trouble with my bms

[csjones10d]

Hi please could some one on here help with the issues I am having with my set up, I will try to explain the best I can the trouble I am having.
When I start off from a standstill if and use to mutch throttle my bms shuts down when I disconnect the power and plug it back in it will work again if I start with a little throttle all works ok
till I reach a steep hill and as soon as the motor has to work harder again the bms cuts out.

It is the bms that is at faut I think because when I run the bike battery with out the bms I have no issues at all, bellow is what my set up is

3000wat motor

bms 60v lifepo4 (72v) 20 cell 60a/150a

battery lifepo4 prismatic cell x 20 total 60v nominal and 72v at full charge

controller 72v 45 amp / controller low cut of is 55v

if you need more info I will try my best to give it to you

 

[dogman dan]

It might be that under load, your cells sag in voltage enough to trip the bms.

Your controller could be the problem, too high an lvc, but in nearly every case, if you have to unplug to restart, its the bms that turned it off.

 

[fechter]

It could also be your controller has some overshoot in the current limit and it goes over the trip point for the BMS. Most of the BMS current sensors have a very fast response, so even a short spike in the current can trip them. I had this problem with one of mine. I ended up adding more shunt resistors to the BMS to increase it's current trip point.

One or more cells sagging below the LVC point under load is also possible as Dan points out. This is a little harder to test, but most BMS will automatically reset when the voltage gets back in range, so I suspect the current limit more.

 

[csjones10d]

thanks for your advice, when it comes to my controller how can I test it and how do you add more shunt resistors or am I best just getting another controller

 

[fechter]

The shunts I was referring to are on the BMS board. If you can take a picture of your BMS, we might be able to point them out. As an example, here's what mine looks like:



Each resistor is 10 milliohms. On this board, there were blank spots for two more, which I used. Each resistor adds a certain amount to the current trip point. Adding more resistors will increase the current trip point. If there are no blank spots, I've placed new resistors on top of the existing ones. They could also be replaced with lower resistance ones.

 

[Sunder]

Sorry for thread hijack, but you've raised the issue here.

Is there a down side to raising the current trip point? Will anything else overheat or burn out? I notice that BMSes with a higher current carrying capacity tend to be a little more expensive, so presumably bigger components are used somewhere.

Would also like to point out that LiFePo4 gains internal resistance quite dramatically:

1. Due to age,
2. Due to lower temps.

As in, a 15*C drop in temp, (from 25*C to 10*C), changes the current my pack can deliver without BMS tripping from about 80A to about 35A. These are very old Thunderskys.

 

[e-beach]

My 2¢.... The battery sags and causing a LV cut out.

Is your battery in good condition?
How old is it?
Is it balanced?
Did everything work fine in the past or is this a new bike/battery or controller?

 

[csjones10d]

the battery is 2 years old and in good condition was working ok up to last month then started to go wrong is balanced every time and the controller is the same age as the battery.

 

[fechter]

You could possibly have one low cell that causes the BMS to trip. One low cell won't be obvious when bypassing the BMS.
Test by running the pack until it cuts out, then measure the voltage of each individual cell. They should all be about the same. Look for a low one.

 

[999zip999]

Check the batteries state of charge by checking each parallel groups voltage and write down like this.
1. 3.45v
2. 2.99v
3. 3.44v

18. Xxx volt
Plus charger voltage and battery voltage.

 

[e-beach]

And if you can't measure each cell without opening you battery, test each cell through the bms wires.

 

[csjones10d]

hi I have found a bad cell hope this is what is the problem will let you know

 

[e-beach]

What did the voltage read compared to the other cells?

 

[999zip999]

Hopefully not lower then 2.5 volt ? You can't keep a secret so don't try. We have ways of finding out.

 

[amberwolf]

Sorry for thread hijack, but you've raised the issue here.

Is there a down side to raising the current trip point? Will anything else overheat or burn out? I notice that BMSes with a higher current carrying capacity tend to be a little more expensive, so presumably bigger components are used somewhere.

The only downsides happen when downstream systems (controller) draw higher currents than the modified BMS (or cells) can handle.

Discharge FETs have to handle the current; if they are higher RDSon types then there will be more heat during discharge at a higher current, so the PCB has to carry that away if there's no heatsink for them. If it can't, the FETs themselves heat up, and if it's all shrinkwrapped / boxed up it can be enough heat at sustained higher currents to melt the solder. Or the FETs can overheat and fail because the RDSon gets higher the hotter they are (which makes them even hotter if the current that was overheating them is sustained).

Similarly, the copper traces on the PCB have to be sufficient to sustain the higher currents without heating that would cause problems.

The cells have to be able to handle the sustained current increase.

If there is no sustained current increase because the downstream systems aren't drawing more than the BMS and pack was designed to handle to start with, then there aren't any downsides.

 

[csjones10d]

hi I ran the battery down till all the cells reached 3.2 and the one that was bad was down to 1.6 have tried to bring it back up but wont hold a charge goes up to 3.6 then falls fast to 2.6 so have removed it from the back and now charging the battery again will let you know if this has sorted out my problem.

 

[fechter]

The low cell could be defective and discharging internally or your BMS could have a bad channel that drains whatever is connected to it. If the voltage was dropping even with the BMS disconnected, then it would indicate a bad cell.