battery failed in a funny way, any explanations?

jimmyhackers

10 kW
Joined
May 11, 2015
Messages
600
i have 3x 6s 12ah lipos i use in series for 72v on my bike. i only every charge upto 4.15v per cell and rarely (if ever) discharge them past storage voltage. they use rectangular pouch cells, two 6ah in parrallel per cell (its usually only one of the two cells per cell that fail, which is what happened this time).

i charge them in parrallel but before connecting them for charging i check each battery/cell individually and make sure they are all similar voltages (within 0.1v of each other). i also do the same individual test after charging.

every now and then a cell will fail and i replace it with a spare i have. no problems.

but this last time.....after/during a ride, a cell failed (was at 2.1v). 4 of the others were at 3.98v and the one next to the failed cell was at 4.13/4.14vs

why/how would/could one cell be nearly full?
 
If you mean a cell that's in parallel with lower voltage cells is at a higher voltage, that means it has become electrically disconnected from those other cells.

When this happens, it means the capacity of that cell isn't available to the group, which is why the other cells in the group end up overdischarged, because they had to work harder to make up for the "missing" cell, and use more of their own capacity, than the other groups that still had all cells connected.

This is one of the problems with having to connect and disconnect and reconnect cells for charging / discharging--eventually a connector or wire will fail to make the connection at all (which is what yours has to be), or have a high enough resistance to cause current flow problems (which will cause heating in the problematic connection, and sometimes insulation failure and/or fire, if current demand on it is high enough).

jimmyhackers said:
i have 3x 6s 12ah lipos i use in series for 72v on my bike. i only every charge upto 4.15v per cell and rarely (if ever) discharge them past storage voltage. they use rectangular pouch cells, two 6ah in parrallel per cell (its usually only one of the two cells per cell that fail, which is what happened this time).

i charge them in parrallel but before connecting them for charging i check each battery/cell individually and make sure they are all similar voltages (within 0.1v of each other). i also do the same individual test after charging.

every now and then a cell will fail and i replace it with a spare i have. no problems.

but this last time.....after/during a ride, a cell failed (was at 2.1v). 4 of the others were at 3.98v and the one next to the failed cell was at 4.13/4.14vs

why/how would/could one cell be nearly full?
 
not exactly, i mean during a series discharge (18s 72v) one cell has died, yet the one next to it has remained almost untouched, while the other 16 cells have behaved normally.

there were no wiring issues and the "not discharged" cell is behaving normally too (internal reistance is like the other cells that behaved normally).

i would of expected all the other 17 cells to of depleted more rapidly when one fails...like normal.

but as i say.....one cell stayed at almost full charge.
 
If that cell is wired in parallel with another cell, and the other cell discharged to a lower voltage than that cell, the parallel connection is no longer connected (also breaking the series connection for the one that's at higher voltage), or they would have been at the same voltage, because parallel connected cells CANNOT be at different voltages (especially when testing at a time when no current is flowing for interconnect resistances to cause differences).


The other cells arent' discharged as low as the low cell because they didn't have any more than a normal load, because their parallel connections to their paired partner were intact.

All of what I'm talking about is for cells that are setup as parallel pairs, as described here:
two 6ah in parrallel per cell (its usually only one of the two cells per cell that fail, which is what happened this time).


The interconnect failure coudl be cracked solder on the board that connects all the cells in a pack, if there is one, or it could be at the tabs of the cells, (torn off, cracked, etc), or wires failed internally somewhere along their length, etc.

But the problem *must* be an interconnect failure between the two paralleled cells, to cause the symptoms you see.


If you have some other wiring setup than parallel-pair sets of cells, you will need to post a drawing showing the specific connections you have, for us to figure out how you could get the results you see.



jimmyhackers said:
not exactly, i mean during a series discharge (18s 72v) one cell has died, yet the one next to it has remained almost untouched, while the other 16 cells have behaved normally.

there were no wiring issues and the "not discharged" cell is behaving normally too (internal reistance is like the other cells that behaved normally).

i would of expected all the other 17 cells to of depleted more rapidly when one fails...like normal.

but as i say.....one cell stayed at almost full charge.
 
i get what you are saying but that doesnt explain why one cell was at 4.14v when it should of been at 3.98v like all the other normal cells. im not sure if im not explaining right or your not reading it right.

like i said....i check every single cells individual voltage before and after parrallel charging (when they are not connected in parrallel), that was fine before the charge and fine after the charge.

i.e. if you depleted a (fully charged to 4.15v per cell) 6s and killed one cell and the other 5 are all at, say 3.7v.....that's normal

but if you depleted a (fully charged to 4.15v per cell) 6s and killed one cell and only 4 of them are at 3.7v and another one is at 4.14v.....thats not normal.

i am NOT asking why one cell failed, I AM ASKING WHY THE ONE OTHER CELL DIDNT REALLY DEPLETE ONE BIT.
 
maybe this pictures worth a thousand words.
 

Attachments

  • weird cell death.png
    weird cell death.png
    10.4 KB · Views: 299
jimmyhackers said:
i am NOT asking why one cell failed, I AM ASKING WHY THE ONE OTHER CELL DIDNT REALLY DEPLETE ONE BIT.
That's what I just explained, *assuming* that the problem you are describing is that one of the two cells in a parallel group in just one of the 6s2p packs is very low, and the other is still full, and all the other pairs of cells that are in other groups seriesed with this problematic group are at their expected voltages for this point in the discharge curve.

Some different words that say the same thing:

Because it's connection to the rest of the pack on one of it's terminals failed, it isn't supplying any current to the rest of the pack. So no energy comes out of it, so it stays at the same voltage.

And even though you're not asking this, it's still relevant to this specific scenario: Because that disconnected cell isn't supplying energy, the other cell in parallel with it supplies that energy instead, along with the energy it would have supplied normally, which depletes it twice as much as it would have if it still had the second cell in the group paralleled to it.

Again, this *only* applies if the problem you are describing is that one of the two cells in a parallel group in just one of the 6s2p packs is very low, and the other is still full, and all the other pairs of cells that are in other groups seriesed with this problematic group are at their expected voltages for this point in the discharge curve.

See my attached diagram at the end of the post for an example of how the problem could occur.


If the problem you are describing is anything else, then my explanation is probably not relevant.



If the diagram you posted is of the problem you describe, then you're saying the entire end cell group (both parallel cells) is dead, discharged much farther than normal, while the next-inward cell group (both parallel cells) is still essentially full. This is different than the wording you used in the first post, which says: " (its usually only one of the two cells per cell that fail, which is what happened this time)" and is what my explanation above covers.


There isn't any way one cell group in series could stay full while the rest are drained even partly, if they are connected the way they're shown, during a discharge session. (it *could* happen if they're just sitting there and one of the cells in each group, *except* for the one that's still full, failed in a way that drains the other parallel cell down, *and* the lowest group had both cells fail so they drain even faster).

weird%20cell%20death[1].png

It could only happen during discharge like this if something disconnects the still-full cell group from the series connection, so no current flows thru it, but doesn't short across the cells in the group, yet does bypass the group by connecting the other two groups normally seriesed with it. See the last example in my attached diagram at the end of the post.

What would then cause the last cell group to be more than fully discharged, I don't know; another wiring fault that shorts (or partially shorts, placing a high load on) that group out could do it, or just a random internal failure, but it would be highly unlikely for a random internal failure to happen *at the same time* as something that disconnects the other cell group from the pack.

Without being there to do a visual inspection and test measurements myself directly, I can't tell which specific problem is occuring, to then figure out which problem could have caused it.

jimmyhackers said:
i get what you are saying but that doesnt explain why one cell was at 4.14v when it should of been at 3.98v like all the other normal cells. im not sure if im not explaining right or your not reading it right.

like i said....i check every single cells individual voltage before and after parrallel charging (when they are not connected in parrallel), that was fine before the charge and fine after the charge.

i.e. if you depleted a (fully charged to 4.15v per cell) 6s and killed one cell and the other 5 are all at, say 3.7v.....that's normal

but if you depleted a (fully charged to 4.15v per cell) 6s and killed one cell and only 4 of them are at 3.7v and another one is at 4.14v.....thats not normal.

i am NOT asking why one cell failed, I AM ASKING WHY THE ONE OTHER CELL DIDNT REALLY DEPLETE ONE BIT.


The diagram below shows just the wiring diagram for one 6s2p pack on the top. It only includes one of the three packs you are using. Below that is the pack before discharging. Third down is the pack as it would be after a normal discharge. Fourth down is what it could look like if there were one parallel cell in a 2p group that got disconnected and didn't discharge while the other one overdischarged. Last one is the problem you appear to be describing in the diagram you've posted. If I've got that wrong. please correct me.
 

Attachments

  • parallel interconnect failure diagram1.png
    parallel interconnect failure diagram1.png
    35.7 KB · Views: 285
i meant that when a cell has failed before (in the past)....it usually is one one of the two in parrallel that failed....and this also happened this time.

when i seperate the paired duff cells....one cell will hold a voltage (be ok) and the other will usually be way lower (duffed).
the spares i replace with come in pairs so i replace both at the same time.
so far i have 6 ok but "questionable" quality 6ah pouches left as a result of this for future spares.

there were no broken internal tabs, wires or anything. no shorts either.

my scenario is actually a combination of the last two scenarios you have drawn. (with no shorts, bypasses or dropped connections)

so one of the two parrallel cells failed in one cell, and i stil ended up with the paired cell next to it in series being completely full.

again.....this isnt a normal scenario. its weird.

only rational explanation is the cell that was still full was bypassed.....and in conjucntion/coincidence i was just unlucky that the other half of a cell pair let go at the same time.

the problem is.....ther was no bypass.
 
jimmyhackers said:
so one of the two parrallel cells failed in one cell, and i stil ended up with the paired cell next to it in series being completely full.

Unless the connections between the parallel cells are broken, it is impossible for them to be different voltages.

Even if you can't see the break, it must exist, or they would be the same voltage.

If you can read a voltage on the cell, it can't be an internal break (unless it is intermittent), so it would have to be an interconnect between the two cells.


The other part, a cell group that remains fully charged when the others in the same series connection are discharged, can only be explained by them not providing any current when the others have done so, and that can only be explained by shunting the current around them without shorting them out, which requires taking them out of the series circuit.

The only other possible way you could see an "apparent" full charge on a cell group in series with others that are discharged, is if the other cells have much much lower capacity than this one, so that when the system shuts off because those cells are low (either thru your intervention or a controller LVC, don't know since you haven't described the system and it's specifications/limits/etc), this group is still near full voltage because it didn't need to use much capacity to this point. But this scenario would mean you get very very little system usage time or power, because if you used much, that full cell group couldn't be full, because it would be drained by however much power you actually used.


Additionally you can see all sorts of wonky no-load voltages on cells that are not working correctly, but if you place a load on them and watch the voltage, any cell that is damaged is very likely to drop in voltage significantly more than it should for that load.

If you have an RC charger you can set it to discharge mode and watch the readouts, and cells that are problematic will drop in voltage faster than others that are better.

Similarly, setting it to charge mode you would see voltage rising faster on worse cells.
 
Back
Top