end cells always fail first, why?

Batteries, Chargers, and Battery Management Systems.
Post Reply
User avatar
jimmyhackers   100 W

100 W
Posts: 229
Joined: May 11 2015 3:33pm

end cells always fail first, why?

Post by jimmyhackers » May 10 2019 2:57pm

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
if you do learn from your mistakes, i should be a genius by now.

User avatar
Schlafmutze   1 W

1 W
Posts: 61
Joined: Apr 23 2014 4:53pm

Re: end cells always fail first, why?

Post by Schlafmutze » May 10 2019 3:17pm

Either because of mechanical or temperature reasons.
Say if there are connections soldered onto those areas that heat cells above safe levels during production.

User avatar
liveforphysics   100 GW

100 GW
Posts: 14134
Joined: Oct 29 2008 1:48am
Location: Santa Cruz, CA, USA

Re: end cells always fail first, why?

Post by liveforphysics » May 10 2019 3:26pm

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.
Each carcinogen vapor exposure includes a dice roll for cancer.

Each mutagen vapor exposure includes a dice roll for reproductive genetic defects in your children.

Each engine start sprays them into a shared atmosphere which includes beings not offered an opportunity to consent accepting these cancer experiences and defective genetics life experiences.

Every post is a free gift to the collective of minds composing the living bleeding edge of LEV development on our spaceship.

User avatar
jimmyhackers   100 W

100 W
Posts: 229
Joined: May 11 2015 3:33pm

Re: end cells always fail first, why?

Post by jimmyhackers » May 12 2019 12:29pm

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.
if you do learn from your mistakes, i should be a genius by now.

User avatar
Matador   10 kW

10 kW
Posts: 882
Joined: Jun 29 2016 10:00pm

Re: end cells always fail first, why?

Post by Matador » May 21 2019 11:47pm

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

User avatar
flippy   100 kW

100 kW
Posts: 1215
Joined: Aug 12 2015 3:07pm

Re: end cells always fail first, why?

Post by flippy » May 22 2019 12:11am

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.
Lithium beats liquid dinosaurs.

Punx0r   100 GW

100 GW
Posts: 5129
Joined: May 03 2012 8:16am
Location: England

Re: end cells always fail first, why?

Post by Punx0r » May 22 2019 9:05am

That would be very dependent on whether where you live is -40°C or +40°C

User avatar
fechter   100 GW

100 GW
Posts: 13858
Joined: Dec 31 2006 3:23pm
Location: California Bay Area, USA

Re: end cells always fail first, why?

Post by fechter » May 22 2019 11:25am

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.
"One test is worth a thousand opinions"

User avatar
spinningmagnets   100 GW

100 GW
Posts: 11078
Joined: Dec 21 2007 10:27pm
Location: Ft Riley, NE Kansas

Re: end cells always fail first, why?

Post by spinningmagnets » May 22 2019 2:15pm

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.

User avatar
Matador   10 kW

10 kW
Posts: 882
Joined: Jun 29 2016 10:00pm

Re: end cells always fail first, why?

Post by Matador » May 22 2019 11:35pm

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

Santacruz   10 mW

10 mW
Posts: 20
Joined: Jun 18 2015 6:28pm

Re: end cells always fail first, why?

Post by Santacruz » May 24 2019 9:34am

So, would it make a difference / be important - to keep all the BMS cell wires the same length?

User avatar
flippy   100 kW

100 kW
Posts: 1215
Joined: Aug 12 2015 3:07pm

Re: end cells always fail first, why?

Post by flippy » May 24 2019 10:09am

no.
.
Lithium beats liquid dinosaurs.

User avatar
jimmyhackers   100 W

100 W
Posts: 229
Joined: May 11 2015 3:33pm

Re: end cells always fail first, why?

Post by jimmyhackers » May 26 2019 10:52pm

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.
if you do learn from your mistakes, i should be a genius by now.

User avatar
gogo   10 MW

10 MW
Posts: 2442
Joined: Mar 17 2008 2:20pm
Location: Iowa USA

Re: end cells always fail first, why?

Post by gogo » May 27 2019 10:04am

A thermal imaging camera might be helpful in collecting information.
"A conclusion is the place where you got tired of thinking." -Steven Wright

User avatar
Matador   10 kW

10 kW
Posts: 882
Joined: Jun 29 2016 10:00pm

Re: end cells always fail first, why?

Post by Matador » May 28 2019 4:38pm

Santacruz wrote:
May 24 2019 9:34am
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.

User avatar
flippy   100 kW

100 kW
Posts: 1215
Joined: Aug 12 2015 3:07pm

Re: end cells always fail first, why?

Post by flippy » May 29 2019 5:35am

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.
Lithium beats liquid dinosaurs.

Post Reply