10440 Lithium-Ion Cells

[Benm]

That depends on how you set it up, but it could work. I'm not sure as to what the -exact- cutoff voltage should be, but 2.8V per cell sounds like a reasonable safeguard.

 

[a_pyro_is]

I was hoping 2.8 V would be ok, And I figured it would be a little bit higher due to the drop of whatever transistor I decided to use.

 

[Benm]

I've drawn up a little something.. its a current source described also in the experiments section, but with addl cutuff circruitry for lithiums... should turn off rapidly once cells go below 6V. There will be a bit of leak current below 6v, so you still have to really turn it off.

 

[a_pyro_is]

Almost exactly what I had drawn! I guess even a blind pig finds an acorn once in a while.
Am I right that when it hits the cutoff point (I'm guessing about 6.1 V) the current will drop to just ~2-5 mA, at which time I need to turn it off else the battery will drain until they're to only ~2 V per cell which is MUCH too low for the cells to survive a recharge?

What do you think about adding an LED to indicate that the power switch is still on? Just an LED and current limiting resistor parallel with the rest of the circuit?

 

[Benm]

That might be a nice idea to add if your enclosure has a place for it. Most importantly though, even without a led you'd notice it just shutting off when using it.

These current sources otherwise will drain batteries until totally dead before you notice any change in output. Fine behaviour on alkalines, but not so good on lithiums