How can I test LICO capacity simply?

[VegasF6]

Is there a way I can get an approximate idea on how much capacity a lico cell may have left in it's life? Buddy of mine seems to feel he has reduced run times and short of just putting it in a flashlight and testing against other cells, I was wondering what I could build from stuff I may have in my junk drawer.

Ideally I would like to put a constant current load of 350 mA across the cell and measure voltage/time. I had wondered if there would be a way to do it with a resistor vs CC but as V drops of course I would drop as well, and if there is a mathmatical formula to extrapolate that, then it is so far over my head!

Would it be possible with the AMC 7135? I understand it is a linear driver and when running an LED, once battery voltage is lower than VF of the LED at that driver current, then it will be unregulated and current will drop. It is possible that VF would be 3.0V at 350mA, I dunno, but either way, I hate to use an LED just for that. What could I use for a load on the driver? A bulb? A resistor?

Obviously I don't really understand electronic theory, I am just self taught, primarily from this board, but I don't have a problem soldering and tinkering if I could get some direction here.

And of course, if there is something dangerous about what I want to do, please speak up!

 

[TigerhawkT3]

Sounds like you could use a CBA II.

 

[45/70]

Vegas, short of doing something like what Tigerhawk suggested, it seems like you want to go to a whole lot of trouble for, well, not much. I'm guessing you're aware of the voltage after resting method of figuring the percentage of charge of a LiCO Li-Ion. This, more or less, applies to remaining life left in the cell also. No, it's not an exact science, but it's close enough for what you're trying to figure out for your friend.

If the voltage after coming off the charger at 4.20 Volts, and resting for an hour (I use 24 hrs, but there really isn't much difference, if any) is less than 4.00 Volts, it's time to retire the cell. 4.00 Volts also represents about 75-80% capacity. With Li-Ion's this doesn't really change throughout the lifetime of the cell. In other words, if a newer cell comes off the charger at 4.20 Volts and stays there for an hour, it's in like new condition or 100%. If you take that same cell and run it in something till the OC voltage is 4.00 Volts (after a 1hr rest), it too is at 75-80% capacity. Age doesn't affect this measurement, whether a new cell or an old cell, 4.00 volts is 75-80% capacity. It's just that, in the first example, the cell's voltage dropped without a load on it, and this represents a worn cell that, in effect, is only 75-80% of the original capacity. This is also the recommended point to retire a LiCO cell.

It's important to note, that this only applies to LiCO cells. It won't work for other types of Li-Ions.

Hope this helps.

Dave