EU). On the other hand, the slimming that is needed (remember the little tightness inside?) can cut somehow the cost. In any case it is much better than the products I have seen in eBay.

This battery will work with HP92, HP97 calculators and diverse HP41c printers: HP82143A, HP 82162A IL printer, etc.

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Battery pack for HP printers and HP92, HP97 machines


By now, all original batteries made for Tomcat machines and HP41c printers cannot hold a charge anymore. There are several replacements in the market, all of them using the ubiquitous Sub C batteries, in different energy capacities. These batteries are cheap, and most of the packs are held in place by some rubber foam corners and adhesive film. You can find them in eBay mainly. 

I have tried to break apart two one of the original batteries that I have, to refurbish the original cells for new, current cells, but it was impossible without doing a lot of damage. This cannot be the right way to do more than one of them, and forget about series production! So we decided to make our own case

We have designed a two-halves case for these batteries, with precise holes for the connectors, to be 3D printed. Here the main challlenge is the cost of the parts in any 3D provider: it is far too expensive and it eats most of the cost. The parts are clipped one onto the other - and like the original, it is a single part used for both sides. We have not decided yet whether to offer it as a kit or send them fully assembled - I am quite frightened with batteries as opposed to passive, non-energy storing parts. There would be some soldering involved, in order to connect the cells among them and to the terminals.

There may be still some work to do in optimizing the part to reduce material - I can see a couple of areas where material can be taken out. However is the sheer size of the piece that determines most of the cost, I think.

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