end cells always fail first, why?

jimmyhackers

10 kW
Joined
May 11, 2015
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600
ive noticed over my life experiences that whenever i diagnose a "dead battery" its almost always the cells closest to the negative or positive terminals thats died. with the middle cells usaually being fine/unaffected.

is there a known reason for this? i have ideas asto why, but id like to confirm or disproove them.

is there a name for this "effect"?

id like to read up on it

thanks in advance
jim
 
Either because of mechanical or temperature reasons.
Say if there are connections soldered onto those areas that heat cells above safe levels during production.
 
That's a fantastic question, and you do see end cells die more commonly than any other cells in most pack designs.

This is because of a current path that's not sharing current uniformly at the end terminations, and hence not heating uniformly at the ends. If you don't have temperature uniformity and current density uniformity in your battery, it will pre-maturely fail at the areas that concentrate thermal burden.

One of the ways to overcome it is to make your end terminations seemingly 'over-kill', and from the perspective of just handling the 50amps (or whatever) of the pack it is overkill, but from a perspective of having uniform current density entering and leaving the cell string, and not adding non-uniform thermal gradients through the pack, it's not over-kill, but perfect.
 
i never thought of it like that, thanks for the info. I was thinking more so that the end cells have the highest "relative" voltage going through them.

i.e. that say on a 3s lipo measureing from the middle cells to either the positive or negative end of the whole batery will still show plus or minus 8.4vs.
i was thinking that this higher voltage was causing degradation but if this was the case i guess i would of see it happen more often in higher voltage batteries.....but i havnt so it musnt be the case.

along this line also i though the spark that happens when connecting the batteries was causing some much larger current/damage spikes that get dissapated on the outside cells and in doing so are nutralised before it reaches the middle cells.

now you say thermals it does make a lot more sense. in addition to what your saying, the outside cells will have half of their surface area exposed to the air. meaning half of those cells are cooler than the other half. this would also explain why middle cells are generally ok as they are insulated by the other cells and heat and cool down a lot more evenly.
 
Could also be that some cheap BMS have unequal quiescent current drain, sometimes on the first cell in the series of cells... Sometimes it's the first three cells.

Same thing with the CellLog from progressive RC, display and electronic are powered by battery it monitors, but the device does not drain all cell equally.

BUT, this typically take a loooong time to occur... But not impossible, especially if your battery was in storage for months with BMS still connected.

Matador
 
temperature. the outer cells are the coldest wich means they have the hardest life compared to others that are nice and snug and warm inside the pack.
if you would check the individual cells you would see the outer edge cells also having lost more capacity then the inner ones from the same set.
 
Besides uneven current distribution, another thing that can happen is the voltage drop in the wiring adds or subtracts to the voltage "seen" by the BMS so the end cells tend to balance to a different voltage than the cells in the middle.
 
There are so many factors to consider. I fully charge my packs on occasion so the balancing function engages, but most of the time I charge to 4.1V per cell, instead of 4.2V, so...have you noticed this phenomenon mostly on packs charged to 4.2V?

Is is more often the positive end or the negative? (Or always both the same?)

I also agree with Luke that the common battery packs do not have good end-collectors, which is likely a major contributing factor.
 
I'd say all of the above I think play a roll.

Unequal current sharing if using too resistive conductors.
Heat more promient at the deep core of battery (can't dissipate as easily), kind of depends on battery geometry too.
Cheap BMS with unequal parasitic quiescent current drain
Use of unmatched cells, or cells of different batches.
Heat damage from unconsistent spot welding techniques

Matador
 
So, would it make a difference / be important - to keep all the BMS cell wires the same length?
 
no.
.
 
ive noticed it on all types of batteries, 12v lead acid, rc car nicads and nimh,lipos.

i dont use a bms on my batteries. just an isdt 600w balance charger . its very accurate at its balancing. (i only charge to 4.19v per cell)

the numerous cheaper/fake imax b6s ive had in the past didnt balance too well (some even failed in puffs of smoke).
my turnigy accucel was a lot better, but the isdt is pretty much perfect.

in a lot of the cases when i dissasemble a broken battery to harvest the 18650's, i find its the bms board thats died and taken some of the cells with it. in these cases though it never really seems to show a tendency to the outer cells, its more random.

for my/any bms-less cells, uneven thermals or anything causing excess thermals like a poor spot weld seem to be the most likely culprit.

balance lead wire length doesn't appear to play much into it. when i test my chargers balancing accuracy i find it best to use a good multimeter and test each cell individualy at the balance plug. then compare the readings to what my chargers osd said.
i have experienced no noticable ill effects from random length balance leads.

would be nice to know how to counter thermals, circular batteries :D.
 
A thermal imaging camera might be helpful in collecting information.
 
Santacruz said:
So, would it make a difference / be important - to keep all the BMS cell wires the same length?

Yes, theoretically, it would make a difference, as longer wires have higher resistance, you get more sag on longer wires. Meaning the BMS would percieve a lower voltage than the reality for those cells that are wired to the bms by longer lenght of wire. However, in real life, the load on the balance leads is generally bellow 100mA, thus very small, with very little noticeble voltage sag.
 
look better: a programmable BMS measures the voltage (basically zero voltage drop) THEN it activates the balance resistor for a few seconds and disabels it again and then it measures again. rinse and repeat until the voltage is at the proper level.
the cheap non smart bms dont do this.
 
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