Replacing 4 cell pack

[vh2q]

About to drop the battery to replace a bad cell. It is one of the cells in a 4 cell pack so i plan to replace the entire module. Could not buy just one cell. Question: is the replacement 4 pack plug and play or am i going to get into an eprom nightmare?

[coulomb]

You will encounter problems unless
* You are lucky and the replacement block of 4 cells has the right CMU id. Since there are only two 4-cell blocks in the pack, I figure that your chances are about 50%, or
* You have a scanner that can emulate a MUT-III and issue the magic command to renumber the CMU ids.

[pbui19]

how do you plan to bring the new cell module to more less the same SOC as the rest in series ?

[vh2q]

The replacement pack is at 16v. The rest of the traction battery is fully charged. So they are all circa 4V. The balancing function will level it all out, I hope. The bad pack I have one cell that is about 3.6V and another about 4v when the rest of them are at full 4.1V. The BMU shows only 4 bars at full charge due to these two weak cells .. it shuts the car down to protect the weakest cell.

[vh2q]

What is the consequence of guessing wrong when switching out the 4 cell module?

[coulomb]

What is the consequence of guessing wrong when switching out the 4 cell module?

My guess (and it's only a guess, I have no direct experience with this) is that the battery will refuse to work (can't go to ready), and you'll get a DTC about non-responding CMU or the like.

Edit: but you could swap the CMU boards in that case. A lot of work once the battery is reassembled.

[Tiborx8]

Why not change only the two bad cells?

[vh2q]

Well that is the plan, to just switch out the cells. unless there is some way (not requiring sophisticated diagnostic equipment) to confirm that the 4 cell replacement pack is a plug and play for the bad pack that's causing the problem. Last thing I want to do is to pull the battery twice. Since two of the cells in the 4 pack are the laggards, I am somewhat concerned that the other two might be dubious, or that the electronics themselves are at fault (eg the balancing function) and not the cells.

Planning to get on it in a couple of weeks.

[Pepin]

I made a bottom balance.


https://www.youtube.com/watch?v=UJx7qL6Eo08
https://www.youtube.com/watch?v=o-9LMiZdoNc



DbMandrake and others after replace cells observe a no relevance gain. I´m not an expert but an electric mechanic in Spain "80% electricos" has a lot of experience special with Smart and Citroën C-zero (i-miev) from carsharing companies. Following his information is important reset BMS and BMS history. Are two steps.

Don't believe the display of charge and the new autonomy!!!!!!! . And drive only with the old autonomy. You can drive with the Obd plug in and reading the voltage for more security.

After this, I charged and finally discharged finding the minimum voltage diference between cells. Wait two hours. Drive a few Kms. Wait two hours. Charge maybe only 15-20 min. Wait. Drive. Etc.....Charge a few minutes and you can see with Hobdrive how up the voltage but the "Ah BMS remaining state" up very slow. You can go down the voltage and finally discharge the car with balanced cells. With no charge in display of the car, I started charge. Then, before inject intensitivy , the voltage go very fast to 3.8v. When the display marks 3 steps all change. Out of reserve, start the intensivity. After this is very dificult to balance charge for the BMS.

I use Hobdrive watching BMS state. Min Max voltage Ah remaining etc....

The voltage and charge has a correlation but are not the same. 4.1v maybe is not a full charge for some cells.
The goal is a battery pack balanced not only with full charge, with bottom charge too.

Car Scanner App "can" make a "smoothing voltage" free. But I dind´t use yet.
This is my experience not a recomendation.

[vh2q]

OK interesting .. the BMU in the car does try to balance the cells but only at the top? (And this balancing function is rather limited requiring LOTS of cycles to achieve a balance).

To clarify my situation .. I have two cells (both in the same 4 pack) that lag behind the remaining cells when the charging cycle ends. They don't get to the same voltage as the rest. Thus, the fuel gauge shows only 4 bars when the car is fully charged. And if you drive the car, the laggard cells reach the lower limit quite fast thus limiting the range before the "turtle" comes on.

Are you saying you can force a bottom balance by following the steps you have described? ie drive, wait, small charge, drive, wait ...

If so perhaps I don't have to replace my cells, perhaps they are just out of whack with the rest. (I am in a location where I cannot take the car to the dealer for an external balance?) And if I do replace my cells, it sounds like unless I am v careful, I will just end up where I started with 2 cells out of line with the remainder?