|Re: NiMH bateries for HP-41 and 71|
Message #7 Posted by Randy on 10 July 2007, 8:54 p.m.,
in response to message #4 by Garth Wilson
NiMH batteries are 1.2V each, the voltage at which my HP-41 says the batteries are low
True, NiMH cells are nominally stated as 1.2 volts. But, that's somewhat misleading as most NiMH cells reach 1.4 volts when fully charged. Referring to the 41 documentation, HP has this to say about the supply voltage:
2-38: The power supply circuit monitors the input voltage level by comparing it to a reference voltage generated internally. A differential amplifier in the IC senses whether the voltage has fallen below 4.2V at the IC, and if so, it grounds the LLD line connected to the CPU.
4.2V distributed over the four cells would mean that the low battery indicator turns on at 1.05 volts per cell, well above the nominal 1.2 volt level of NiMH cells. But, it has been my experience that the 4.2V level in practice is a bit low as I see units turning on the BATT indicator at 4.4 and 4.5 volts which works out to ~1.12 volts per cell. All things considered, it's still well below the 1.2 volt level of a NiMH cell.
So, that works for me... and yes, I use both NiCD and NiMH N cells to test 41's. Even when buying case lots of alkaline N cells, those little monsters are still quite expensive.