Battery problem 36V12Ah

[Latsabb]

8 months ago I bought the front wheel set with 500W power.
It worked fine the first months, then the capacity of the battery started to go down. When I received the kit, it took app 8-10 hours to charge the battery. Then the charging time was reduced from one day to the next (recient months), but now the battery will not take charge at all. When I plug in the charger, the led (in the charger) does not change from green to red indicating charging.

I did some measurements:
Output from battery: app 31V
Output from charger: 41,5V
Output from battery when charger is connected: 41,5V

Where is the problem? Charger, battery or battery electronics?
I have e-mailed GM, but there has been no reply.

Can anyone advise me what to do?

Latsabb

[Bikemad]

Latsabb,

Looking at your voltage measurements, it would appear that the charger is working OK, so I guess it will be a problem with the Battery Management System, connections going to/from it, or faulty cells.

You will need to email Tom (tom@goldenmotor.com or zhourenli@goldenmotor.com) and he should be able to advise you on what needs to be done next. Keep the email short and try to explain the fault in simple terms to avoid any confusion.

He may ask you to measure the voltages of the individual cell groups inside the battery, but wait until you hear from him, before you dismantle the battery, to ensure that you don't invalidate the warranty.

Please let us know the outcome of your battery problem.

Alan
 

[Latsabb]

Update

Sent a mail directly to Tom, and he replied within 12 hours, telling me to performe more measurements:

I made the measurements described, and some control measurements.
Two of the cells appear to be faulty. Nothing seems to be burned and all wiring/soldering looks ok.

From   To      V         From   To     V       Diff
P-      B1     3,90         P-   B1     3,90    3,90
B1      B2   -0,38          P-   B2    3,50   -0,40
B2      B3     3,95         P-   B3     7,50    4,00
B3      B4     3,95         P-   B4   11,40    3,90
B4      B5     3,95         P-   B5   15,40    4,00
B5      B6     3,95         P-   B6   19,30    3,90
B6      B7     3,95         P-   B7   23,30    4,00
B7      B8     3,95         P-   B8   27,30    4,00
B8      B9     0,10         P-   B9   26,90   -0,40
B9      B+     3,95         P-   B+   30,90    4,00
                31,27                  


I am awaiting reply from Tom

[Bikemad]

It looks pretty certain to me that you have six failed cells (two groups of three) that will need to be replaced.

You will have to wait and see what Tom's solution to the problem will be, but I'm guessing it will not be a straightforward repair job, because replacing the faulty cells could require specialised spot welding equipment to remake the connections onto the new cells.

Thanks for the update so far, and please let us know what happens.

Alan
 

[TOM]

Hello,you should replace two packs of cells,B2 and B9
Please replace it following my drawing.

[Latsabb]

Update

I recieved 6 new cells, and replaced them. It was not easy as the old cells were "spot" welded, and I have not the equipment to do this. But I still managed to make a good connection. The battery was then charging, but stopped after 15-20 minutes. Since the new cells was fully charged and the existing almost empty, I figured the electronics stopped the charging since two cell clusters was full and it would correct itself when balance within the battery was achieved. I tried riding the bike with low-medium load so the cells could be equalized, but after 5-10 minutes the battery electronics started to cut the power. I tried to charge it again, but after 5 minutes the chargerlight turned green.
I am wondering if the electronics is causing the problems and somehow damaging the cells. I will have to talk to Tom again.
Any ideas?

Latsabb

[Latsabb]

Update

Tom's response to the problem is to remove the BMS. I will try this, but the BMS is there for a reason, will I now kill the cells rapedly??
My plan is to disconnect the connection between the BMS and the cells, and solder on a lead to bypass the BMS.
Does anyone see a problem with this solution?

Latsabb

[Cornelius]

I think you're safe, as long as you don't exceed the End-Of-Charge-Voltage for each cell, which are 4.2V +/-0.05V? I think these cells can manage 4.65V for a very short period, but that's NOT recommended...

[Latsabb]

Update

I dissconnected the BMS and the battery was charging fine.
Due to other problems with my bike I lost intrest and the bike has been stored in my garage the last 6 months. Now I had the bike serviced, replacing bearing, crank, casette, chain and some other parts. I was ready to hit the road.
The battery was charged in 15min increments, giving the cells time to balance.
After 8-10 minutes of use the controller went into safe mode and I had to pedal this heavy bike the rest of the way to work and back. It is compleatly dead now, won't take charge, battery indicators won't light up.
I have probably killed every cell when mixing used and new cells, charging and discharging without the BMS.
So if you are thinking of dissconnecting your BMS, my advice is: don't do it, get Golden Motors to send you a new BMS instead.

Latsabb

[aawaterloo]

Mixing the old cells with the new cells is always riskly.
In your case, most likely old and new cells were at different State of Charges (SOCs).  The function of the BMS is to not only monitor the cells (most likely voltages and temperatures) during charging and discharging but also balance them (assuming the BMS has a balance circuit). 

If the BMS has a balance circuit which works while charging then you need to wait for a long time as it will hold the cells at top of the charge which reach there first while bringing the remaining ones to the top of the charge. 


One way to mix old cells with new one is to manually bring each cell group to top of the charge in the battery pack. You will need a power supply to do this.  Top of the chage is defined based on the battery chemistry.  It seems for the cells in your battery, it is 4.2 Volts. I would also recommend using constant voltage charge and when the charge current drops to C/20, then stop. 

Because of the different age of cells, you may also see a capacity difference.  Every cell loses capacity over time.  Some chemistries do more than others.  Li-ion batteries are generally lose around 2% capacity every month at room temp. At higher temp, the capacity loss increases.

Just a word of caution.
You will most likely permanently damage the cells if they are over-discharged (again depends on the the cell chemisty but most likely under 2.0V). 
You will most likely permanently damage the cells if they are over-charged (again depends on the the cell chemisty but most likely around 1.2 times the maximum charge voltage).  Overcharging can also result in a more serious themal event.

Also, when you put cells which are at various SOCs in series, you draw even amount of current from each group of cells in series when you run your bike. Which means cells which are at low SOC will risk being over-discharged.  However, a good BMS should detect the low voltage and protect the battery pack by cutting off power.