Battery pack of HP 25 calculator

[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