Smart Pie, 36V10Ah WINA battery, Battery Management System problem --> Dead cell

[Redmer]

Together with my college we have been building an electric cargo tricycle. We both have the same bike, the Babboe Big:


The bike will mostly be used to transport our children so speed is not important. That is why we chose the Smart Pie by Golden Motor

The battery was made with 12 GWL / Power WINA LiFePO4 3,2V 10Ah cells:

Hi-Tech industrial battery cell based on lithium technology - LiFePO4, high energy density and metal housing.

Nominal voltage of the cell is 3,2 V and the operational voltage is 2,5 - 3,65 V.
The maximum charging voltage for initial charge is 3,8 V.
Recommended subsequent charging is to 3.65 V.
The minimum voltage is 2.5 V.
Maximum discharge current is 2C continously and 3C peak (<15sec)
Operating temperature -20°C - 60°C (discharging)

More info: http://www.ev-power.eu/LiFePO4-small-cells/WINA-LiFePO4-Power-3-2V-10Ah-aluminium-case.html

So this should make a 36 V battery.
We charged all the cells individually like it is told on the EV-Power site. Initial charge = 3,8V 2,5A per cell

As a BMS / PCM we have the PMC-12S, 36V, LiFePO4, Phosphate BMS, 20-40A.
This was bought here: http://www.greenbikekit.com/bms-pcm/12s-e-bike-lithium-battery-bms.html
(the 20-40A version)

We connected the cells and BMS according to the diagram that came with the BMS.


We programmed the Smart Pie with the software you can download on the Golden Motor website. H

Our settings:
-   Max Current Continuous 12 A
-   Max Current Peak 15 A
-   Motor type – BLDC (brushless) 120 degr.
-   PAS high
-   Motor Voltage 36 V
-   FWD speed 100%
-   REV disabled
-   Regenerative Brake 116% (maximum)

The PAS sensor did not fit properly so the PAS does not work at this moment. But the Smart Pie came with a thump throttle so we could ride the bike.

I was the first to finish building the bike. The EV-bike did what it should. I went for a test drive and I was amazed about the speed it had. I drove 12,5 km in 34 minutes, that is with stoplights and other things that slow you down. I was very content with what I had build.
My college finished his bike a bit later. But his showed problems. After he connected his battery and got on the bike to drive it would shut down within a 100 meters. After disconnecting the battery he could use it again but only for a few meters.

He checked all the wires but could not find anything. Because the bikes were the same and mine was working fine I said, give my your battery so I can check it.

After connecting it to my bike is also shut down within 100 meters.

Now the problems started with my bike as well.

I put my battery back on and the bike drove fine again but my regenerative braking was gone. After a few kilometers I went over a small hill and the 15A fuse (in the Battery) gave in. I had a 20A fuse with me so I put that one in and drove on.

So it was the battery pack of my college that was not working like it should. We checked the battery and found that the cell on the negative end of the battery was dead 0,0V. ??

At first I could drive 20 km easy with one charge but ever sins I connected the other battery the first battery indicator light on the thump throttle went out after a few 100 meters. So I also checked my battery and we found the same cell that died.

We checked all the wireing but everything was connected correct.
We checked if the BMS worked like it should by putting a laboratory power supply in the place where a cell was. Set it to 3,4V, hooked up the battery to a load and the turned the voltage down. At 2,1V the BMS shut the battery down like it said it would in the specs. Checked the other BMS and the same result
So both BMS are OK. (or not???)

My college never programmed a maximum current. After connecting his battery to my bike the regenerative breaking settings were gone. So maybe all the settings were gone. So I reprogrammed the Smart Pie with the settings from above. We rebuild one battery at the expense of the other battery, charged it and put it on my bike. Again it drove like a charm. I went for a drive of about 15 km and everything seemed good. A day later we checked the battery and again the cell on the negative side was dead (0,0V)

What the hell is going on we are at a loss here. Can anybody help us?

[Bikemad]

I think that the BMS is continually draining current from that one cell while not in use. You can check the current consumption by connecting an ammeter between each of the cell's balance lead connections and the BMS balance leads.
I suspect that B1 will be drawing a lot of current (>400mA) if it has completely drained a 10Ah cell in 24 hours.

How much current was the laboratory power supply having to supply to keep the voltage at 3.4V compared to the load on the battery?

Have you measured the individual cell voltages before and after a full charge to check for consistent "balanced" readings?

It might be worth talking to your BMS supplier about this problem, as it has now happened with three different "new" cells (all located in the same position) it really does sound like a BMS fault to me.

Alan
 

[Redmer]

Ok so I’ve disconnected the BMS to see if this was the problem. I disconnected the BMS and put the battery on the charger. After the charger said it was full measured the cells.

To my surprise the cell on the negative side was 4,4V (specs say maximum voltage is 4,2V so that is another cell destroyed). The rest of the cells was between 3,4V and 3,6V.

I did put it on the bike and drove for a few kilometers the bike worked like it should. Measured again and the negative cell went down to 3,3V the other cells were 3,3V.

The negative cell that had 4,4V on it was deformed. So I replaced that one and did some more testing but now kept a closer eye on the charging process. This battery is becoming more and more expensive  :cry:

So with a new cell put it on the charger and this is the list I got:
3,33V  Negative pole
3,34V
3,74V
3,72V
3,74V
3,62V
3,70V
3,71V
3,60V
3,68V
3,69V
3,68V Positive pole

After a few minutes I disconnected the charger because of the big difference of Voltage per cell.

I charged the 2 cells closed (marked in red) to the negative side separate with the lab charger (3,5V 2A)
An hour later did a measurement (after just letting the cells stand still for an hour)
 
3,52V  Negative pole
3,48V
3,60V
3,57V
3,57V
3,54V
3,57V
3,57V
3,53V
3,56V
3,56V
3,56V Positive pole

So the cells go down in Voltage. Is this supposed to happen?
How come when I put the charger on the battery (cells in series) the voltage range is so different?

Another test now with the BMS on the battery gave me this information:
With the BMS on the battery, the voltage dropped from 3,48V to 3,3V but is did not go down any more than that. This was for the cell on the negative pole.

I still don’t know what to do with the battery. Is it the cells, the charger, the BMS

[Bikemad]

To my surprise the cell on the negative side was 4,4V (specs say maximum voltage is 4,2V so that is another cell destroyed).

The maximum cell voltage of 4.2V is for LiPo and LiMn, not LiFePO4.
The specification on your cells states:

Charging Voltage Limit 3.7 V The cells is damaged if voltage exceeds this level

Your charger's regulated output voltage should be set to 43.2V (3.6V/cell) to match the BMS:

Charging voltage  DC:43.2V     3.6V/Cell
Single cell balancing voltage 3.60±0.025V

If it is any higher than this it it should not be used as it could overcharge individual cells if the charging current is higher than the drain current of the BMS.

The cell voltages may drop slightly immediately after a charge and this is not unusual, but I would expect all cells to drop a similar amount.
In your case, the BMS appears to have brought the voltages just below the maximum 3.6V/cell that it is supposed to be.

I would suggest you bring the two lower cells up to a similar voltage to the other cells with the lab power supply set at 3.6V until the voltage of all of the cells are nice and evenly matched.

When they are nicely balanced, keep an eye on the cell voltages over a few hours to make sure they are still even and haven't dropped too much.

If they are uneven (or much lower voltage than they started at) you will have to carefully balance them again and then check them over a few hours with the BMS disconnected to see if there is any marked difference.

If the BMS is making them uneven, or pulling the voltage down unacceptably, it will probably need to be replaced or left disconnected while the battery is not being used or charged.

If the charger voltage is correct and any of the cells exceed 3.65V it could be due to slightly different cell capacities or possibly the BMS.

All you can do is make sure everything is 100% correct regarding the BMS wiring connections and the charger voltage and keep a very close eye on what is happening during charging and discharging to try and pinpoint the problem.

The cell information clearly states that "There is no Selfdischarging effect" so if a cell's voltage keeps going down it is likely to be the BMS that is causing it if the cell is not faulty.

Alan