HELP - Problem with my Cells (TS)

Hi and welcome,
Can you give us details on tha BMS and charger setup.
Gerhard

Clever said:

Hi all, I am a beginner in electric transport. The question is a few months ago I bought a used chinese electric scooter. Is a 5Kw, motor is in rear wheel, 72V/40Ah Thunder sky batteries (24 cells) with BMS cell by cell.

Until last week everything was perfect, the scooter have 3000kms and I can ride 70kms per charge at 85kms/h.

The problem is now range is 35kms per charge. I have tested voltage cell by cell and one of them is 0 and cell is swollen. When scooter stops voltage cell by cell is 2,6V, When scooter is full charged voltage cell by cell is 3,5/3,6 except damaged cell with 0 volts.

The question is how is this possible if I have a BMS?, maybe it does not work? How can I know if BMS is working or not?

Thanks.

Click to expand...

Wow, another BMS (BS) not working properly and allowed a battery to die. If the BMS is not more reliable than the battery it is a moot point to use one. If your controller will limit the charge voltage to like 3.6 volts and then shut down like a good charger can do and your controller limits the end voltage to like 2.4 volts then you will have no reason to need a BMS (BS) system.

Remove the bad battery and replace it with another known good battery. Bottom balancing is going to be required to bring in a new battery to the pack.

It would be good to know if the charger is stopping the charge or if its a dumb charger being controlled by a (BS) system. If all charge discharge functions are being controlled by the (BS) system then you may have this issue again in the future. Check your BS system. Not sure how to know if its working or not. My guess is its not working. Doh!

Pete

There's a lot of debate about BMS. Failures in cell mounted devices have ruined a lot of batteries. I'm not planning on having one at all but I am going to monitor them in a different manner.

You should read this thread. It's lengthy but then you'll understand why more and more people are getting away from cell level monitoring devices.

What you're going to have to do is replace that damaged cell to begin with and figure out if the BMS device is why it failed.

You have a dead cell. I wouldn't just replace that cell without testing the BMS carefully, especially the one on that cell. I suspect you have a battery regulator designed to dissipate some of the power when the cell voltage gets a bit high during charge. I suspect the load got stuck on and ran the cell completely dead. I'm actually a fan of cell level monitoring of Lithium, but poorly designed and implemented monitors are worse than nothing at all.

What is the cutoff voltage for the charger? What is the charge algorithm used? CC then CV till what amperage and shutoff voltage? Should be a smart charger there with a decent algorithm. If you don't know ask Kelly Control. They should be able to tell you. You should know.

Your new battery won't be at the same soc as your pack. Your new battery will be around 50% soc. It may still read the same or near the same voltage as your pack but will not be at the same level. You must either bring the cell to the level of the pack or the pack to the level of the cell. I just discharged all cells to near the same level and then charged up the ones that were off just a tad. Then charged up the pack.

Pete

I happen to have a charger that will charge either packs or single cells. It is a manual charger and must be watched and I also built a setup that will discharge my pack 4 cells at a time as needed or if needed.

If you can drive do so and bring the cells down to about 3V or so together. Then use a head lamp from some vehicle, maybe two lamps in parallel and connect them to one battery at a time to bring them down to about 2.8V or so. Pick a voltage close to the minimum voltage of your cells, maybe 2.7V. You want to get each cell to this same low voltage after resting them for a time. Then you can insert the new one and charge the whole pack.

ElectriCar said:

If you can drive do so and bring the cells down to about 3V or so together. Then use a head lamp from some vehicle, maybe two lamps in parallel and connect them to one battery at a time to bring them down to about 2.8V or so. Pick a voltage close to the minimum voltage of your cells, maybe 2.7V. You want to get each cell to this same low voltage after resting them for a time. Then you can insert the new one and charge the whole pack.

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Hi ElectriCar, you mean if my new cell is for example 2,9V I must to have the others 23 cells in 2,9V?.

Clever said:

Hi ElectriCar, you mean if my new cell is for example 2,9V I must to have the others 23 cells in 2,9V?.

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Your new cell won't be 2.9V. You must bring all of the cells down to 2.9V by discharging them, possibly individually, so they are at the same capacity point.

gottdi said:

What is the cutoff voltage for the charger? What is the charge algorithm used? CC then CV till what amperage and shutoff voltage? Should be a smart charger there with a decent algorithm. If you don't know ask Kelly Control. They should be able to tell you. You should know.

Click to expand...

Thanks gottdi I will try to answer to you. When scooter rides and charge ok complete set battery charges at 84V. Sorry impossible for me to know Charge algorithm.
What means CC and CV?, I am a beginner

Clever said:

Hi ElectriCar, you mean if my new cell is for example 2,9V I must to have the others 23 cells in 2,9V?.

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Yes that's the "balancing" part. But I probably would take them lower than 2.9V, close to the minimum recommended. Calb recommends 2.5V minimum so I'll probably stop at 2.7 or so.

CC = constant current
CV = constant voltage
Normally you charge at CC until voltage starts to climb a bit faster then switch to CV holding voltage and decreasing current to finish charging the cells.
I don't think you need to go much below 2.9 V as capacity falls off pretty quickly so all the cells will be at the same SOC, (state of charge), around that voltage. You can go lower for greater accuracy but there won't be much difference.
You only need to replace the bad cell, unless there are others that are damaged.

Hi all,

I am in trouble again, new TS cell is here. The problem is without BMS (I am not sure it is working) complete voltage after charging is now 81V (23 cells at 3,3V and the new in 4,3V, the new cell is the first cell in set)(so range is about 30kms) Last week when everything is working rightly (even BMS, I think!) I had 86V full voltage, this was 3,6v per cell and range was 60kms.

So how is possible without BMS the cells are charging only 3,3V?, any idea?

Clever said:

Hi all,

I am in trouble again, new TS cell is here. The problem is without BMS (I am not sure it is working) complete voltage after charging is now 81V (23 cells at 3,3V and the new in 4,3V, the new cell is the first cell in set)(so range is about 30kms) Last week when everything is working rightly (even BMS, I think!) I had 86V full voltage, this was 3,6v per cell and range was 60kms.

So how is possible without BMS the cells are charging only 3,3V?, any idea?

Click to expand...

After you bottom balance your cells, you have to track your cells as they charge and determine which cell has the smallest capacity, i.e. will hit 3.6V first. Then measure your pack voltage at that exact moment and set your charger accordingly.

Your BMS used to top balance and get all of the cells to 3.6V. Bottom balancing is better. It's okay that all cells finish at different voltages as your pack is only as strong as its weakest cell.

My guess is that you either didn't bottom balance your new cell or that it has significantly lower capacity than your older ones. Don't let it charge to 4.3V btw, it'll definately hurt it if you keep it there for long.

Clever said:

Hi all,

I am in trouble again, new TS cell is here. The problem is without BMS (I am not sure it is working) complete voltage after charging is now 81V (23 cells at 3,3V and the new in 4,3V, the new cell is the first cell in set)(so range is about 30kms) Last week when everything is working rightly (even BMS, I think!) I had 86V full voltage, this was 3,6v per cell and range was 60kms.

So how is possible without BMS the cells are charging only 3,3V?, any idea?

Click to expand...

It sounds like the battery management system shut down the charger because one cell hit the maximum allowed voltage. If you take a TS cell past 4.3 volts it will be damaged.

You need to remove some charge from just the new cell. I'm going to guess it cell has about 15 more amp hours in it compared to the others. You can only charge until the first cell is full and discharge until one cell is empty. So, with one cell more charged than the rest you only have about 25 amp hours you can use right now. You are going to have to manually balance the pack, just installing a new cell was only half the repair job.

We said you need to bottom balance your pack and single cell to bring that cell into the pack. You can't just stuff it in. It won't work. That new cell is 50% charged right out of the box. There is no way to know for sure if your whole pack is at 50% because of the way they discharge so flat. You must take the pack down past the knee then take that cell past the knee to match the others at resting voltages. You can't determine as soon as you pull off the charger or load. You must let the batteries rest.

You have taken that battery to way higher voltage and into the danger zone. You also need to watch the voltage of the new battery while charging to be sure you don't go over. Your BMS is not working so don't rely on that. As you can see it did not protect your NEW VERY EXPENSIVE CELL. Is that worth the cost of the BS system?

I am not sure why you did not monitor your new cell closely. Seems odd to me.

sounds like you need to either top or bottom balance the whole pack, including the new cell. Putting the new cell in when it was at 50% DOD from factory,a nd the rest of the pack in unknown state was not a good idea.

As painful as it is, I think you are going to have to disconnect series, re-wire in parallel, and top (or bottom) balance to whole pack together since you probably have cells all over the place now.

gottdi said:

You also need to watch the voltage of the new battery while charging to be sure you don't go over. Your BMS is not working so don't rely on that.

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2 very important sentences right there.

1. You will have to monitor the individual cell voltage while charging until you have the new cell "in line" with the existing cells throughout a charge. Even with a bottom balance you should still watch the first charge to make sure. You could also do this by top balancing but if you actually succeed in removing the BMS bottom balancing is better.

2. Your BMS seems to have some problem (it is *possible* you just started with a bad cell.) Ae BMS shouldn't be relied on to keep things working, it should only be called on in the event something is going wrong. It actually seems that your BMS is working at the moment because you said the charge terminated at 81 volts with the new cell at 4.3 volts when it used to charge all the way to 86 volts.