Please help with BMS to fix the BIKE

[Gotto]

He there! My name is Vadim and I ordered a Chinese 8000w 40Ah bike some time ago. It was all great, I was going back home after I have done 73 km on a single charge and I nearly drained the battery. I decided to accelerate full throttle and the battery died. I pushed the bike back home for 1 km and it refused to charge anymore.

I contacted the supplier and they tell me that it's definitely the BMS. OK! I took it to the repair shop to check the issue and received the letter from the repair guy:

Before ordering a replacement BMS with the same parameters, my husband checked all the components and found the following problem:
The battery consists of Panasonic NCR1865BD cells in a 20s13p configuration. According to the NCR1865BD cells specification, the maximum capacity is 3180 mAh and the maximum load is 10A per cell. This means that the battery capacity is 72V/41.34 Ah with a maximum possible load of 130A. This battery is completed by a BMS for a maximum current of 100A. A controller with a maximum current of 150A is used for the 8000W motor.

This means that the battery with a 100A BMS is not able to supply the necessary current for the motor and a 150A controller under load, which can lead to disconnection or damage to the BMS. It would be possible to add a more powerful BMS to the battery but not more than 120 - 130A, which is still insufficient. A more powerful BMS cannot be used in order to protect the battery cells that are capable to deliver a maximum of 130A. Under a load above 130A, the battery would overheat, reduce the life of the cells, and possibly damage them.

This motor and controller would require a battery that is capable of delivering at least 150A under load. Replacing the BMS with a new one would put the battery into operation, but it is possible that the BMS would be damaged again at maximum load and for this reason, we cannot assume liability for this repair.

So, the manufacturer provided an option to send 160A BMS to solve the problem and the repair guy said that 160A will kill the battery and that bike might catch fire.

Please give an advice what to do!

 

[Chalo]

Don't push the battery to the limits of its rating. Nothing good can come of it.

 

[Gotto]

Very true! I made a mistake and will try to avoid it next time. The question is should I put 160A BMS or 120-130A?

Should I use the same 150A controller or should it I use 130A controller?

Battery if the part that will remain the same.

 

[E-HP]

Very true! I made a mistake and will try to avoid it next time. The question is should I put 160A BMS or 120-130A?

Should I use the same 150A controller or should it I use 130A controller?

Battery if the part that will remain the same.

The manufacturer sized the BMS to protect the battery from you, and it did. I'd stick with 100A and either program the controller to limit current or program your throttle hand to control your impulse to run full throttle on a depleted battery. Sounds like otherwise the bike is running fine.

 

[Gotto]

Very true! I made a mistake and will try to avoid it next time. The question is should I put 160A BMS or 120-130A?

Should I use the same 150A controller or should it I use 130A controller?

Battery if the part that will remain the same.

The manufacturer sized the BMS to protect the battery from you, and it did. I'd stick with 100A and either program the controller to limit current or program your throttle hand to control your impulse to run full throttle on a depleted battery. Sounds like otherwise the bike is running fine.

That is the case! Now the manufacturer said that they will provide 160a BMS. I am confused because the battery is only 130A.

 

[amberwolf]

The manufacturer sized the BMS to protect the battery from you, and it did. I'd stick with 100A and either program the controller to limit current or program your throttle hand to control your impulse to run full throttle on a depleted battery.

^^This.^^

Don't put a bigger BMS in there, use the same size as what you already had, or *less*.

Don't use the bike as hard as possible--be more gentle with it, *especially* when the battery is running low.

And:

Reprogram the controller so it can't pull more than the rest of the system is designed for (preferably less, so it doesn't strain the parts).

You should also make sure the controller's low-voltage-cutoff (LVC) is set *higher* than the battery's LVC, as the one in the battery is a last-ditch safety shutoff that you only want to hit if there is a cell problem. If you make the battery do the shutdown every time, you're pushing the cells very hard and they won't last as long, and risk of damage is higher. This means you get less range, but your battery won't have to be replaced as soon, and risk of catastrophic failure is lower.


The reason the limits are there is to protect things. If the limits don't match, then you have parts that may be damaged by other parts providing or taking more than other parts can handle.

When protection fails or is removed, then parts can be damaged, destroyed, or catch fire.


Battery damage from using it too hard or running it too low doesn't always cause an immediate failure or fire. Sometimes that happens later, when you're not even there, when it isnt' even being used, and that can be when buildings burn down.


Something else to consider is that running things at their max specs is like riding on the edge of a cliff all the time. It's much more likely to crumble under you and leave you tumbling to the ground than if you ride farther from the edge.

 

[Gotto]

The manufacturer sized the BMS to protect the battery from you, and it did. I'd stick with 100A and either program the controller to limit current or program your throttle hand to control your impulse to run full throttle on a depleted battery.

^^This.^^

Don't put a bigger BMS in there, use what you already had.

Don't use the bike as hard as possible--be more gentle with it, *especially* when the battery is running low.

Or:

Reprogram the controller so it can't pull more than the rest of the system is designed for (preferably less, so it doesn't strain the parts).

You should also make sure the controller's low-voltage-cutoff (LVC) is set *higher* than the battery's LVC, as the one in the battery is a last-ditch safety shutoff that you only want to hit if there is a cell problem. If you make the battery do the shutdown every time, you're pushing the cells very hard and they won't last as long, and risk of damage is higher. This means you get less range, but your battery won't have to be replaced as soon, and risk of catastrophic failure is lower.


The reason the limits are there is to protect things. If the limits don't match, then you have parts that may be damaged by other parts providing or taking more than other parts can handle.

When protection fails or is removed, then parts can be damaged, destroyed, or catch fire.


Battery damage from using it too hard or running it too low doesn't always cause an immediate failure or fire. Sometimes that happens later, when you're not even there, when it isnt' even being used, and that can be when buildings burn down.

Really appreciate your comment! That is why I got so surprised about the manufacturer's comment that I should use 160a BMS. How can they advise such things??? So I will use BMS with less A then a battery! So if the battery is 130A - 100-120A BMS MAX!!!

 

[amberwolf]

I'd use the original 100A, because as the battery ages it becomes less and less capable.

If it was manufacturer cell specs at 10A per cell, that's for a single cell in open air at room temperature in a lab.

Built into a pack it can't get rid of the heat as well, so current should be lowered for that reason.

As it ages it gets hotter for the same current, so current should also be lowered for that reason.

So...lower is better, and I would say *less* than 100A is probably a better idea than *more* than 100A.

 

[john61ct]

Something has to be the limiter to protect the battery

if your brain+throttle hand cannot.

A fuse / CB can do it but that's a very blunt instrument.

Some BMS just burn when overloaded, others are protective and just shut down.

A good controller can be programmed to do it.

Or a CAv3 can be added to do the job.

Finally, say instead of the above, you just kept increasing the battery power - then either the controller or the motor itself

would become the bottleneck and burn if you kept pushing past its limit.

 

[Chalo]

Finally, say instead of the above, you just kept increasing the battery power - then either the controller or the motor itself

would become the bottleneck and burn if you kept pushing past its limit.

The controller is inherently power limiting. That is its function, fundamentally. For this reason, I prefer to use a battery and a motor that are both rated higher than the controller. I can run at full power until the battery is depleted without anything getting hot or going awry.

 

[john61ct]

The controller is inherently power limiting. That is its function, fundamentally. For this reason, I prefer to use a battery and a motor that are both rated higher than the controller. I can run at full power until the battery is depleted without anything getting hot or going awry.

Yes, but for future-proofing I'm thinking, so long as a CA or the controller programming can be tuned to do the limiting

that limit might be far below what both the controller **and** the other two "could" handle.

IOW can upgrade either of the other two, or both, but keep the controller.

Battery always being protected no matter what, my preference being for that to always be better/bigger than actually needed, never let the bottleneck be there.

If I started wanting to bump up against motor limits that's when the thermal derating feature - IMO a must-have - comes, into play.

 

[Chalo]

In re future revisions, I'll note that one can buy a selection of different basic controllers with different power ratings for the cost of a user programmable controller, and a handful more yet for the cost of a CA3. At the point where I'm spending extra just so I can monkey with settings that don't need to be monkeyed with, I have a hobby:life ratio problem.