Converting HP-25 to NiMH batteries, possible?

[cubytus]

Hi there,

I found a working HP-25 calculator, and quickly learnt that it requires a battery pack installed to be working properly. Only, the calculator was designed well before NiMH batteries were standard. Fast-forward 45 years, NiCad batteries are getting harder to find, mainly due to lower capacity and higher toxicity.

Using NiMH batteries kind of works as long as they're charged on a different charger, but can't be charged using the stock HP power adapter.

Given the extremely simple charging circuit, is it possible to convert this calculator to NiMH batteries? Or am I better off trying to find NiCad batteries?

[KimH]

I can say that I have done this on my 25C - found the NiMH without the "buttons" - jogged them in a charger - put them in the "battery Cradle" and it worked - charging should be OK as long as they charge slow and they do.

No guarantee for you, but for me it worked.

[Maximilian Hohmann]

Hello!

I operate these calculators with NiMH batteries only. Works nicely as the batteries have 4 to 5 times the capacity of the original NiCd batteries. And always charge them externally, never ever connect the original charger to the calculator. Never.

Regards
Max

[Dave Britten]

I agree with Maximilian on this. I use a couple of flat-top AA NiMHs in my 25, and charge them with an outboard charger. The power design of the Woodstocks just makes it not worth the risk to charge in the calculator. I don't think I even own an HP charger for the 25. Plus the outboard charger will be faster, and you can just swap between a couple sets to "recharge" the calculator instantly.

[Wes Loewer]

(07-20-2022 04:26 PM)Maximilian Hohmann Wrote:  And always charge them externally, never ever connect the original charger to the calculator. Never.

I believe you, but I am curious as to the reason behind this advice. NiMH batteries have similar voltage as NiCd batteries, so I would assume it is related to the internal resistance. If so, I'm surprised that at relatively low current that this would make an appreciable difference.

Or perhaps it's something else I haven't thought of.

[Maximilian Hohmann]

Hello!

(07-20-2022 06:06 PM)Wes Loewer Wrote:  Or perhaps it's something else I haven't thought of.

Yes, it's something different. There are countless threads on this topic but it is always worth reminding fellow collectors and calculator users: The chargers of the "Woodstock" series calculators have a poorly stabilised output voltage. Connecting one to the calculator without a battery in place will almost instantly kill the calculator. The same applies if the battery connection has a higher than normal resistance, which is often the case when the battery contacts have developed some degree of corrosion (they all have after 40 years). I am almost certain that most non-working Woodstocks have been electrocuted by their own chargers.
To save yours from this fate it is the safest option to store the charger as far away from the calculator as you can and charge the batteries outside the calculator.

Regards
Max

[cubytus]

Wow, I never expected such a quick response! Thanks all!

Well, since this HP-25 is not the "continuous memory" version, swapping the batteries would result in any program being erased. No big deal, but worth mentioning. No need for "flat-top" versions as these seem even harder to find than NiCd, plus they wouldn't fit a standard NiMH charger. I used a battery contact spring to close the circuit in the battery box, but was surprised to see the calculator contacts weren't spring-based back then.

In the meantime, I read that NiCd and NiMH don't have the same charging profile, and that NiCd, while being lower capacity, are far more tolerant of overcharging, and this is precisely why the charging circuit uses it as a voltage regulator.

So I understand that no-one attempted to convert the on-board charging circuit to NiMH?

[BobVA]

(07-20-2022 07:48 PM)cubytus Wrote:  ...Well, since this HP-25 is not the "continuous memory" version, swapping the batteries would result in any program being erased.

... I used a battery contact spring to close the circuit in the battery box, but was surprised to see the calculator contacts weren't spring-based back then.

...So I understand that no-one attempted to convert the on-board charging circuit to NiMH?

1. All memory is erased in an HP-25 when the power switch is turned off, so taking out the batteries doesn't cause any (more) problems. :-)

2. Was there a battery pack in the calculator when you got it? It's a small plastic box that holds the two AA cells and has a spring-loaded interconnect on the end opposite the battery contacts in the calculator. The spring isn't mechanically connected to anything so, even if you get the pack, they are frequently missing the spring if the cells have been removed. The spring provides both the series connection between the cells and the spring tension to push the other end of the cells against the circuit board contacts.

3. Not that I know of.

4. Welcome to the forum!!!

Bob

[cubytus]

(07-21-2022 12:59 AM)BobVA Wrote:  2. Was there a battery pack in the calculator when you got it?

Yes, but it has been altered to take regular, standalone AA batteries. So I got this spring from another device.

About using Ni-MH batteries: even if it's not ideal, if I let them charge using the original charger, at some point display segments flash randomly. Question: as I don't have a scope on hand, would it be reasonable to assume the Ni-MH chemistry doesn't provide the regulation function like the Ni-Cd did?

[Maximilian Hohmann]

Hello!

(07-21-2022 01:40 AM)cubytus Wrote:  ... at some point display segments flash randomly...

That's usually the beginning of the end for any Woodstock calculator. When the battery is nearly fully charged, the unstabilised voltage from the charger keeps rising. The flickering ist proabably caused by dried out capacitors in the charger. A sure recipe for terminal damage...

Here ist the NiMH battery pack I use for most of my Woodstocks (I only have one such battery pack for all calculators). It has 2500mAh cells, so five times the capacity of the original 500mAh NiCd cells!
If you cut away the central beam across the pack and use the original spring, the batteries can be removed and put back into it without tools in a matter of seconds. Be sure to mark the polarity somewhere as reverse polarity will kill the calculator instantaneously!

Regards
Max

[BobVA]

(07-21-2022 01:40 AM)cubytus Wrote:  About using Ni-MH batteries: even if it's not ideal, if I let them charge using the original charger, at some point display segments flash randomly. Question: as I don't have a scope on hand, would it be reasonable to assume the Ni-MH chemistry doesn't provide the regulation function like the Ni-Cd did?

+1 to Maximilian's comment. The flickering display is not a good sign, but I don't think it's caused by the difference between NiMH and NiCad cells.

Either you've got an out-of-spec component in the charger or the calculator, or a possibly a poor connection to/between the batteries. (In particular the connection to rivets on the circuit board, which form the battery contacts, can degrade over time so there may be some resistance there.)

The safest path is to just not to connect the charger to the calculator and to charge the cells with an external charger appropriate to the battery chemistry.
It only takes a second to swap the cells and modern, high capacity NiMH batteries give very long run times. Just watch the polarity.

This advice may sound overly cautious since HP clearly sold a lot of Woodstock calculators and never changed the design. But there's some speculation that this is more of an issue now, due to degradation of passive components and perhaps increasing fragility in IC's as they approach 40-50 years of age.

[Danito-Danone]

My contribution; it works fine, D-D    

[Wes Loewer]

(07-20-2022 06:21 PM)Maximilian Hohmann Wrote:  Yes, it's something different. There are countless threads on this topic but it is always worth reminding fellow collectors and calculator users: The chargers of the "Woodstock" series calculators have a poorly stabilised output voltage. Connecting one to the calculator without a battery in place will almost instantly kill the calculator.

Yes, I was aware of the Woodstock problem with connecting the charger without the battery, but I didn't realize that this could happen with even a different battery in place.

Yep, better safe than sorry.

[cubytus]

This is the battery holder now. It was cut by its previous owner and there was no spring inside, most probably lost, but no trace of leaks. We can see the glue spot I put to hold the spring in place, as well as the crack on the inner short edge (helps when extracting batteries). Given the wear marks on the holder, likely the previous owner used a hard tool.
   

As for the polarity, I wrote it on the inside, but soon found out it was moulded in the plastic. I'm still a bit puzzled by its awkward positioning. Did HP first design the holder in two parts for disposable batteries, then glued them together around rechargeable ones?
   
   

Interestingly enough, I just tried a different set of NiMH batteries: instead of the Eneloops, I used a set of higher capacity EBLs (2800mAh instead of 1900mAh). Now there are no artefacts flashing on screen, even with the original charger connected, voltage on the battery terminals is 2.65VDC without any AC. With Eneloop, it's 2.9VDC with unstabilized 0.9VAC present.

Granted, I understand NiMH won't last long using such a "dumb" charger, but why would there be such a difference between the Eneloops and the EBLs? Are there diferent kind of NiMH chemistries after all?

[rprosperi]

(07-26-2022 04:02 PM)cubytus Wrote:  Interestingly enough, I just tried a different set of NiMH batteries: instead of the Eneloops, I used a set of higher capacity EBLs (2800mAh instead of 1900mAh). Now there are no artefacts flashing on screen, even with the original charger connected, voltage on the battery terminals is 2.65VDC without any AC. With Eneloop, it's 2.9VDC with unstabilized 0.9VAC present.

It's been said and said again, but I'll caution one more time to NOT connect the original charger, even with known good batteries inside. Even a small accidental drop onto the table can momentarily disconnect the cells inside and POOF, your Woodstock is now a doorstop.

[BobVA]

(07-26-2022 04:02 PM)cubytus Wrote:  ....As for the polarity, I wrote it on the inside, but soon found out it was moulded in the plastic. I'm still a bit puzzled by its awkward positioning. Did HP first design the holder in two parts for disposable batteries, then glued them together around rechargeable ones?

Nice find!! I must have re-cell'ed a zillion of those things and never noticed that molded in polarity marking!

Rechargeable batteries were pretty much expected in "professional" calculators, so I'd be *very* surprised if HP ever considered a disposable AA pack for the the Woodstock series. I think those marks are for the assembly workers.

[cubytus]

(07-26-2022 07:23 PM)rprosperi Wrote:  It's been said and said again, but I'll caution one more time to NOT connect the original charger, even with known good batteries inside. Even a small accidental drop onto the table can momentarily disconnect the cells inside and POOF, your Woodstock is now a doorstop.

Agreed, I understand the risk I'm taking. The batteries act as a voltage regulator. The question as about why two different NiMH brands would give different results.