Protected cells and regulated lights

[bridgman]

So... when I use protected Li-Ion cells in an incandescent or direct drive LED I can get an idea of when to recharge because the brightness dips a bit as the cell voltage drops. However...

I'm about to start using AW's protected cells in a couple of regulated lights (Scorpion LED with 1x17670, Malkoff M60F with 2xR123A or 2x17500 or 1x17670) but not sure how I am going to tell when to recharge other than "every so often whether they need it or not". The obvious issue is that the regulator will generally hold the brightness constant as the cell voltage dips, so there is a good chance of running the cells down until the protection circuitry trips. I don't know if this is a "bad thing" (other than the sudden darkness ) or if this is how the cells are designed to be used. I suspect that I should avoid tripping the protection circuit but not sure how to avoid it other than frequent and paranoid recharging.

I imagine the M60+1x17670 will be OK because the LED will start to drop out of regulation before the cell runs down, but not sure about the rest.

 

[bridgman]

Bumping my own thread... and I'm not even selling anything. How lame is that. Anyways, just looking for ideas here. There's probably a D'oh!! moment coming when someone points out the obvious thing I missed, but I can live with that.

I had pretty much decided to stick with incandescents or a Malkoff on single 3.7V Li cell (so I could see the dimming) but of course by the first day someone is asking "so how long does the Malkoff run on a single cell ?" and now I'm back trying to figure out where to draw the line when doing non-destructive runtime testing on a protected Li cell.

What is the thinking here ? Normally it's pretty easy to see the rapid dimming but with a regulated LED I assume that won't be happening...

 

[Chodes]

Interested in answers to this too...

 

[mdocod]

Your Scorpion LED on the 17670 will probably dim as it runs, just like the M60 will on a 17670. I don't have any reason to think that the scorpion uses anything but a buck regulator, considering it is designed around 2xCR123s.

As for the 2xli-ion options pushing the M60. You pretty much just have to try to put forth some effort and try to keep track of how much you have used the thing and try to recharge frequently rather than infrequently. You can bump up against those protection circuits a few times and it really isn't going to be a big deal, they are there for a reason. But it's much healthier for the cell to try to charge it up before it is drained that low. If you DO bump up against that protection circuit, the sooner you get it on the charger, the healthier it will be for the cell. Having a volt-meter on hand is your best bet for determining the state of charge on the cells. You should own one if you own loose li-ion cells as it is very helpful.

4.2V 100%
4.1V 90% (close enough to full, don't bother charging)
4.0V 80% (also close enough to full IMO to hold off on a charge unless you need to have a full charge for your next excursion)
3.9V 60% (Anything from this point or less is a good place to go ahead and charge)
3.8V 40%
3.7V 20%
3.6V basically 0%, but there's technically still some left, but don't discharge below this point if you can avoid it
3.5V or less, try to get the cell to a charger as soon as possible, it's still safe, just not ideal.
3.0V or less, over-discharged, a few accidental discharges to this level will not kill the cell but it's definitely something that should be avoided, if you do this to a cell a few dozen times, it might be worth considering a replacement.
0V, the protection circuit has kicked in and the cell is sitting at less than 2.5V behind the safety of the circuit. It has been discharged too far, this should be avoided. Cells can be recovered from this but don't expect new like performance from the cell. Charging a severely discharged li-ion cell can be dangerous and further testing after a successful charge should be performed to determine if the cell is still viable. Perform the charge in a fire safe area with good ventilation. Check voltage after the charge, and after the cell has had a chance to rest for awhile, make sure it will hold above 4.0V.

 

[bridgman]

Excellent, thanks !! The voltage vs. %charge relationship is exactly what I was looking for.

I have at least 1/2 dozen voltmeters "somewhere" in the house or garage, including at least 2 good ones. The challenge will be finding them

 

[jirik_cz]

0V, the protection circuit has kicked in and the cell is sitting at less than 2.5V behind the safety of the circuit. It has been discharged too far, this should be avoided.

This is true only for cells discharged with small current. Under heavier load (typically in regulated lights) the protection kicks in sooner. Usually around 2.75-3V under load. Then the cell voltage recovers quickly to more than 3.3V. This shouldn't harm the battery in any way.