Protected Li-Ion - How to avoid discharging below low-voltage cutoff?

Hi everyone,

I'm doing runtime tests with the new XTAR 18700 protected cells that they sent me for review. I understand that protected cells are supposed to have circuitry that prevents the cell from discharging below the point that would damage the cell. But I have read several reports here of protected cells that have been over-discharged, after which most chargers will refuse to recharge them. How can this happen, if the protection circuit is supposed to protect against that? Or does this only happen with junky protected batteries that have flawed protection logic? I get the impression that this occurs when the battery has been discharged at very low load, such as by parasitic drain from an electronic switch. In my case, I tend to run flashlights on their lowest setting until the battery dies. I wonder if this would also let the voltage drop below the cutoff point without the protection circuit noticing? I ask because I have been running the MRV Lumapower clone for about 43 hours on Low, and it was still going strong. I just now pulled the battery and am now charging it with my XTAR WP2. It seems to be charging it, so it looks like I didn't ruin it. Any ideas?

Thanks very much!

I don't own any "high end" cells, only a couple of Trustfires with flames on them and some cells I harvested from dead laptop batteries. I don't trust the protection circuits at all, they shut off way to late, just before you really ruin the cell. 2.7 V under load is waaay to low, as far as I have read. So I try to recharge my cells as soon as they reach 3.5 V, which is supposed to give you a much longer lifetime for the cells. So, in flashlights that don't have proper voltage control (which I think in my case is only the S-mini and the drop in I made with a S-mini driver, which are supposed to have good battery protection on their own), I check the voltage every now and then, and drop them in the charger when too low. On the other hand, most of my flashlights that use 18650 cells run in a single cell configuration and with a driver that is buck only (mostly AMC7135-based). These types of drivers can't raise the voltage on their own, so when the voltage of the cell sags, they get near the forward voltage of the led, and the light gets dimm, reminding me to change the cell.

So, depending on which kind of driver your MRV is using, it might not be a good idea to let the battery run flat. Wheter or not you damaged your cell, can only be determined in its lifetime. Deep discharging reduces the possibly achievable recharge cycles (as does overcharging). Complete discharging, and totally overcharging, will destroy them, possibly (but not necessarily) causing Something Bad to happen, namely venting of the cells with or without flames, or explosion. But I don't think this happens when the cell gets discharged very slowly.

I'm not an expert in any way, so feel free to ignore my dribble, but up to date none of my cells died under my treatment (I have been playing with Li-cells for over a year now).


Thanks Confusius! I'll keep this is mind.

Other opinions?

I've run cells dry in an MRV. Some have protection circuits that cut in, many don't.

From my runtime tests on the MRV you have an hour and a half on high - you will see the brightness dropping like a stone.

On medium I'd say you'd be safe for 6 hours and on low a day and a half.

That is part of the reason I do runtime tests.

How I use them is that if I've used the cells for any time on high, I top them off in the charger. I tend to keep my cells fully charged though since it is still light at 10pm and light again by 5am now I'll probably discharge most of them to storage voltages for the summer.

OK, so we can say that it all boils down to the logic of the protection circuit? I would hope that these XTARs have a better protection circuit, they are marketed as a higher quality option.

Did you check it with a multimeter? I think most protection circuits don't cut off if the power drain is small (even on AW batteries), but I don't have a reference for that right now.

The only protected cells I have are the TF Flames, of which I have 4. I've not checked the voltage that they cut off at, but they show evidence of cutting off correctly.

Just as the light emitted starts to noticably dim, it cuts out completely. Turning it back on a few seconds later will show a short burst of light and then again cut off.

I'll check the voltage next time I have one cut out.

Hmm, no, I didn't check it with a multimeter. I know the power drain is minimal.

Another question: How long should it take to charge a single 18700 battery (presumably fairly well drained) in an XTAR WP2? Mine only took about 3 hours for the green light to come on.

The reviews say that it pushes 600-650mA into the cells. So that's not far off 2000mAh pushed back in. Seems reasonable to me.

OK, thanks very much for the answers!