DBMandrake
Posts: 190
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Battery Capacity Testing

Wed Sep 25, 2019 4:48 am

Forgive me if I've missed it as I've only skimmed this thread, but in all your recording of battery measurements for your car, have you ever checked the spread of capacity of the individual cells, or are you only recording and plotting the overall usable pack capacity reported by the BMS.

This is a very important factor, because without checking the capacity of individual cells (which can be roughly estimated by their relative voltages at a low SoC like 20%) you can't be sure whether what you're seeing is normal expected degradation (all cells degrading roughly equally) or whether you're seeing one or more substandard or failing cells bringing the capacity down below what it should be.

For example in my car the spread of cell capacity goes all the way from about 33Ah to 40Ah in the same pack from worst to best cells, yet I have seen at least one other C-Zero where the cells were all almost perfectly balanced in capacity even at a relatively high mileage.

Clearly in my pack some cells have not been as reliable or durable as others, and this possibly points the finger at poor quality control during cell manufacturing or testing.

If you're lucky and you get all "good" cells in your pack it will last well, if you're unlucky and you get one or two "bad" cells in your pack the usable capacity will degrade a lot faster than normal...
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

CZeroOwner
Gold Member
Posts: 34
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Wed Sep 25, 2019 7:00 am

Hi Simon

The guy at the shop said that there was little difference between the cells.

I believe that the degradation I'm seeing is due to the expected degradation of otherwise health cells from cycling and storage time but I'll check on the voltage difference between cells at low SoC.

CZeroOwner
Gold Member
Posts: 34
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Thu Sep 26, 2019 11:48 pm

DBMandrake wrote:have you ever checked the spread of capacity of the individual cells?


Yesterday evening, just before charging the car, the SoC was 12.5%. At the same time, the highest cell voltage was 3.730 volts and the lowest was 3.700.

David

CZeroOwner
Gold Member
Posts: 34
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Tue Oct 01, 2019 3:14 am

DRMandrake asked for more details on the voltage differences between the cell in our car. Here 3 graphs showing the cell voltages at difference SoCs. The data for the first graph was collected on June 17 2019 and the second and third graphs are from April 13 2019. Note that the difference between the top and bottom of the y axis in all 3 cases is 0.1 volt.

Image

Image

Image

The difference between the max cell voltage and the minimum cell voltage increases dramatically as the SoC decreases. The cell module 3 cell E has the lowest voltage at all 3 SoCs and cell module 12 cell D has the highest voltage.

David

DBMandrake
Posts: 190
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Battery Capacity Testing

Tue Oct 01, 2019 5:50 am

20mV spread at 16% SoC is very good!

From memory mine is now about 125mV at 20% Soc.... :(

Above 2/3rd of that spread is just due to the few cells I will be replacing however so I'm expecting it to drop to around 40mV at 20% SoC after my cell swap.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

CZeroOwner
Gold Member
Posts: 34
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Tue Oct 01, 2019 7:18 am

It sounds like replacing the cells is a very good idea.

David

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