Classic Battery Packs and Voltage



I rebuilt a 67 battery pack with new cells. I charged the batteries offline then put it in the calculator and it showed the low power light. My first assumption was that it was due to high voltage drop across the wires I had used to the replace the external power / battery leads (these had melted when a line had been cut and shorted !). The calculator worked well except for the light. After a few weeks, numbers started to flash across the screen as if the charge was low. I recharged it and it didn't make any difference.

I then changed the pack to one I had rebuilt for a 45 and the 67 then worked perfectly and the 45 started to behave strangely when used with the 67 pack. The interesting point of all of this is that the 67 pack (bad) has a voltage of 4.1 V while the 45 pack (good) has 3.7 V. Can there be problems with too high a voltage or is this likely to be something else (bad contact) ? I'd appreciate any comments.



No, too high a voltage (within reason) won't cause problems -- 4.1V will be fine
What you need to do is measure the voltage _on load_ inside the calculator (you could start, actually, by measuring the voltage between the middle pin on the charger conenctor and either of the outside 2
(which are shorted together if the charger is not plugged in) with the machine turned on. If you get too low a voltage there, check your battery contacts. If that's fine, check the voltage at the logic PCB pins (the pinout is in my 'HP67 notes' in the articles forum on this site I think).

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