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Charging times for HP-41 rebuilt battery pack - Printable Version

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Charging times for HP-41 rebuilt battery pack - Timo - 01-06-2011

Hi there, it's been quite a while since I last posted here on the Forum but only recently I came across a question regarding a rebuilt HP41 battery pack that I purchased. More precise, the question concers the charging time required to charge it to full capacity again.

The pack contains 4 150mAh KAN (NiMH) accumulators and I'd like to charge it using the standard HP charger. Now, the charger has the well known 3VA 8VAC output which translates in a current of 375mA. However, it's AC and it's converted to DC by the rectifier built into the pack. Since I do not know which type of rectifier is used in the pack I can't really figure out how much DC current is effectively used for charging the cells, therefore I can not judge how long I have to charge the pack.

Can anybody give me a helping hand in figuring out the charging time?

Thanks a lot in advance for your efforts and best regards,

Re: Charging times for HP-41 rebuilt battery pack - Dieter - 01-06-2011

Timo, the rating of 3VA and 8 Volts is just a nominal value, and of course the charging current provided by the small PCB inside the battery pack is much, much lower. Just imagine the original 70 mAh batteries being charged with 375 mA... #-)

As far as I know (and also measured by a friend who rebuilt such a battery pack some time ago) the original charging circuit is set to a charging current of 15 mA, maybe a tiny bit higher. For a 150-mAh-battery this means that a completely empty battery pack requires a charging time of 150 * 1.2 / 15 = about 12 hours. Assuming the original PCB was not modified.

By the way - the schematics of the PCB inside the original HP battery pack can be found somewhere out there "on the net". For instance here. The Zener diode limits the voltage to not much more than 6 Volts when the 41 is powered from the AC-adapter. You can also see a small voltage regulator IC for 12 V output and 100 mA max. current. Another reason why "8 V at 375 mA" is far off. :-)


Re: Charging times for HP-41 rebuilt battery pack - Timo - 01-07-2011

Hi Dieter!

Well, thanks a lot for this detailed answer. Especially that circuit diagram explains a lot. I did not rebuild the pack myself. However, I can not remember that the guy who sold it to me mentioned modifications of the charging circuit. This would have been dumb anyway since he sold me the original HP charger as well... ;)

Anyway, in the meantime it seems that I also have a problem with the charger. I have started another thread about this, maybe you can help me out with this one as well.

But thanks again for your help on this problem!

Best regards,