# BMS problem (I think)



## JohninCR (May 6, 2008)

I'm initiating myself with an electric bike after successful EV's for the kids using wheelchair motors and controllers. For my bike I'm using a Kelly 36V200A controller. Overkill I know but I plan to use it later on something more powerful. I have the Kelly programmed down to 20% performance, so I don't damage my 36V15ah Lipo4 packs purchased direct from Ping in Shanghai.

My problem is that the BMS built into the packs shuts the battery down when I reach a certain speed. I don't believe it's an over current issue as it never occurs under strain going up hill. It happens even going down hill under power at 20-25mph.

My electronics knowledge just about fits in this post, but I'm fairly confident that I have everything wired correctly. The Kelly is one of those with only one connecting post to the motor, the negative poll of the motor, so the positive goes from the battery pack to my fuse and then to the controller and motor. The motor is one of those chinese MY1020's.

What I think is happening (because the pack cut off once as I let off the throttle near maximum speed), is that some kind of electrical signal (back EMF maybe, whatever that is) is sent from the motor back at the battery pack's BMS causing it to shut off battery output until I reset it by disconnecting the battery.

Is the solution likely to be putting a diode on the battery cable (or both).

John


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## mattW (Sep 14, 2007)

Back EMF only really applies across the motor, I don't think the batteries wouldn't know anything about it. Does the battery turn back on straight away when you slow down again?


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## JohninCR (May 6, 2008)

Thanks Matt,

The BMS sees the "+" side of the motor directly, via a direct connection other than the fuse. That's what has me thinking back EMF.

No the battery doesn't come back automatically. I have to disconnect it before the BMS turns it back on. That took a while to figure out, since a battery smart enough to turn itself off wasn't intuitive. I thought I had a controller problem. No that I know the BMS isn't quite as smart as it thinks it is, how do I outsmart it. Since I have no clue about it's setup, is a diode good start, and would offer some protection to the BMS anyway?

John


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## mattW (Sep 14, 2007)

Usually you wire the + - of the batteries to the controller input and a seperate + - to the motor. Is the Kelly a different design? Otherwise your wiring might not be right.


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## JohninCR (May 6, 2008)

mattW,
Yes, as strange as it sounds, with this controller there are three primary connections to the controller. + and - to the battery and a third M- to the the motor. To be exact, yes is do have the motor's "+" connected to the controller's "B+" which goes to a fuse then the battery. The controller somehow controls what goes thru the motor via the 3rd controller connection, "M-".

This am, I put the second battery pack in parallel with the other, despite recommendations not to do so by Ping, who puts these Lipo4 packs together, and I passed 30mph, not bad since I weigh over 100kg.

I do want to try maximizing performance, and while a 2nd pack is a temporary fix, I want to understand what makes a single pack to shut down, and how to address the issue. I understand that the BMS is responsible for maximizing/balancing performance from individual cells in the pack, but I want to understand how best to protect these packs because they aren't cheap. That's why I ask about the diode(s). Would a diode protect my packs from stray currents created by the motor? ie, if this happens with a small motor, will the same occur but in larger scale with my much larger motor for my electric motorcycle that is next?

JOhn


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## mattW (Sep 14, 2007)

Well if it works with two in parallel then it would be something to do with the current not the voltage since the voltage would stay the same. Back emf is only to do with the voltage across the motor so I don't think that's it. Maybe you could connect an ammeter (or a multimeter if you pull <10A) to see whether the bike is pulling too many amps out of the batteries, their voltage might be sagging too low so the low voltage cut off switches them off.


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## BradQuick (May 10, 2008)

Does the Kelly controller you have have regen? Maybe it is feeding back to try to recharge the batteries. Regen can generate a lot of current, probably more than the batteries' maximum charge rate. Maybe the batteries are smart enough to not let you charge them too fast.

The amount of current being fed back might be too much for one pack, but OK for two packs (half the current to each pack.)

- Brad


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## JohninCR (May 6, 2008)

Thanks guys. Ping, the Lipo4 pack maker, got back to me and he said I must be hitting over-current. With it acting like a speed cutoff even going downhill, I'm not so sure, especially with it not cutting off going uphill at slower speeds where it seems like more current is demanded by the load.

It's not regen other than for a split second when I release the throttle, since there's a freewheel on my sprocket. I picked up a diode and will report back if that makes a difference.

I switched to an EVWarrior controller at 24V and lead, and I'm getting great torque but overheating on hills, so first I'm ventilating the motor better by swiss cheesing the end cap and adding an air scoop. 

I've read about people over-volting controllers as well as motors. Hopefully this Curtis 1505 rated for 24V isn't smart enough and can handle 36V, then I can use the Kelly for something bigger.

Ping recommends against series or paralleling his Lipo4 packs. Series I understand, don't want that other current going thru the BMS, but how could parallel be a problem, other than problems if 1 pack gets lower than the other? I tried them in parallel and no shut downs, so I'd like to go that route, but don't want to blow my packs.

John


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## Pierre spiteri (Apr 13, 2009)

HI i have come across the same problem, i bought my pack from sun-thing it was working perfectly, now when i go uphill or even on level road and push the throttle to full or even more than half full it cuts off. i checked the voltage immediately it was almost 53v. the manufacturer told me that i had to remove the bms but i dont have clue how. Can somebody help me?


thanks for your help

Pierre Spiteri


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## Pierre spiteri (Apr 13, 2009)

HI i have come across the same problem, i bought my pack from sun-thing it was working perfectly, now when i go uphill or even on level road and push the throttle to full or even more than half full it cuts off. i checked the voltage immediately it was almost 53v. the manufacturer told me that i had to remove the bms but i dont have clue how. Can somebody help me?


thanks for your help

Pierre Spiteri


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## Sangesf (Apr 21, 2011)

Pierre spiteri said:


> HI i have come across the same problem, i bought my pack from sun-thing it was working perfectly, now when i go uphill or even on level road and push the throttle to full or even more than half full it cuts off. i checked the voltage immediately it was almost 53v. the manufacturer told me that i had to remove the bms but i dont have clue how. Can somebody help me?
> 
> 
> thanks for your help
> ...



Did u check each cell group to see their voltages?

You probably have once cell group way lower than the others...


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## chinabusiness (Feb 6, 2012)

I think that depends on whether the BMS manage the whole pack or each single cell . 

When the whole pack comes at 53v , then each cell is about 3.3V .It seems that there is no reason the BMS will cut off the pack . 

probably BMS manage each single cell , then the key point might be there is some quality problems( the parameters of the clles are not consistent) with cells- ie , there are one/some cells which are too weak compated to others ,then BMS cut off . 

After BMS cut off , maybe you try to measure the voltage of each cell to see whether there is one cell whose voltage is too low - then replace it with a new one . 

FYI 









Pierre spiteri said:


> HI i have come across the same problem, i bought my pack from sun-thing it was working perfectly, now when i go uphill or even on level road and push the throttle to full or even more than half full it cuts off. i checked the voltage immediately it was almost 53v. the manufacturer told me that i had to remove the bms but i dont have clue how. Can somebody help me?
> 
> 
> thanks for your help
> ...


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## Pierre spiteri (Apr 13, 2009)

hi infact on of the cells is slightly lower than the others. all the others are at the same level. i will try to have the cell changed.

thank you


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## chinabusiness (Feb 6, 2012)

Only slightly lower.... 

Anyway ,good luck




Pierre spiteri said:


> hi infact on of the cells is slightly lower than the others. all the others are at the same level. i will try to have the cell changed.
> 
> thank you


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## Pierre spiteri (Apr 13, 2009)

hi i did not understand your comment. can you please explain your comment i checked the voltages of the individual cells and all of them 3.3.4 v but one of them was 2.94v.


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## chinabusiness (Feb 6, 2012)

Maybe you can try to change this cell with low voltage to see the result . 




Pierre spiteri said:


> hi i did not understand your comment. can you please explain your comment i checked the voltages of the individual cells and all of them 3.3.4 v but one of them was 2.94v.


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