li-ion safety question

[moon lander]

When a li-ion cell goes below a certain voltage (like 2.75v) it shouldn't be charged again, for safety reasons. Ok but the voltage measured will be different if you wait a while before testing it right?

Example: I ran my Jetbeam Mk2x until it went dim, and the 14500 read 1.85v on my multimeter, so i thought i had ruined the cell. i noticed however that the voltage was rising as the cell rested. two hours later, the voltage reads 3.14v. So i guess im back in the safe range...or am i?

I usually don't check the voltage until just before i re-charge the batteries, so wouldn't have even noticed that the cell voltage had dipped so low if i hadn't tested it immediately after using it for a long period of time.

So is my cell ruined? And do i need to be testing the voltage right after using lights with unprotected cells?

 

[DM51]

If it's back up over 3v you should be OK, but keep a close watch on it that it doesn't overheat when charging, and see if it has held a good charge next time you use it.

I always check all cells before charging, even though I hardly ever run them down to near the low safe limit. It gives you a better idea of how they are doing, and how much charging time they will need.

It is better for a cell to be charged from half full twice than from almost empty once, so you're not actually getting better value by running your light until it goes dim each time.

 

[SilverFox]

Hello Moon Lander,

The damage from over discharge depends on a few things. The depth of discharge, the amount of time spent in the over discharged condition and the current draw when the over discharge occurred.

Under heavy loads, you can drive the voltage lower, and it will rebound once the load is removed. Under light loads, the rebound is less.

When you over discharge, you loose cell capacity because one of the electrodes starts to dissolve. In the worst case, you for copper shunts that short the cell out during the next charge cycle. This can result in an explosion and fire.

If the over discharge is done during high loads, and is immediately discovered, it is possible to recover the cell. You need to initially charge at around a 0.05C rate, or lower, until the cells voltage rises above around 3.5 volts. Then you can charge at normal rates, but being watchful for cell heating and any other problems.

You will have reduced capacity, but it may still be above 80% and usable.

When checking for over discharge, remove the cell from the load and let it sit for around 15 - 20 minutes. After that resting time, the cell voltage should be above around 3.4 volts ideally. Different brands seem to respond to low voltage differently. Sometimes the cell will only rebound to around 3.2 volts, but higher is better.

Your cell recovered to just over 3 volts, so there may be hope for it. I would start off with a slow charge rate of around 50 mA to bring it up to 3.5 volts. If you don't have an adjustable rate charger, put the cell in your charger and charge for 5 minutes. The pull the cell and let it sit for 15 minutes. Then measure the voltage again. Repeat until you are above 3.5 volts, then charge normally. Keep an eye out for cell heating and toss the cell when it goes below 80% of its normal capacity.

Tom