Uneven battery drain

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KevinL

KevinL

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At World's End
I ran down a pair of cells in my SF 6P the other day, the light turned orange, ran for a couple of seconds, then didn't light up so I changed the cells. (yeah I know it's not advisable to run them truly 'dead'. Leave that to the Lux..)

Just out of curiosity, I put both cells on my DMM, measuring both voltage and flash amps. One cell clocks in at 2.33V, with 0.5A flash amps and dropping fast. The other cell surprisingly measures 2.6V with 2+ flash amps, drops but not as rapidly. To double check my readings, I put the cells in my E1e+KL1 (one cell at a time, of course). The 0.5A cell starts in moon mode from the get go, but the 2A cell puts put significantly more light - it's running regulated, but noticeably dimmer than with fresh cells. Moon mode produces just enough light to see by, but the 2A cell can still throw a beam and hit a wall from 10+ feet away.

Does anybody else notice uneven drain in their cells? Obviously the 6P must have been drawing more juice from one cell than the other, but why? These are generic lithiums that don't seem to withstand high current loads as well as the SF or Energizer cells I've used before.
 
The same amount of current was being drawn from all the cells in your light, because the cells were in series. All of your cells were being discharged evenly.

What you observed is a well known fact: no two batteries are identical. There are minute differences in every cell. Thus, one cell will become discharged before any of the others. In most cases, the difference is small enough that it doesn't make a difference. In rare cases (both in non-rechargable primary cells, and in rechargables) one cell has significant defects which cause it to be much weaker than other cells, thus causing it to "die" early (I'm looking for a better word, but can't think of one). Thus, when a set of cells appears to be dead, it may, in fact, only be a single cell in the set that has expired, while the rest may have some extractable energy left.

In the case of rechargables, if a set of cells has a weak cells, and is used to its fullest every discharge cycle (i.e. fully discharged), then that weak cell will eventually be destroyed by the rest of the cells - because the cells continue to pump current through the dead cell, the dead cell will reverse. Cell reversal is bad - it can cause primary cells to vent and/or leak, and will slowly destroy rechargable cells. A cell reversing voltage will cause it to lose some capacity, causing it to discharge earlier, increasing the likleyhood that it will be reversed in future discharge cycles, further decreasing the cell's capacity, ad infinitum until the cell eventually becomes useless.

It is likely that most of the cases of "memory" that people report with NiCD packs is not infact the memory phenomenon, but rather a weak cell that became reversed, and basically discharges almost immediatly, dropping the pack voltage by about 2.5V, making it appear that the pack is dead. In those cases, probing each cell in the pack under load would probably yield one or more cells reversed.

enough rambling
 
[ QUOTE ]
KevinL1 said:
The 0.5A cell starts in moon mode from the get go, but the 2A cell puts put significantly more light - it's running regulated, but noticeably dimmer than with fresh cells.

Does anybody else notice uneven drain in their cells? Obviously the 6P must have been drawing more juice from one cell than the other, but why?

[/ QUOTE ]

First off, if it's 'noticeably dimmer than with fresh cells', I can't agree it's regulated. By my understanding of things, when it's dimmer it's out of regulation by definition.

Yes, I've seen this, in fact it always happens to some extent. Perfect match between cells is rare indeed. While there's no such thing as 'juice' (at least not in engineering terms), in terms of *current* (probably the best way to define it), series batteries like the 6P draw *exactly* the same current from each cell.

My bet it the 'good cell' in your experiment was at most a few percent of total life span away from the 'bad' one. That is when Li based cell voltages drop (and their current delivery ability does as well) to a significant degree, they're really at the end (or nearly so) of their available fuel (and therefore service life). That is I bet the 'good cell' doesn't stay out of moon mode long.

Doug Owen
 
That probably explains why the 6P dropped from bright to dim to nothing at all very, very quickly and refused to relight. Anyway, a change of fresh cells later, the 6P is back in business.

Perhaps I should have used a better word than regulation when referring to the KL1. It was probably just out of regulation and still reasonably bright, but it does drop fast, and I should have said current. I'll throw the cells into the battery eater 1W lantern I built, they direct-drive the Luxeon at something like 300mA (measured) so no resistors needed here, and it's still plenty bright. I wouldn't try this with FRESH lithiums though, just nearly-dead ones.

Once again, thanks to both of you guys for enlightening me /ubbthreads/images/graemlins/smile.gif
 
Hello KevinL 1,

The next time your batteries run down, mark them as to their position in the light when you take them out.

Several of us have found that the battery closest to the lamp is the one that is the most used up. I am still not sure why that happens, but it seems to be repeatable.

Tom
 
I use two 1/5A 400mAh cells in a Arc4+ and charge them in pairs till I discovered the above mentioned! I now charge them one at a time.
 
[ QUOTE ]
SilverFox said:
Hello KevinL 1,

The next time your batteries run down, mark them as to their position in the light when you take them out.

Several of us have found that the battery closest to the lamp is the one that is the most used up. I am still not sure why that happens, but it seems to be repeatable.



[/ QUOTE ]

This is most interesting.

Any feel for how big the difference is? Since there's no electrical reason for it, and random chance should put the weaker one equally in front or rear, my guess it heat?

Doug Owen
 
Hello Doug,

I believe Kevin's experience (reported in the first post) is pretty typical. It seems to hold true for all battery chemistries. I have noticed it in 2 and in 3 cell lights, but have not gone beyond that.

It is my opinion that heat speeds up the chemical activity of the forward cell, but I don't understand how that effects each cells performance in a series circuit.

Tom
 
Good idea.. I'll mark the batteries the next time and keep you guys posted as to whether it still holds. I don't use my 6P that often, so it may be a while.
 
From my experience with other batteries at work I think the heat from the LED is heating the closest battery which can change the internal resistance of that battery which could change the rate at which that battery is drained. This is not fact but mere speculation.
 
In a series pack the drain is the same on each cell, regardless of age, internal resistance, chemestry, whatever. A NiMH, alkkaline and zinc carbon cell all in series drain at exactly the same rate, they just reach the end of their useful lives at different times.

The only reason I can see is heat shortens the service life of the front cell. It may be made far worse in some lights with higher internal resistance and the very high currents involved contribuiting increased self heating to the mix. I'd be most interested in how much the difference is in real terms. I suspect very slight.

Doug Owen
 
OK, time to throw more lithium into the fire (figuratively. We don't pour oil into flashlights /ubbthreads/images/graemlins/grin.gif).

The 6P has just eaten another set of crap cheap 123s for breakfast. This time I removed the cells before the light completely went out, the beam was starting to yellow severely and put out less than half its normal output - that's not good for the lamp. This light probably hasn't even done half an hour. *smacks self for buying wierd batteries* /ubbthreads/images/graemlins/ohgeez.gif

Cell 1, closer to the business end of the P60 - 2.62V, 3.5 flash amps
Cell 2, closer to the tailcap - 2.29V, 1.7 flash amps
 
Lithium 123's FOR SURE have lower internal resistance when hot. On tests with high current draw switching regulators I have seen 123's self heat and I have watched their output voltage (under load) rise for a while. The switching regulators are current regulated at the output so their input power draw is basically constant. I have allowed a somewhat used up hot lithium cell to cool down and once cool (room temp) it could no longer provide the output current to start the switcher up (it's terminal voltage would drop under load). Coaxing the same lithium to warm up would allow it to work again. We're talking quite warm to the touch - so maybe 50C.

So, definitely the lithium cells nearest the hot end of your light would have lower internal resistance and provide more power to the load.

From my tests of various brand Lithiums they all showed increased output voltage (hence lower internal resistance) under load when hot.

I assume the measurements you took on battery voltage and 'flash amps' was with the batteries straight out of the light?

george.
 
Yup, straight out of the light, almost as fast as I could unscrew the tailcap and tag the cell with a marker before putting them on the DMM.

Just to check, I've let the cells sit for a few hours and re-measured :

Cell 1: 2.74V, 3.5 flash amps
Cell 2: 2.6V, 2 flash amps


Does this mean we should 'rotate' the cells occasionally for extended battery life? Kinda like rotating tires, except that the car folks tell me it's not needed very often nowadays due to improvements in computer-guided alignment..
 
What you're saying is something I've done "for a reason I could not determine" but I guess it started with the 12PM that I would empty to help the batteries cool down or replace with a fresh set...

I often swap the batteries round in my lights. Especially my EDC lights. I figured I was imagining the improved output.

I'm looking at the L2's batteries. Both have marks from the TailCap spring contact.

It's a habit more than deliberately.

Al
 
Old thread, but I have more info.

I think you're right. Halfway through the set of generic 123 cells on my C2, I took them out and took the measurements as usual. The cell nearer the tailcap measured out to be weaker, so I rotated the cells.. light continues to be EDC'ed and used. The cells are not yet weak enough to make the light noticeably brighter, I think they were around 4-5 flash amps. Note that this is intermittent everyday use, not a continuous runtime test, so it reflects my actual usage pattern of the light.

Now that the cells are nearly dead, both clock in at 2.5V and have the same 2 flash amps. They're equal.

I'm outta generics, time to load up some real bright red SF badged cells for some serious power /ubbthreads/images/graemlins/smile.gif
 
Glad to see that rotation is allowing you to squeeze some more life out of your cells. Where were you getting your cheap generics from?
 
ebay, but there's no reason to consider generics nowadays when Batterystation has good cells that cost less and perform better. I consider Batterystation cells "branded" (as opposed to generic).
 
nice, check your PM's Kevin. i think i sent you one early early this morning that i didnt get a reply to.
 
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