HP-67 Battery Pack


What is the best way to open the HP-67 battery pack so I can replace the three rechargeable AA cells? The battery pack seems very monolithic. I plan to replace with nickel metal hydride rechargeable batteries (2000mAh, 1.2V) any problem there?



I have done this, but used NiCads. There should be no problem with NiMH batteries.

I used a sharp craft knife to ease the 2 halves of the plastic appart. The batteries are then coupled together with metal strips. CAREFULLY remove and keep these, noting the way they are connected and the way round the batteries are (take some pictures).

I would use some smaller batteries WITH SOLTER TAGS (get the ones which are the same diameter as AA cells but a bit shorter). You may have to get these via the web or mail order.

Reassemble, you will have to do a neat job as there isn't much room and the battery is kept in place by pressure from the battery cover.

PS I don't recommend you solder to batteries, in hindsight I was a bit silly to do so. Batteries may explode or give off nasty fumes if heated.


I did not open the original pack. I just soldered 3 AAs together, and put them in.
I had to replace the battery contacts of my HP-67, so I used stiff silver ones. These keep my battery pack exactly in place. If you have the original contacts, you might have to put some foam or cloth in, to prevent rattling of the battery pack.
My solution is working for 3 years now.

Soldering the batteries is easy with a hot soldering iron, but Tom is right: it is easier to use bats with solder tags.


Klaus wrote:

> Soldering the batteries is easy with a hot soldering iron, but Tom is right:
> it is easier to use bats with solder tags.

No just easier, safer too. The warning about the danger of leakage or explosion is real.



You are right, rechargeables may be damaged (or may explode) if soldering is done improperly. If done properly, there is almost no risk. Spot-welding the tabs is a hot process, too.
I know some guys fond on RC-devices, and they all solder their packs.
Motto: If you know how to do it, do it yourself. If you are not sure, ask a professional.


Just take the pack to your local Batteries Plus or Batteries, Etc store. They can open the pack and replace the cells with a properly welded up replacement. Should cost $15-$20.


David - I got the pack open very cleanly. Took one look at the Rube Goldberg connections and said to myself oh no -- no way. I'm not a "Joe Solder". I will take it to a specialty shop that works with batteries. Thanks to eveyone for their help.



One way to minimize the amount of soldering is to use one of the three-paks sold for portable telephones. Typically they come with all three cells welded together just as you want them and with tabs welded at the positive and negative ends to which wires are attached. You can remove the wires and cut and bend the tabs to mate with the battery pack connections of the calculator battery pack. I repaired a number of different calculator battery packs this way.

I don't buy new telephone packs unless they are on sale. I get them from discarded telephones, etc. Of course, if you are going to use the discarded items you need to check the condition of the cells before you do the work of installing the set in your calculator battery pack. Until three years ago there was an electronics store in Fletcher, NC which sold the discarded packs for a dollar a piece and the buyer was allowed to check the voltage output before buying. In the last few months I have managed to buy several telepohone packs at clearance sales at Wal-Mart for three to five dollars.

There are some things that you need to watch for:

1. Some telephone packs look like they contain three cells but actually contain only two!

2. Some telephone packs include a fuse inside the pack.


I have the all the proper tools to do the battery packs... but I still take them to my local battery store. Costs me a few bucks more, but they do a very good job and it ain't worth the hassel of doing it myself.


Now that I've sent the batpak away for replacement, can I safely opererate the calc on the AC power converter without the pack in place?



I would advise against using the 67 on the adapter with no battery pack. The card reader control board is hard wired directly to the battery circuit (it doesn't get switched over to the adapter's 5vdc power supply by the wafer switch when the adapter is pluged in like the rest of the calculator does.) The no load voltage on the constant current battery charging circuit out of the adapter is around 14vdc (if memory serves), and with no battery there would indeed be no load. It may be OK if you don't use the card reader, but IMHO, I wouldn't risk it.
Best regards, Hal


Yes, don't use the AC plug without a good battery pack. However, you can hook a DC power supply on the battery contacts.

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