HP-67 battery advice needed


Hi all,

A few months ago, I bought an HP-67 on eBay. I've been too busy to do anything but gaze at it lovingly since, but now I would like to actually start using it. (OK, play with it, at least!)

The problem is, I don't have a charger for it. So, I figured I'd do the same I did with the HP-25 I bought at around the same time -- which I don't have a charger for, either -- that is, I'll just take apart the battery pack, get rid of the old NiCd cells (they're probably shot anyway), and replace them with NiMH cells which I'll charge outside the calculator.

This approach worked fine with the HP-25: I managed to pry the pack apart with my fingers, so if I ever want to sell it, I can glue it back together and it will be as good as new -- I didn't have to break out the piece of plastic in the middle like you see on a lot of old Woodstocks. All I have to do is glue the thing together with a few inches of Scotch tape whenever I change the batteries.

No such luck with the HP-67 battery pack, though: I can't figure out how to pry it apart without damaging it! The plastic seems to be a lot softer than the stuff they used for the HP-25 pack; when I try to pry it apart with a screwdriver, I just end up cutting notches into it, and when I try to pull it apart with my fingers, it gives a little, and then snaps right back the moment I let go.

I'm thinking I should just build my own pack -- fortunately the Classics don't have a pack that doubles as the battery compartment door, like the Woodstocks, so it's OK if it isn't pretty -- and then, if I do it right, I'll be able to take it apart and charge the batteries outside the calculator.

I'm wondering: is there a better way? I have heard there are companies that supply rebuilt Classic battery packs -- are there any that supply battery packs that are easy to take apart, so charger-less individuals like myself can charge the batteries using a regular off-the-shelf NiMH charger?

Thanks for your insights --

- Thomas


Hello Thomas,

to get a new battery pack, I just soldere 3 AA type cells together (without the plastic pack). To prevent "rattling" of the pack inside the calc, you can put anything from clothes to polyfoam between the lid and the battery pack. Soldering such a pack takes 5 min, so you could solder 3 batteries together, or use NiMH and take the pack apart for charging.

There might be easier ways, but this way, you don´t need to damage the old pack!



I just thought of another possibility -- you can get AA battery holders, made of some rather soft plastic and with a coil spring on one end; I think they're meant to be mounted on PCBs etc.

I played with these a long time ago, and I remember discovering that three of those holders will actually fit side by side inside a Classic's battery compartment.

The soft plastic is a pain to work with, and I think I'll need to cut the bottoms off the holders to make the whole assembly flat enough, but I think I'm going to try this -- hopefully I'll be able to constuct a pack this way, that fits inside my HP-67 and from which the cells can easily be removed for charging in a standard charger.

If I get something to work, I'll post a picture later on. Probably next week or so...

- Thomas


I've opened a couple of battery packs (mine and a friend's) and the best way seem to be to use a Xacto knife or single edged razer blade. I clamped the pack in a vise and carefully sliced along the seam. The trick is to take time, making multiple strokes along the seam and slowly cutting the welds.
The first time I did one I was a bit too impatient and dinged up the plastic prying when the welds weren't totally cut. The second one I took my time and you can hardly tell.

BTW, In my unit I had replaced the NiCADs twice. I finally replaced them with NiMH batteries and after testing I found the charger handled them without problem--though it took longer to charge--but it's been working 2+years without a problem.



I buy HP-67 batteries from Mark Hoskins (Waterhosko on eBay). His email is:

waterhoskoNOSPAM@NOJUNKaol.com (remove ALL uppercase letters).




The other replies to your post describe how to open the pack and other possible replacements. One thing, the original pack is welded only along the flat ends (the ends that the battery terminals "point" to). The side seams are not welded.

As far as battery packs that you can take apart to enable outside recharging, I have never seen or heard of such a thing. To provide the necessary stability, the individual cells have to either be glued to each other or in a shell, with the necessary connections between the terminals. This would make removal and separation of the cells for charging somewhat difficult. Perhaps you could modify a standard pack to do what you need, by gluing the needed connector strips into the shell such that the batteries could just be inserted. Not sure how you would insure contact pressure, but you could probably come up with something. The shell could be taped together while in use.

Best regards, and keep up the good work on Free42!


I just found this page:

It describes how to build a charger for packs of 1..7 cells

Unfortunately the description is in german, perhaps you can find a similar project in english, or you can ask me per eMail...


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