Out of balance batteries in MiniMag

[apete2]

I use 2 L91s in an incan minimag for EDC, and every time it comes time to change the batteries, they are horribly out of balance. Last time the two batteries read 1.56 volts and 0.394 volts. What could cause this?

 

[901-Memphis]

Sounds like one of the cells is bad to me. Pretty low reading for 1 of the 2, but obv i would wait to see if someone else has anything to add.

Which batteries are they btw?

 

[Benson]

I don't mess much with primaries, but it sounds fairly normal to me. Unless the cells are perfectly matched, one will always drop off first, and the Energizer Li primaries are supposed to hold voltage nearly constant till a brick-wall drop-off. So one cell drops off, the light dims, and you change the battery. If you had kept on (despite the dimming) for a little while, the stronger cell would presumably have rolled off quite soon as well.

Sounds like one of the cells is bad to me. Pretty low reading for 1 of the 2, but obv i would wait to see if someone else has anything to add.

Note he said every time. It would be pretty interesting luck if exactly half your cells were defective, and somehow you never grabbed two good ones.

Which batteries are they btw?

He already said; L91s.

 

[berry580]

energizer e^2 or........ something like Ever Ready L91?

i actually have a similar question myself.

Under what conditions would batteries discharge at different rates? Is a faulty sample (battery/ies) the only situation where one would experience such phenomenon?

thank you

 

[uk_caver]

Presumably a simple experiment would be to save the 'good' cells, pair them up, and try those pairs in the light and see how long it runs for.

If it doesn't run for long, then [at least] one of the good cells was almost dead, and was only saved from going flat originally by its first partner dying slightly earlier.

 

[tpolley]

i am having the same issue with some ultralast cr123r's i bought at batteries plus. they both read about 3.7 volts off the charger. i use them for a week or two and the light dims. one battery will read .5v while the other reads 3.1+-v. i thought they should both maintain the same charge untill they both died so i replaced the "defective one" and the new one does it to. i assume its normal.

 

[Kestrel]

There was at least one thread not long ago that noted the same behavior in CR123's in an incan 2-cell 6P and/or a 3-cell C3. Apparently the 'hot' cell (nearest the heat-producing incan) is more efficient than the colder cell IIRC, and results in the total watt-hours that are obtainable from each cell differing by a demonstratable degree. Perhaps others who know more on this might post here. A search for 'CR123', 'two-cell' and 'balance/balanced/imbalanced' perhaps?

BTW (going very slightly OT) yet another reason to use:

two protected LiIon cells instead of two unprotected LiIons (to avoid the possibility of reverse charging the depleted cell)
​

or

a configuration utilizing a single (large) cell instead of two (small) cells
​

IMO at least, :shrug:
K

 

[MorePower]

There was at least one thread not long ago that noted the same behavior in CR123's in an incan 2-cell 6P and/or a 3-cell C3. Apparently the 'hot' cell (nearest the heat-producing incan) is more efficient than the colder cell IIRC, and results in the total watt-hours that are obtainable from each cell differing by a demonstratable degree. Perhaps others who know more on this might post here.

BTW yet another reason to use:

two protected LiIon cells instead of two unprotected (to avoid the possibility of reverse charging)
​

or

a single (large) cell instead of two (small) cell configurations
​

IMO at least, :shrug:
K

Regardless of the fact that a warmer cell will typically maintain a higher running voltage under load, and thus provide more watt-hours, the current going though 2 or more cells in series would be the same, and that is what determined depth of discharge.

Benson has stated the most likely explanation; unless the cells were perfectly matched to begin with, the voltage of one cell drops first and the small amount of remaining capacity in the other cell will result in a much higher OCV than in the dead cell.

 

[Kestrel]

Benson pretty much always makes the most sense to me regarding cell info, I just wanted to cite some additional information that could possibly be related. As I had posted previously, perhaps someone who was more familiar with that older thread might have something to post about this, that's all...

Edit: Like, say, SilverFox & NewBie,

NewBie mentioned that the heat from the lamp may contribute to what is going on. Perhaps we could load the lower capacity cell nearest the lamp in an effort to heat it up. That way the weaker cell may be able to keep up with the stronger cell because of the increase in chemical activity (due to the heat) of the electrolyte

Again, I'm not sure, just doing a cursory search for this behavior, that's all...

Edit #2: Here. NewBie was one of the most knowledgeable cell experts CPF has ever had:

Notice, in this case, how the ZTS "weaker" cell (10%) out performed the 100% cell, in the middle of the run, but towards the end, the 100% cell keeps it's voltage up longer.

I have a theory I've considered, which has to also do with cell temperature and placement in a flashlight body, we all know how the head section runs alot hotter than the rear section. This may also would contribute to failure conditions. When I partially deplete a cell, as in my previous tests, and then hook everything up, that partially depleted cell is warmer than the other one, just like it would be in a flashlight. As we all know, chemical reactions usually happen faster/"better " when things are warm.As we all also know, cells tend to perform better when warm. This may lead to a further imbalance, causing the warmer cell (the one by the head), to deplete sooner.

(emphasis added)

I'm not saying that this is what is happening, just posting about a potential possibility, that's all. However, in my opinion, CPF's top three experts on Lithium cells are/were SilverFox, Mdocod, and NewBie.

 

[Mr Happy]

This is a debate that will never be resolved without some controlled experiments to back it up.

It is true that two cells in series have the same current passing through them, and so all other things being equal, they will be depleted of charge at the same rate.

However, it is been reported over and over, with great consistency, across many cell chemistries, that in multi-cell configurations the cell nearest the head discharges faster than the others.

Thus we have theory vs observation. When observation consistently gives different results from theory, it is an indication that the theory is incomplete (or the observations are incorrect). If the observations are correct there is presumably another factor needed to explain them.

Step one is to perform some controlled experiments to verify the observations in repeatable circumstances while eliminating other variables. (E.g. we do the test with rechargeable cells and swap the cells around between tests to eliminate differences between cells.)

My opinion is that one would be foolish to assert "does" or "does not" with certainty right now. Even a trained professional chemical engineer (I am one) has to acknowledge that sometimes there are surprises or factors not considered. The more years of experience one has, the less one is surprised by the unexpected.

 

[TooManyGizmos]

So ..... maybe rotate your cells from back to front , every 20 minutes of use.

Kinda like your tires