BMS restricts current way too much

[J A G]

Hi all,

I'm building my first ebike and have run into a problem. Yesterday I hooked everything up for the first time, and the self-learning Brainpower controller was able to turn the wheel in the right direction. So I turned the pedal to activate the PAS, and the wheel started to move. But as I increased speed on the pedal, the power cut out and the wheel slowed down. I have a watt meter, and it looks like when the current gets up to about 0.25 A, it cuts out down to the baseline current, and then once it goes down, can come back up, but only to 0.25 A. At first I thought something was wrong with the PAS, because when I hooked the blue (sensor) wire up to a voltmeter and turned the crank, it starts with 5V at rest and decreases with turning. But it doesn't decrease proportionally to the speed of the turning of the pedal. It goes down to 1A at the beginning of turning, then jumps up to 3 or 4, then if you turn fast, it bounces around 2.5-2.7A. So the signal from the PAS is more or less mirroring what I'm seeing with the turning of the motor. I also removed the crank to fiddle with the directionality of the PAS magnet wheel and sensor. When I flipped one or the other, it made the wheel run backwards, but it still would not draw more current.

But then I turned to the battery. I bought a step-down converter so that I could have a USB port on my battery to charge my phone. It is rated for up to 3A. I hooked it up, and it is drawing 0.16A (6.9W) and giving 0.48 A (2.5W) to the phone. Normally the phone draws at least twice that current.

In this picture, battery with BMS is to the left, phone being charged is to the right


So I suspect that the PAS is acting flaky because it can't get the current it wants from the battery.

I pointed my FLIR camera at the setup while it was charging the phone, and neither the BMS nor the cells were hot, although the step-down converter and watt meter were heating up. So the BMS (brand new) is not shutting off due to excess heat. It is allowing the voltage through, but not much current. It is rated for 15A. When I first hooked it up to the controller, I put a 5A fuse right after the watt-meter to make sure I wasn't going to accidentally fry the controller and/or motor hooking things up.

Battery packs at top, watt-meter and step-down converter bottom left, phone bottom right


I tried resetting the BMS by removing the strip of sensor wires for a few minutes with no change. Although I disconnected it from the main power before I did so. Does a reset require that the BMS have the larger wires connected?

Thanks in advance if anyone has any thoughts for how to troubleshoot further.

ETA: This is the BMS: https://cgi.ebay.com/ws/eBayISAPI.dll?ViewItemVersion&item=153160323513&view=all&tid=1899351684005

 

[flat tire]

Bypass the BMS to test your hypothesis. And get a torque sensing solution if you want to do throttle by pedal. It's the only way to go.

 

[dustNbone]

500mA is baseline USB charging current, so that just indicates the phone isn't negotiating a higher rate. That could be a limitation of the USB board (converter) or the USB cable, but it's probably not because the input current to it is being limited.

The only way for it to limit the current would be to lower the voltage when it approaches said limit, and you should be easily able to see that happening.

As flat tire said, bypassing the BMS by running the same tests but taking power directly from the battery terminals would rule that out.

Also a way to bypass the PAS for testing would be helpful.

 

[john61ct]

I do not understand why you are using USB charging of a phone in your testing setup.

Is that supposed to act as a tiny load dump or something?

Incandescent light bulbs or resistors with a decent ammeter,

depending on your test goals, even cheap adjustable electric loads would give a wider power range to test.

 

[john61ct]

Better link to the BMS

https://www.ebay.com/itm/Balanced-BMS-10S-36V-15A-25A-35A-45A-60A-Li-ion-Battery-PCM-for-Ebike-Escooter/153160323513

 

[J A G]

Thanks. Just removed BMS and attached voltage converter. The phone didn't pull any more than it did before, so I attached a newer phone, which is able to pull about 1 amp at 5v, which is 0.3A on the 36V side.

I then put the battery without BMS on the controller. Same problem. It won't pull more than .25A. So maybe it's the PAS after all.

 

[J A G]

I do not understand why you are using USB charging of a phone in your testing setup.

Is that supposed to act as a tiny load dump or something?

Incandescent light bulbs or resistors with a decent ammeter,

depending on your test goals, even cheap adjustable electric loads would give a wider power range to test.

It's what I had. I don't have resistors. I found an old light bulb and base and could try that. But I've already shown that the phone can draw more than the controller, which points to a problem on the PAS/controller side, rather than the battery.

 

[amberwolf]

Just a note:

FWIW, BMS do not limit current, that's what the controller does.

All a BMS can do is completely shut off the output if the current draw is higher than it's limit.

That limit is there to protect the cells, so the BMS should be matched to the cells' capability, and any limits it has should be left as they are.

 

[amberwolf]

I hooked the blue (sensor) wire up to a voltmeter and turned the crank, it starts with 5V at rest and decreases with turning. But it doesn't decrease proportionally to the speed of the turning of the pedal. It goes down to 1A at the beginning of turning, then jumps up to 3 or 4, then if you turn fast, it bounces around 2.5-2.7A.

Amp, or Volt?

The PAS sensor should not be drawing significant current, so if you do mean Amps, and it is drawing that much, something is wrong with it or it's wiring and that's likely the source of the problem.

If you mean volts, then it's normal. Most of these sensors are just outputting voltage pulses. The faster you go the less time the meter has to react to the pulse so it averages what it does see, and it comes out to about half the supply voltage. An oscilloscope would let you see what's actually there.


If the PAS sensor is hooked up to the throttle input instead of the PAS input, the system will be confused by the pulsing voltage and not operate correctly. So you might check to be sure the right plug is being used.

 

[kdog]

It's not the speed limit on the controller is it? wheel in the air wouldn't draw much, then cut out and slow down etc.

 

[J A G]

Amp, or Volt?

The PAS sensor should not be drawing significant current, so if you do mean Amps, and it is drawing that much, something is wrong with it or it's wiring and that's likely the source of the problem.

If you mean volts, then it's normal. Most of these sensors are just outputting voltage pulses. The faster you go the less time the meter has to react to the pulse so it averages what it does see, and it comes out to about half the supply voltage. An oscilloscope would let you see what's actually there.


If the PAS sensor is hooked up to the throttle input instead of the PAS input, the system will be confused by the pulsing voltage and not operate correctly. So you might check to be sure the right plug is being used.

Sorry, I meant Volt. The plug is correct, assuming the vehdor's wiring diagram is correct. Maybe I should try switching it.

 

[J A G]

It's not the speed limit on the controller is it? wheel in the air wouldn't draw much, then cut out and slow down etc.

I did mess with that and it didn't change anything noticibly.

 

[J A G]

So the throttle came yesterday. I hooked it up, and I'm getting no wheel movement with it at low or medium setting. If I give it full throttle, the wheel moves slowly. I counted and it's about 20 RPM. Current is about .15A. If I put my hand on the wheel, I can stop it, and current goes up to about 0.7A before it stops. Maybe the motor is bad?

 

[kdog]

Hmm I'd still be looking at something else in the controller settings/ signal/sensors/inputs as your culprit. Try every variable you can find in a methodical manner. I'd bypass the bms first just to rule it out. It's not the bms limiting current but I guess there could be a battery issue (such as a connection /unbalanced/sensor wire issue)
Keep posting , you'll get there eventually!

Edit fix auto correct!

 

[J A G]

Hmm I'd still be looking at something else in the controller settings/ signal/sensors/inputs as your culprit. Try every variable you can find in a methodical manner. I'd bypass the bms first just to rule it out. It's not the bms limiting current but I guess there could be a battery issue (such as a connection /unbalanced/sensor wire issue)
Keep posting , you'll get there eventually!

Edit fix auto correct!

Thanks. The BMS has been removed now. I've been going through the controller settings. A few are a but cryptic. But I'll keep trying.

 

[J A G]

More progress: In messing with the controller settings, I realized that the PAS level, which can be adjusted with +/- buttons on the display, controls both the throttle and the PAS. That is, when it is set to zero, neither work at all, and when it is set to max, both work as I would expect. The weird thing is that I was changing it before, when I couldn't get the PAS alone to work well. It appears that hooking up the throttle has made the PAS work better. Maybe the wires are crossed in the controller.

 

[J A G]

Another update: the controller has a setting to turn off either the throttle or PAS. When I do that without unplugging anything, the throttle works without the PAS on, although it is still tied to the PAS settings. But the PAS does not work correctly with the throttle turned off. It cuts out like it did before.

 

[kdog]

That the motor turns strongly suggests that motor is working. the controller seems to be functioning (but maybe not properly)
Some photos of the setup might help.