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Hsilop · 10-20-2016, 04:50 PM

Hi!

I am looking for an old HP 34C or similar calculator, and was wondering about how to go about charging it, what (replacement) battery packs to use, etc.

I am guessing that rechargeable batteries in those days were quite different from those today, and would require careful maintenance in order to perform their best.

Should I charge them as seldom as possible, letting them run down completely and then charge them as fully as possible - or should I charge them as often as possible and try to maintain full charge as much of the time as I can?

Could I damage the batteries or the calculator while charging, e.g. by overcharging?

Most grateful for all help - especially from people who remember how things worked in the good old days ... :-)

Maximilian Hohmann · (This post was last modified: 10-20-2016 05:22 PM by Maximilian Hohmann.)

Hello!

(10-20-2016 04:50 PM)Hsilop Wrote: I am looking for an old HP 34C ...

Good luck with that, the 34C is one of the most difficult to find. And the prices asked are simply ridiculous. This whole "30 series" is not one of the best product lines from HP. It is almost impossible to find one of these without cracks in the (very brittle) plastic case or some degree of corrosion in and around the battery compartment.

(10-20-2016 04:50 PM)Hsilop Wrote: I am guessing that rechargeable batteries in those days were quite different from those today, and would require careful maintenance in order to perform their best.

This calculators use the same 500mAh NiCd AA cells as most earlier ones. Nothing special there. Apart from the "memory effect" (https://en.wikipedia.org/wiki/Memory_effect) these batteries do not much differ from present day NiMH cells, apart from their much lower capacity. The only problem is that charging them with the original charger will take much longer because the charging current matches the lower capacity original cells.

Personally I replace the original NiCd cells (be careful when disposing them, Cadmium is very poisonous, best thing is to take them to a dedicated recycling station) with NiMH cells and charge those out of the calculator in a dedicated charger. Thereby the batteries will be charged they way they should be and there is no risk of damaging the calculator either.

Regards
Max

Hsilop · 10-20-2016, 05:45 PM

Thanks, Max!

(10-20-2016 05:20 PM)Maximilian Hohmann Wrote: ... NiMH cells and charge those out of the calculator in a dedicated charger.

Are there complete NiMH battery packs for the 34C, with chargers, or would I have to get plain cells and tape them together (but then they wouldn't fit in the charger without disassembling, would they - and would they also need to be soldered together in series)?

(10-20-2016 05:20 PM)Maximilian Hohmann Wrote: This whole "30 series" is not one of the best product lines from HP.

Would you recommend the 25[C] or 29[C] over the 34C?

Dieter · 10-20-2016, 05:53 PM

(10-20-2016 04:50 PM)Hsilop Wrote: I am looking for an old HP 34C or similar calculator, and was wondering about how to go about charging it, what (replacement) battery packs to use, etc.

This has been discussed here several times, for instance in this thread.

Post #6 describes the way I use regular NiMHs in my 34C.

Dieter

Maximilian Hohmann · 10-20-2016, 06:15 PM

Hallo!

(10-20-2016 05:45 PM)Hsilop Wrote: Would you recommend the 25[C] or 29[C] over the 34C?

Personally I prefer any of that "Woodstock" series over the "30 series" unless you really need the extra functions of the 34C (integration and solver). But a working 29C will be very very difficult to find, even more difficult than the 34C.

If you want a real good undestructable classic HP calculator, look for an HP-67. It doesn't get much better than that. And there is someone on eBay USA who still makes replacement battery packs for a very reasonable price.

Regards
Max

hibiki · 10-20-2016, 06:23 PM

I also use NiMH rechargeable batteries (Duracell) in my 34C. Its battery contacts are broken and although I have the battery pack skeleton, I use a thin copper sheet to make the contacts between the two cells in the opposite side of calculator contacts. The batteries fit well inside the calculator and I recharge them out of the calculator (I have also the power adapter).

Hsilop · 10-20-2016, 06:45 PM

Thanks again!

So two AA size, 1.5V rechargeable cells will do, if I just connect them in series? Will non-rechargeable alkaline batteries work also in a pinch?

PANAMATIK · 10-20-2016, 06:48 PM

(10-20-2016 06:15 PM)Maximilian Hohmann Wrote:
(10-20-2016 05:45 PM)Hsilop Wrote: Would you recommend the 25[C] or 29[C] over the 34C?

Personally I prefer any of that "Woodstock" series over the "30 series" unless you really need the extra functions of the 34C (integration and solver). But a working 29C will be very very difficult to find, even more difficult than the 34C.

Another possibility: If you want to have the HP-34C functionality with solver and integration in a woodstock case, you can buy a cheaper and easy to find defective HP-21 or HP-25 and replace the ACT by the new ACT programmed with HP-34C. It will be delivered with HP-34C vinyl overlay.

Bernhard

hibiki · (This post was last modified: 10-20-2016 07:08 PM by hibiki.)

(10-20-2016 06:45 PM)Hsilop Wrote: Thanks again!

So two AA size, 1.5V rechargeable cells will do, if I just connect them in series? Will non-rechargeable alkaline batteries work also in a pinch?

The original battery cells are flat in both side, what means that they are smaller than regular AA cells. As my calculator spring contacts don´t exist (but enough to make connectiow with the batteries), the space is enough for the batteries. Try to find the smallest AA cells (in length not in capacity), yes, they are not the same size depending on the manufacturer.
Fresh alkaline cells have more than 1,5V, NiMH charged has 1,4V max. Better not to risk your calculator (or don´t use fresh alkalines).

Ron Ross · 10-20-2016, 07:14 PM

If you just want a good LED RPN Hp calculator, you might also consider an Hp 37e or 38c. Both are financials, but are about half the cost of the Hp scientific calculators and are often in better shape due to being in an office setting vs often being used in the field for the scientific calculators.
.
You can compare what functions they do have with the museums compare features on the main page.

Hsilop · (This post was last modified: 10-21-2016 06:16 AM by Hsilop.)

I found this NiMH battery pack on ebay. It says rechargeable NiMH, output voltage: 2.4V, and capacity: 2000mAH. The seller is a long time ebay member with 100% rating who advertises on this website, so I assume this is a safe buy, but I would much like the opinions of the forum members.

The capacity is larger than other battery packs I've seen on ebay. Is this a good thing - or a potential problem?

Also, the output voltage is 2.4V, not 2.8V (2 x 1.4V) as suggested in Hibiki's last post. Is this OK?

Would it be OK to charge this pack in the calculator, or should it be charged outside (if so, how - take it apart each time)?

Maximilian Hohmann · (This post was last modified: 10-21-2016 09:42 AM by Maximilian Hohmann.)

Hello!

> ... so I assume this is a safe buy...

110% percent safe :-) I am not related in any way to this seller, but have already bought battery packs and other parts from him - like I would guess almost every other collector of HP calculators on this forum.

> Also, the output voltage is 2.4V, not 2.8V (2 x 1.4V) as suggested in Hibiki's last post. Is this OK?

That's no problem at all. The original NiCd batteries had 2.4V so this is what the calculator expects to get from them.

> Would it be OK to charge this pack in the calculator, or should it be charged outside (if so, how - take it apart each time)?

You can safely charge this pack in the calculator but be aware that it has four times the capacity of the original pack, so will require four times as long to charge (more than a day!).

Regards
Max