Thinking about an HP-41CL

[Dieter]

(07-28-2016 07:50 PM)Dave Frederickson Wrote:  Agreed, but the datasheet is a representative example of a typical 500mAH NiMH N-cell, and it's what Sylvain recommended.

Wait until you see the XCell data sheet. ;-)

(07-28-2016 07:50 PM)Dave Frederickson Wrote:  There are some values for dT/dt in the GP50NH datasheet.

The GP datasheet lists some cut-off conditions for different types of charge control. So if (!) the temperature gradient is evaluated, charging should stop at the given value. AFAIK the BC700 only checks the cell temperature as an additional safeguard: according to the LaCrosse manual it cancels charging at 53°C.

A better and IMHO more reliable way of charge control is the time-proven method of detecting the peak resp. drop of the cell voltage during charge. This is what is implemented in most processor-controlled chargers.

The idea is simple: While the battery is charged, its voltage increases. As soon as the battery is fully charged the voltage cannot increase further. The still flowing charging current now is transformed into heat, which – to a certain extent – is no problem as NiCd and NiMH batteries are usually designed accordingly. The increasing temperature in turn causes the voltage to drop (the higher the temp, the lower the voltage and vice versa). So this is a quite reliable and method for detecting the point where the battery is fully charged. All that's required is a sufficiently high charging current that will cause the battery to become warmer on full charge. Here 0,3 C usually is sufficient, sometimes even lower rates will do. At least if charger is able to detect the less distinct voltage drop.

(07-28-2016 07:50 PM)Dave Frederickson Wrote:  I simply found a datasheet for a SC cell. It provided some ballpark specs until I could find an AA datasheet.

SC and AA batteries can be very different. Sub-C is a common size for high drain applications, e.g. power tools or RC cars. The specs for AA size cells can differ significantly. I strongly suggest consulting the datasheet for exactly the type of battery you intend to use.

(07-28-2016 07:50 PM)Dave Frederickson Wrote:  

(07-28-2016 05:51 PM)Dieter Wrote:  What high-capacity cells do you refer to?

Any cell that is not practical to charge on the original ~50mA charger that came with the calculator.

OK, that's virtually any current battery type. ;-) Due to environmental restrictions NiCds are practically unavailable over here in Europe, so NiMH is what you get. Here the capacity for AA cells typically is 2000 mAh and more, compared to 500 mAh back in the days of the Spice series.

But let's not forget: the same charging time still yields the same operating time. If you charge your 2000 mAh battery with the original charger the same way you did 30 years ago, you'll still get the same usable capacity. ;-)

(No, of course that's not a serious option. I am using 2000 mAh Eneloops in my 34C, and these are charged in a decent processor-controlled charger).

Dieter