Question regarding 2-AA lights and batteries

I have a Surefire 9P setup using 2-AA's and a low voltage XP-G 3 mode drop in. I was using 2 NIMH AA (Older Energizer type) batteries, and trying to get a gauge on run times. When the light finally ran out (lumens down below 10 approx), I took the batteries out and checked them both with a DMM. The bottom battery next to the tail cap was at about 1.02v. The top battery next to the drop-in was reading .01v. I thought that the batteries would run down together? Is this normal to have a battery go down that low? This is the first time that I noticed this, as I usually check every time. They are usually very close in the readings. Is it a bad cell? I charged them back up, and they both came off the charger at 1.24v.

Hi RI. This is the major disadvantage to running any type of cells in series. One cell will almost always run down faster than the other(s) when used in series. The bad part is, that if the "battery" is run down far enough, such as I'm certain has happened in your situation, the lowest voltage cell in the "battery" will be forced by the higher voltage cells into what is called "reverse charge".

When the voltage of the lowest cell drops to zero, the current running through the "battery" effectively, begins to run backwards through the cell, ie. the anode and cathode become "reversed", as the cell is now at a negative voltage potential. This is very bad for any chemistry of cell and should be avoided, if at all possible. In the case of lithium primary cells, and to a somewhat lesser extent Li-Ion cells, this cell reversal can cause a "vent with flame" incident. With your NiMH cells, the result is merely a damaged cell, no fireworks, but this type of damage is permanent.

I'll add here that when a cell has been reverse charged in this manner and is removed from the device, normally the voltage will recover to a minimal positive value by the time the voltage is measured, although not always.

This is why it is very important when running cells in series applications, to make certain that the individual cells are well "matched" for capacity. Even so, it it best not to run cells in series down past the first point where a noticeable loss in performance becomes apparent. Actually, it is best to stop discharging the cells before this, if possible.

Dave

Sounds pretty normal to me... things start happening very fast when you're down to last few mah of capacity and no two cells will ever be perfectly balanced. If you have a good smart charger, cycle/analyze them to see their capacities... match them as best you can.

Don't run them dead like that, I'd look charge them before they get to 1.2V

Thank you for the insight. I used some older cheap energizer nimh rechargeables for this test. They were matched at about 1.23v each prior to the test. I was surprised when I checked them with a DMM. I guess one should reverse the cells about half way through a usage cycle when using any 2 batteries a series. I am glad that I did this with cheap cells, and not Li Ion cells.

Did you say that the cells were 1.24V fresh off the charger, rather than 1.42V? Sounds like your charger is not working, 1.24V is when you would want to recharge the cells...

Generally, the cell closer to the LED module will have a lower voltage because it gets warmer first. During normal use, holding the flashlight in your hand helps dissipate heat, and you probably won't be using it for that much time at once, so it's less of an issue.

Yes, 1.24v. They are old cells. Old junky Energizer rechargeable cells. I was just using them for an experiment. The light never even got close to getting warm. I got 3 hours out of my LSD Duraloops in the same setup. Three 1 hour walks on different days, before they started to lose power.

It's time to recycle those cells...uneven discharge due to heat is usually not a significant issue with good cells, but if your cells can't even hold a charge properly, that's going to make it worse.

If the light is able to drain batteries down to where combined they add up to less than 1.5v you need to concern yourself with watching the output and not run the light to where it dims too much. Nimh batteries typically do well in devices that leave them with an average of about 1v per cell but when you start dropping below about 0.8v per cell average then both batteries are completely depleted and it is very easy for either one to go into reversal at that point. Think of it as having two batteries one starts at 100% and the other at 98%. Under normal situations you would discharge them to perhaps 10% and 8% but if you go down to 1% on the first one the second one goes to less and that could equate to -1% or reversal.

RI Chevy said:

I used some older cheap energizer nimh rechargeables for this test. They were matched at about 1.23v each prior to the test.

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Hi RI. It is not possible to "match" NiCd, or NiMH cells by measuring open circuit voltage. The only way to match rechargeable cells, of any type really, is to measure their capacity when discharged at a rate similar to that at which the cells will be used, and match them up accordingly, generally to within 1-2% capacity.

The open circuit voltage of NiCd and NiMH cells really doesn't tell you much of anything, other than a very rough "go/no go" estimate as to whether the cells are charged at all, or not. For example ~1.20 volts is pretty much dead, and ~1.30+ Volts means the cell has some charge in it. It's entirely possible to have a good cell that reads 1.30 Volts and an aged cell that reads 1.35 Volts, and the better 1.30 Volt cell actually has a considerably larger amount of capacity left.

........ I guess one should reverse the cells about half way through a usage cycle when using any 2 batteries a series.

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If I'm reading you correctly here, it would appear that you have fallen prey to the myth that when cells are stacked in series, the cell, or cells nearest the tailcap are capable of "boosting" the cells in front. This is not true. When cells are discharged in series, no matter their physical configuration, or orientation, all cells drain evenly. The exception may be, as Robin mentioned, if the light gets really hot during discharge. In most situations however, this doesn't occur. The head end of the light has to get really hot, not just warm.

As has already been mentioned, provided your DMM is giving you an accurate voltage reading, a NiMH cell that only charges to 1.24 Volt is either not getting a proper charge, or is ready for the recycle bin. As I mentioned previously, a NiCd or NiMH cell that reads ~1.20 Volt, is pretty much fully discharged.

Also, as Lynx mentioned, when cells are discharged in series it is best to stop the discharge when the open circuit voltage of each cell is around 1.20 Volt, or the voltage/cell is no lower than 1.00 Volt, when under load. This helps prevent a reverse charge situation along with making sure the cells are well matched in capacity.

Dave