Battery pack of HP 25 calculator

[Carlo Meletti]

Good evening,
Since long time a have an HP 25 calculator. It is years it is no more in use, just because it was no more possible to get the Woodstock battery pack (at least in Italy).
So I had got a new HP calculator with botton batteries.

I would like to re use my HP 25.
As I still have the last battery pack I used, I'm now thinking to cut the battery container and substitute the two with new NiCd (?), if I'll get them in the market.

The first problem is to know exactely the voltage each battery stylus should grant. The HP25 User Manual is saying nothing.
The Museum reports: "To reduce the size and weight, the 20 series used two rechargeable cells instead of the previous three. This 2.5 volt supply was converted to 4 volts to operate the display."
So I ask if anybody can help me on two questions:
1) I wonder if the total pack should supply 4 volts, 2 volts each. A strange value for today's standards, I presume. Is it correct ?
2) Which should be a satisfactory mAh capabilitiy both for the calculator and the recharger ?

Thank you in advance
Carlo

[cjmcc]

I am running my HP29c on a pair of AA sized Eneloop batteries. Rather than cut the old battery pack apart, I was able to get a replacement printed ( https://www.thingiverse.com/thing:1276886 ). The red material from the 3D printer doesn't match the case, but the calculator works!

[Sadsilence]

Both battery types NiCD and NiMH have right nominal voltage of around 1.2V per cell, together 2.4-2.5V. Necessary calculator curcuits' voltages are created internally.

You can open battery case with a litte patience on bottom side and glue parts together again after battery exchange, so you do not have to cut out middle bar.

Personally I prefer flat top NiMH cells with low self discharge characterics. Most famous member of this battery type are eneloops (but no flat top). Flat top cells cause genuine ones were flat top, too. So less mechanical pressure on calculator contacts. No matter if flat top or not. So called LSD batteries are still with load even after a few months storage time without use.

This one.

Load current with HP charger is around 50mA, which perfectly matched genuine HP 450-500 mAh batteries. Load time then was around 12 hours.

Cells mentioned above have a capacity of 2100 mAh. Take it multiplied with 1.2 - 1.5 (due to loss during charge procedure) and divide result of around 2500 through loader current of 50 mA. Batteries are fully charged within around 2 days! Advantage is: you simply cannot overcharge and damage batteries that way. Still it makes more sense to charge cells with a modern racing car charger or a lab power unit. Loading "adapter" can easily made by using neodymium magnets. Or indeed cut out middle bar, so cells can easily removed and loaded seperately with a default nimh charger.

[Dieter]

(07-03-2018 01:49 PM)Carlo Meletti Wrote:  The first problem is to know exactely the voltage each battery stylus should grant. The HP25 User Manual is saying nothing.

The HP25 used a pack with two NiCd batteries. Each of these has a nominal voltage of 1,2 or 1,25 V, so that's 2,5 V in total. In real life the voltage of the pack may vary between 2,2 V (nearly empty) and 2,9 V (fully charged).

Today's NiMH batteries have the same output voltage, so there should be no problem if you use these instead of the old NiCds. Their capacity of 2000 mAh (and even higher) is five times as high as offered by the original NiCds.

(07-03-2018 01:49 PM)Carlo Meletti Wrote:  The Museum reports: "To reduce the size and weight, the 20 series used two rechargeable cells instead of the previous three. This 2.5 volt supply was converted to 4 volts to operate the display."
So I ask if anybody can help me on two questions:
1) I wonder if the total pack should supply 4 volts, 2 volts each. A strange value for today's standards, I presume. Is it correct ?

No. The batteries supplied around 2,5 V, i.e. what two regular NiCds or NiMHs deliver. Internally (!) the one or other component (such as the display) may have required different voltages. These were generated electronically by a DC-DC converter circuit.

(07-03-2018 01:49 PM)Carlo Meletti Wrote:  2) Which should be a satisfactory mAh capabilitiy both for the calculator and the recharger ?

The battery capacity is relevant for the calculator's operation time. Every NiMH type in AA size that is sold today offers much more than the original NiCd batteries of the Seventies. Personally I would recommend Panasonic Eneloop cells.

The charger does not have a "mAh capability". It has to supply a charging current (mA). The lower the longer a full charge cycle takes. You can estimate the time for a fully depleted battery by calculating battery_capacity x 1,2 / charging_current. So 2000 mAh batteries in a 200 mA charger will take about 12 hours. Or 4 hours with a current of 600 mA.

I strongly recommend charging the batteries externally. Get a decent "smart charger", i.e. one with processor-controlled charging circuit that automatically stops as soon as the battery is completely charged (so that overcharging is safely avoided). For instance consider the well-reputed BC-700 which is sometimes even sold in a set with four Eneloop batteries.

Dieter

[AndiGer]

And don't forget the regular cited recommendation not to charge batteries inside a Woodstock or at least do not switch on with the charger attached as it can kill the machine (ACT chip) in case of bad/missing battery contact

[Carlo Meletti]

I would like to thank all of you very much for answering and giving me sound information for solving my problem.
I'm now sure about what doing to use the HP 25 again.
Thanks again to everybody.
Carlo

[Graan]

I have bought a HP 25C and when looking at the battery contacts in the calculator I dont see how it will be a good connection to the batteries. It was 2 NiMH batteries (GP cells 1300 mAh) inserted when I got the calculator. The connectors from HP 25C are 2 pcb studs with traces with some slodering. What I can see there is no good contact direct to the batteries. Should it be some other arrangement?
I connected the HP 25C to a power supply and it works fine, but when inserting the batteries the calculator does not seem to work properly. The batteries are charged. Measuerd 1.3 V on battery.

[Dieter]

(08-06-2018 07:42 AM)Graan Wrote:  I have bought a HP 25C and when looking at the battery contacts in the calculator I dont see how it will be a good connection to the batteries. It was 2 NiMH batteries (GP cells 1300 mAh) inserted when I got the calculator. The connectors from HP 25C are 2 pcb studs with traces with some slodering. What I can see there is no good contact direct to the batteries.

Since this hasn't become absolutely clear here: There are the contacts inside the battery pack that connect to the plus and minus ends of the batteries, and there wre the two contacts at the bottom of the battery pack that connects to two tabs inside the calculator.

I assume you are talking about the connection between the two batteries and the battery holder.

This indeed can be a bit tricky, depending especially on the shape of the cells' plus pole. In a 29C I had no problem with usual button-top Sanyo/Panasonic Eneloops, while others recommend flat top cells. If your GP batteries don't provide good contact inside the battery holder you may try a different type. In any case I'd recommend NiMH cells (and a decent external charger with "intelligent" charge control). You may also remove a bit of the pastic inside the holder so that the batteries make better contact.

(08-06-2018 07:42 AM)Graan Wrote:  I connected the HP 25C to a power supply and it works fine, but when inserting the batteries the calculator does not seem to work properly.

What is "does not work properly" supposed to mean? Does it work or not?

(08-06-2018 07:42 AM)Graan Wrote:  The batteries are charged. Measuerd 1.3 V on battery.

That's the open circuit voltage. Which doesn't say much about the battery condition, not even the charge status. You have to apply a load to get a useable result. For instance you could connect a 10 Ohm resistor (1/4 W or more) to each of the two batteries and measure the voltage once again.

Dieter

[Graan]

Ok I understand now. Seems the battery pack have been modified since there were no tabs in the battery holder (connection to calculator), just the battery poles contacting the connectors in the calculator. There is however a spring connector connecting the + and - at the end of the batteries.

When turning on the calculator with the above arrangement the display was flickering. But as said, when connecting to a PS the calculator worked fine.

Yes 1.3 V is open citcuit. Have not tested the batteries more.

As seen in the PS the calculator draws aprox 150 mA.

[Dieter]

(08-06-2018 09:19 AM)Graan Wrote:  Ok I understand now. Seems the battery pack have been modified since there were no tabs in the battery holder (connection to calculator), just the battery poles contacting the connectors in the calculator.

Tabs in the battery holder? That's not what I wrote:

Quote:...two contacts at the bottom of the battery pack that connects to two tabs inside the calculator.

Edit: the mentioned "two contacts at the bottom of the battery pack" are not correct – I confused with with a different pack. The pack and the calculator are connected via the battery poles which in turn make contact with the said tabs inside the battery compartment.

So the tabs are inside the battery compartment of the calculator where the battery holder is inserted. The holder itself does not have any tabs.

(08-06-2018 09:19 AM)Graan Wrote:  As seen in the PS the calculator draws aprox 150 mA.

More or less. That's why I recommended a 10 Ohm load. At 1,3 V this represents a load of 130 mA.

Dieter