Thinking about an HP-41CL

[Dieter]

(07-25-2016 07:58 PM)renif Wrote:  Your aluminum clip works great.

Proven high-tech. <8)

(07-25-2016 07:58 PM)renif Wrote:  Readout after some 2 h of charging: 2.05 mA (initially up to 5 mA), now slowly decreasing by roughly 0.01 mA/min.

This sounds like a simple, unregulated charging circuit: more or less something like a resistor between the charger and the cells, so that the current is the voltage difference between these two, divided by the resistance. Since the cell voltage increases during charge, the difference decreases and so does the charging current. Technically, charging doesn't get much simpler that this. #-)

(07-25-2016 07:58 PM)renif Wrote:  Single cell voltage (open circuit) approx. 1.35 V at that point. The charging is signaled with a green LED, which goes out as soon as the cells are full.

I suspect the LED signal disappears as soon as the fixed charging time is over. Regardless how full or empty the battery is. I think it's safe to assume this low-tech charging circuit cannot detect when the batteries are fully charged. How should it, with such a low current?

Hmmm... assuming an average current of 2 mA, even an old Eighties-style 150 mAh NiCd N-cell would require a charge of roughly four days (!). Are you sure the current is really that low, i.e. 2 mA? Not more something like 20? Even the 1/2 N cells in the 82120A battery pack (approx. 70...80 mAh) were charged with approx. 15 mA.

Just to be sure: you set your meter to DC, not AC, didn't you?

(07-25-2016 07:58 PM)renif Wrote:  Hope that's the information you sought.

Yes, all this sounds like what I would have expected.
Except the charging current – should be much higher than 2 mA.

(07-25-2016 07:58 PM)renif Wrote:  Anyway, I think a modern replacement is to be set on the calculator budget plan at the end of the year...

Virtually anything will be an improvement. ;-)

Dieter