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Car Battery problem : 1st/last cell issue ?
#1
Hi guys,
with this post i trying to hunt for informations about a strange problem of car batteries.

Is there any reason thinkable, that the first/last battery in an xs configuration gets an issue, given
- it is properly produced and balanced at factory status
- it gets charged and discharge for some (more)  usage/cycles (supervised by the original BMS electronics)


and
- gets the issue, that repeatable the first and last Cell/pack proves to empty before the other Cells in a test. Not equal earlier, but earlier.

Any thinkable reasons for this ?
I have two assumptions, but i dont want to focus your thoughts.

No, i do not have more informations until now, but i may get answers to specific questions you might ask.
The question comes out of the car battery industry, from a person involved.
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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#2
Test the cells...

Also what c rate so you Charge and discharge at? Above its nominal?
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#3
(02-13-2019, 06:03 PM)daromer Wrote: Test the cells...

Also what c rate so you Charge and discharge at? Above its nominal?

No results of cell test lnown.

Ist E-Car Battery, BEV, so quite a rough current profile. They have to work, actually.

To specify my very unspecific Request, i just ask for ideas, not for solutions.

Example:

- Electrostatic influence of the Field onto chemical reactions
- would probably affect only 1st or last

- Thermal iinfluence, outer cells hare hotter/Colder than the others
- ??

- BMS connection points, end point(s) possibly different to the connectors between cells
- Depends from BMS "intelligence"


More ideas ?

( For daromer: More Ideas ?)
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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#4
The problem could easily come from differences in internal resistance.

Also, where is the 1st cell? Is it in the middle of the pack or outside?
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#5
The most common issue is that the cells arent the same and therefore act different. For instance if you run them really hard the outer will be affected due to thermal and also often bad design on the busbar/wires causing it to happen.

Im not giving you a solution. Im giving you a way to find out whats going on instead of guess work of stuff not related. Without the facts from start including proper tests I wont be guessing on strange scenarios that might not even be in the ballpark
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#6
(02-14-2019, 06:29 AM)daromer Wrote: The most common issue is that the cells arent the same and therefore act different. For instance if you run them really hard the outer will be affected due to thermal and also often bad design on the busbar/wires causing it to happen.

This adds to the mechanical/thermal specialties of the outer cells. Thanks.

Quote:Im not giving you a solution. Im giving you a way to find out whats going on instead of guess work of stuff not related. Without the facts from start including proper tests I wont be guessing on strange scenarios that might not even be in the ballpark

Hey man, i asked for ideas, not for solutions.... read my post....  Big Grin Big Grin

( i think i am close to make a training here on problem solving systematics....)

(02-14-2019, 03:47 AM)BlueSwordM Wrote: The problem could easily come from differences in internal resistance.

Also, where is the 1st cell? Is it in the middle of the pack or outside?
It happens regularly on the packs in questions. Its new custom development.
Thanks, position of "outer" cells will be checked.
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
Reply
#7
I have been thinking about this for different chemistries and I think it has to do with voltage leakage paths via the electrolyte, casing and internal resistance, which in some instances creates a very slight voltage gradiet through a stack of cells which over time creates a distortion of charge that you are then seeing.

For lead acid the other factor may also have to do with the way the current distributes along the end plates as it will not be as even due to the plate reistance (more so at starting currents) and the second / third plates the distribution will be more even and then show less degredation compared to the ends.

I like pictures.... ok, technically not physically correct, just for concept explanation....

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#8
Thanks for all answers so far.
I got more informations after your input.

- it is pouch cells
- first and last cells are not only out of sync, they are damaged (reduced capacity)
- effect is repeatable, happened on several banks.
- 1st and last cells are "outer" cells.
- balance wire ist (electrically) connected in the middle of the current path between two pouch terminals. Connections are welding.
- balance wire at the end MAY be different regarding the current path and resistance, not enough details until now.
- there may be a problem with the potential of the Pouch, wich is conducting itself, and a pole, where electrical corrosion happens.
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
Reply
#9
(02-28-2019, 01:59 PM)Cherry67 Wrote: Thanks for all answers so far.
I got more informations after your input.

- it is pouch cells
- first and last cells are not only out of sync, they are damaged (reduced capacity)
- effect is repeatable, happened on several banks.
- 1st and last cells are "outer" cells.
- balance wire ist (electrically) connected in the middle of the current path between two pouch terminals. Connections are welding.
- balance wire at the end MAY be different regarding the current path and resistance, not enough details until now.
- there may be a problem with the potential of the Pouch, wich is conducting itself, and a pole, where electrical corrosion happens.

Hello :

  It has to do with the resistance of the connections and the cell. Using aluminum or nickle connections only compound the
problem. Most Bms's use a passive resistor to bleed off the extra power, But they are rated way lower to the amp's being put
into them to be any good. Putting the higher capacity cells on the outside of the string helps.

 Active selective Bms's charge  each cell to a specific voltage and stop. These are hard to find and but work very well.

I use a cell voltage equalizer and they keep each cell within .02 v , they themselves do not use a lot of power.
There are several versions of them on the Chinese market. They do not have to work hard, because a out of balance condition
usually takes several cycles .

https://www.aliexpress.com/item/QNBBM-6S...autifyAB=0

I do not use a Bms system on my LTO 6 cell battery, only a equalizer. LTO can go to zero volts and go to 3v for some time and not hurt them . I have cells that were at 0v for over a year and charged up to full capacity. In a car starting setup , have had zero problems, from 95 F to -15F. starting a 8l diesel tractor.

Lead acid batteries have 1 advantage , they can boil off electrolyte to solve the out of balance - resistance problem , except for
 the gel cell versions.

The pouch cells need to be compressed to keep the foil together , you lose capacity if not touching together .                             @ 5 psi according the my research. Round cells don't have that problem.


Jim
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#10
(03-02-2019, 04:25 PM)Jim Jr. Wrote:
(02-28-2019, 01:59 PM)Cherry67 Wrote: Thanks for all answers so far.
I got more informations after your input.

- it is pouch cells
- first and last cells are not only out of sync, they are damaged (reduced capacity)
- effect is repeatable, happened on several banks.
- 1st and last cells are "outer" cells.
- balance wire ist (electrically) connected in the middle of the current path between two pouch terminals. Connections are welding.
- balance wire at the end MAY be different regarding the current path and resistance, not enough details until now.
- there may be a problem with the potential of the Pouch, wich is conducting itself, and a pole, where electrical corrosion happens.

Hello :

  It has to do with the resistance of the connections and the cell. Using aluminum or nickle connections only compound the
problem. Most Bms's use a passive resistor to bleed off the extra power, But they are rated way lower to the amp's being put
into them to be any good. Putting the higher capacity cells on the outside of the string helps.

 Active selective Bms's charge  each cell to a specific voltage and stop. These are hard to find and but work very well.

I use a cell voltage equalizer and they keep each cell within .02 v , they themselves do not use a lot of power.
There are several versions of them on the Chinese market. They do not have to work hard, because a out of balance condition
usually takes several cycles .

https://www.aliexpress.com/item/QNBBM-6S...autifyAB=0

I do not use a Bms system on my LTO 6 cell battery, only a equalizer. LTO can go to zero volts and go to 3v for some time and not hurt them . I have cells that were at 0v for over a year and charged up to full capacity. In a car starting setup , have had zero problems, from 95 F to -15F. starting a 8l diesel tractor.

Lead acid batteries have 1 advantage , they can boil off electrolyte to solve the out of balance - resistance problem , except for
 the gel cell versions.

The pouch cells need to be compressed to keep the foil together , you lose capacity if not touching together .                             @ 5 psi according the my research. Round cells don't have that problem.


Jim
Jim, thanks for interesting informations.
The connections are copper welded to the compound of the pouch cells. The cells are compressed.

Can you please go in more detail what problem of the connections cause the problem, or what to care for to avoid the capacity loss problem ?
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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