ZS EV Mk2 51kWh LFP Battery - Not Balancing Massive Cell Deviation

[T1 Terry]

Having dealt with a neighbour’s out of balance pack before, the only solution was to open up and then bring up the low voltage cells to match the rest of the pack.
Specialist garages like Cleevely can help with EVs but IMO the pack top voltage is 3.65 volt as BMS is continuously trying to top up the low voltage cells. IMO balancing isn’t kicking off.

It might happen one day, but it will be an after market add on, but active balancing using induction coils is the way to balance an out of balance pack quickly.
It basically takes the current from the high voltage cell, and feeds it into the low voltage cell. All the time there is a differential of greater than 5mV between the highest voltage and the lowest voltage cell, the balancer is active, below 5mV (0.005of 1v) the balancer shuts down.

A comparison, a lossy type resistor balancer to balance a 156Ah cell that was 10% out of balance, it would need to burn off 15.6 Ah of capacity out of the high cell by turning that 15.6Ah of electrical energy, into heat energy using a resistor.

An active induction type balancer, to correct the same 10% out of balance needs to move 7.8Ah from the high cell to the lower cells. Even if it only moved 1 Ah per hr, it would balance the pack in 7.8 hrs. Most induction balancers can shift between 5 Ah per hr if the voltage differential is high enough, but as the voltages start coming closer together, the less it can shift.

In the house battery systems I was building before my workshop was burnt down, the induction balancers would have a 48V 600Ah first charged battery pack, fully balanced, within 8 hrs with all the cells at 3.45V. They work and they work well, lossy resistor type balancers can only handle low current because the heat generated would burn them out.
If they tried to handle even 0.5 Ah per hr, it would take 31.2 h to bring that 10% out of balance high cell voltage down, but the heat generated through the circuitry and that resistor would most likely not last for 31.2 h.

The claim a resistor type balancer can keep a balance pack balanced is even questionable. Those that say once the pack is balanced, it will stay balanced, aren't using all the numbers in the equation. The difference in cell internal resistance creates a different capacity loss each time it discharges and then recharges, resistance at a cell connector generating heat rather than moving current in or out of the cell .... these are just a few of the things that will cause a battery pack to go out of balance. These cells are mass produced, they all have +/- tolerances that are acceptable to remain A grade cells.

Cell balancing will be the next critical step in the EV lithium battery evolution, the higher the capacity of the cells in the battery pack, just exaggerates the problems if the balancer is not up to the task .....

Sorry, another ranting sermon .... I should stick to the humours comments, they are always a lot shorter

T1 Terry

 

[hermitdave]

Haven't come across induction coils based balancing. somehow feels wrong but I know that active balancers bleed excess voltage of high voltage cells and dump it on ones with low voltage cells. The amperage however is the key factor since active balancers are often limited in that manner. Higher the amperage the more expensive they are.

Manufacturers tend to go with resistive as it cheap and cheerful. I did install active balancer on my neighbours pack after bring weaks cells up. The balancer then managed to keep them inline however it wasn't capable of pulling those weak cells to begin with.

My 2p, if 402V is good then great. If it isn't good I am not going to go mad. I will do a decent DoD and then fully charge it each time.

 

[T1 Terry]

ZHC and few copy cat brands out of China are the most common, but a few cell mounted balance boards that are controlled by a master that collects all the data via optic fibre or 2 wire canbus appear every so often where some electronics technophobe builds one for his EV project.

One stand out was a system made by a fellow member of the local chapter of the AEVA. He built it for his extended range Gen 3 Prius battery. Each board had a microprocessor and would monitor the cell either side as well, so a cell board failure was reported yet it didn't shut down the system. Because it used the hive mind principle, it had some amazing capabilities, it would adjust up or down the charge rate on a 300 sec sampling schedule, so the cells charged remarkably fast, yet remained balanced .....

A short from the aluminium cell casing to the aluminium battery box caused a fire and burnt it to the ground .... wife said, that's it, I want a factory EV now, so he lost interest in the whole thing .... shame, they would have been a brilliant system to sell as a package ......

T1 Terry

 

[hermitdave]

ZHC and few copy cat brands out of China are the most common, but a few cell mounted balance boards that are controlled by a master that collects all the data via optic fibre or 2 wire canbus appear every so often where some electronics technophobe builds one for his EV project.

One stand out was a system made by a fellow member of the local chapter of the AEVA. He built it for his extended range Gen 3 Prius battery. Each board had a microprocessor and would monitor the cell either side as well, so a cell board failure was reported yet it didn't shut down the system. Because it used the hive mind principle, it had some amazing capabilities, it would adjust up or down the charge rate on a 300 sec sampling schedule, so the cells charged remarkably fast, yet remained balanced .....

A short from the aluminium cell casing to the aluminium battery box caused a fire and burnt it to the ground .... wife said, that's it, I want a factory EV now, so he lost interest in the whole thing .... shame, they would have been a brilliant system to sell as a package ......

T1 Terry

Sounds like OpenBMS

 

[Peter WA]

If you can see every cell voltage, the give away that a bad connection is the problem, the cell either side of the low cell is also outside the voltage range of the other cells.

Sorry for the late reply, I got no notification :-(

In the first image you see what I get. I don't get individual cells. I get 12 modules. Each module must be 10 cells. For a total of 120 cells. If I multiply the average cell voltage by 120, I get 409V, which is what the car displays. All ties in.

For each module I get the highest and the lowest voltage of any of the cells in that module.

You can see that the one at #14 (which would be module 7, highest cell) is the outlier, with some others also a fair bit above average (16, 18, 20).

What gives me the confidence to be certain that it is a cell that is high, and not measurement error is the second image. The speed with which that voltage rises for the highest cell. That is the typical charge curve of an LFP cell reaching 100% SOC. 10 minutes from 3.4V to 3.65V. At the same time the lowest cell voltage barely moves, indicating that there is a lot of spare capacity left in those low cells.

Now 3.38V for the (many) low cells, dropping back to 3.36V as soon as charging stops, is low enough to worry about the capacity that is locked in those under-charged cells. My best estimate is 10%. Could be as much as 15% or as little as 5%. But 10% matches with the reduction in range I have recently seen.

 

[T1 Terry]

Unfortunately, the cell balancer will not shift the over capacity to the under capacity cells, so the imbalance will continue to get worse. The reported 3.380V for the lowest cell, was unfortunately, while charging, the settled voltage is probably around 3.65V.
The cell reached 100% charged when it hit 3.55V, the big steps in voltage rise after that are a clear indicator.
appears as though the voltage drop when charged stopped was around, 0.02V, the next little step was as far as the balancer intends to pull it down. As soon as that high cell is placed under load, it will rapidly drop to 3.40V, the rest is surface charge with next to no capacity.
In the 5 mins it takes to push the high cell from 3.40V to 3.65V, at least a bit is going into the low cell, but looking at the chat of cell voltages, you only have 1 cell fully charged and 2 others close to fully charged.

I would continue to do short run recharges and see if the balance improves ..... that little bit that is wasted as over voltage charge in cell 14, will gradually bring the low cells up, but it will take a lot of short cycles .... bringing cell 14 up to 3.65V each time will do no harm, you can see by the cell temp display, it is not stressing that cell at all.

T1 Terry

 

[Peter WA]

the next little step was as far as the balancer intends to pull it down.

Unfortunately no. There is no balancing happening at all. The little step down is when charge current drops to 0. Just settling and surface charge dissipating.

Leaving the charger connected for several hours after does not make any difference. And unlike some reports from BYD users, the car does not appear to balance when just parked.

I can't read out voltages unless the HV battery is connected, which means turning on the car. At that time a small load will drain the battery, and show me slightly lower voltages across the board, but nothing indicates than any balancing ever takes place.

I do hope it is just a software bug. But it could be a hardware fault too.

I doubt my dealership will be interested at all at this point. So I'll just have to chance it for now and see how things develop over time. Keeping evidence.