Nickel Cadmium batteries used to be quite common.
They have a voltage of about 1.3 volts when fully charged, but drop down to about 1.2 volts for most of their discharge curve, assuming that the load is compatible withe the cell capacity and temperature is taken into account.
The article here covers it pretty well.
http://www.hangtimes.com/nicdbasics.htmlThese cells have very low internal resistance all the way down to the point where you must stop discharging them.
When the voltage is down to about 1.1 volts, they must be recharged or they can discharge to the point that they reverse polarity and then it is hard to bring them back.
This peril is most dangerous when cells are placed in packs and their charge position is different. Having several fully charged cells mixed with cells that,are not fully charged will result in the charged cells pushing the partly discharged cells into reverse polarity hell and then redemption is difficult or impossible…😬😉
The other snake in the NiCd garden is a reduction of useable capacity called “memory” that results from keepimg the NiCd packs on charge all the time and then only pulling out 10-15 percent of the rated capacity over and over. Then when you need that full capacity, it will not be there…
I used NiCd in portable receivers and solar powered wildlife transmitters, so I,suffered with them for years until Nickel Metal,Hydride packs were available. The suffered not from memory or reverse polarity, but their seff discharge rate was high, so a piece of equipment that sat for a minthmwas often dead.
I am guessing that a fixed resistor or a rheostat would serve to do what you want, govem that themrate will,be higher at 1.3 volts for,about 10% of discharge and then pretty constant. However, the size of the cells will affect capacity at a fixed discharge rate, so small cells will not give good results for 25mA and larger ones will.
10%’of capacity for discharge rate is about right.
73, Mike
