HP-45 Battery/Power Problem

[Accutron]

I have nearly new condition HP-45 which is behaving strangely. It works fine with the external supply, but a new, fully charged NiMH battery pack putting out 4.25V does not work correctly. If I plug the charger in for about 10 minutes, the calculator will then run off the battery for 20-30 seconds, then shut off. Sometimes it shows the low power indication *very* briefly before it turns off. It's acting like the calculator's power fail cutoff voltage is slightly higher than 4.25V. I already confirmed it wasn't a problem with the charger socket clip by shorting the two outer pins. The battery contacts are aligned properly, and all of the gold surfaces on the battery contacts and charger socket pins and clip are clean and bright (factory new). Has anybody else observed this behavior before? I'm assuming there's no easy fix for this, but I'm very reluctant to perform a label-ectomy on a pristine HP-45.

[Dieter]

(06-20-2015 05:38 AM)Accutron Wrote:  Sometimes it shows the low power indication *very* briefly before it turns off. It's acting like the calculator's power fail cutoff voltage is slightly higher than 4.25V.

I do not think that the threshold is set to something like 4,3 V. You also say that the calculator actually turns off, and I think it does so because the voltage is too low. So the first thing you should check is the battery pack. The 4,25 V without load do not mean anything. Try to connect a voltmeter to the battery contacts while the calculator is in use. What happens after the mentioned 20...30 seconds? Does the voltage drop? Then there is a problem with either one of the NiMH cells or the charger. I think we can safely assume that the calculator does not turn off while the voltage is above 3,5 V, let alone 4,2 V.

Dieter

[Accutron]

I discovered the problem, and thankfully it wasn't the calculator. I checked the pack again under no load and it read about 2.75V. I checked it again with a different meter and got the same reading. The middle cell is completely dead (and passing current) and the other two cells are reading normal. I apparently just had a bad cell in the process of dying. The initial reading of 4.25V is what threw me off.