Low voltage constant current battery drain

[vincevl]

Hi Folks,

I've been experimenting with building a Nickel Iron battery and I have my first cell working. I've been able to charge the cell using my bench power supply. However I want to drain the battery using a constant current load so I can test the cell performance.

Presently I've been draining the battery using a Joule thief LED circuit that draws about 25mA but the problem is that as the battery drains the load drops. The charged cell is 1.4V and I want to drain it down to 0.8V at about 50mA.

I searched the web and found LM317 circuits but they are unstable with Vin < 1.25V.

Any suggestions for a circuit would be appreciated.     Please keep it simple I'm a newbie to electronics.

Many thanks,
Vince

[IanB]

There is a classic two transistor circuit for constant current sinks. You can see it illustrated in this recent video of Dave's: