Bestech BMS 80A 10s cutting out board while accelerating/going up steep hills - single motor set up


I have just wired up my Bestech BMS 10s 80A. Board is running fine but when I accelerate from stop giving full throttle/accelerating up steep hill it cuts off whole board and I have to switch it off ( using built in BMS switch ) and switch it on again to resume riding. As it is 80A I’m surprised it cuts me off. I’m sure I’m not pulling 80A.

Here are my specs:

10S 8ah batteries in series ( Zippy Flightmax 30C - 40C Burst - all the same.) SK3 6374 192 KV motor ( 80A) FocBox 15mm belts

Cutting out happens while I have 38V or 42V ( fully charged) so it can’t be over voltage protection ( I have 57V set up in BLDC). Running BLDC mode. I have done motor detection. Do I have to redo it when unplugging phase wires?

Since I have two same BMSs( minimum order two) I have tried both thinking one can be faulty. Unfortunately it is the same behaviour on both.

I’m sure it has nothing to do with FocBox. I can’t even read any fault codes as BMS cuts off power and faults are erased. I did check on bench motor with live sampling and with no load I’m pulling 3-5A.

All connections ( phase wires, battery wires, etc) are shrink wrapped and taped with electrical tape to avoid any possible shorts.

Using @Ackmaniac software - moderate settings- here are screenshots.

Bestech specs

Any idea on what can cause the cut off? Fuse on BMS is rated for 90 Celsius rather than Amperage but don’t quote me on that.

@Namasaki have you had similar problems?

I’m sorry I can’t shed any light on your particular problem, although it sounds very similar to my issue I’m having occasionally…

When I’m full throttle going up some kind of incline, my power will suddenly cut, throwing me forwards if I don’t catch it in time…

I believe the VESC is cutting out though as when it happens I have no control, then a few seconds later I have control again so I think it’s the VESC re-booting.

This also happens full brake going down hill sometimes… which is a little unnerving! :confused:

I completely loose all the power and have to reset switch and it all works fine afterwards. Maybe if I will wire BMS with bypass on discharge and test it again and it then cuts me out I will know it is Vesc.

Actually before I had BMS wired up I was riding on same setup, same settings, same hills and never had any problems, no cut offs - it must be BMS since it all started happening as soon as I wired it in. Strage :confused:

I have not had this issue even when accelerating up steep hills. I believe the problem your having is due to abnormal voltage sag in one of your battery cells. If any cell drops to 3,0v the bms will shut down. One cell going too low likely won’t trip the Vesc protection. I suggest not bypassing the bms. I suggest testing your batteries for internal resistance. I suspect at least one of your 10 cells will show high internal resistance. Replace the pack with the bad cell and your problem should be solved. One time I ordered 4 Zippy batteries and tested them and found 2 packs with bad cells right out of the box.

Up to 5 milliohms internal resistance is acceptable If any cell is showing 10+ milliohms I would replace that pack.

1 Like

Thanks @Namasaki - what resistance should make me worry ? I’ll connect them to balance charger and check all the values now.

I just added that info with edit

@Namasaki Thanks! I just took pictures and here they are. I know they are not balanced - I have tried with Bestech BMS but my charger just stopped feeding any current at 42V so for the moment I will be balancing them on Turingy balance charger separately.

Everything is wired as per your diagram using 12 AWG wires to BMS and VESC ( apart of charge port - 22 AWG wires)

Internal resistance although poorly matched is within acceptable limits. Still, voltage sag is the only suspect that I can think of. Do you have some way of monitoring voltage while riding?

@Namasaki Yeah, I have voltage meter attached. What should I look for? Voltage drops a little depending on how I accelerate but only around 0.1 0.2 of voltage before acceleration. It also rises then breaking - regen.

I’m on the board now and up a steep hill, slow acceleration I had a drop of voltage from 41.7V to 4.12V. I think that’s normal right ? If it was much lower than this then I would be worried …

Do the full throttle thing that makes the bms shut off and see what the voltage drop is. I think it’s likely that cells with 5 milliohms are sagging more than the rest.

So you have 2 3s/8000/30c packs and 2 2s/8000/30c packs all in series?

1/2 volt drop from total pack voltage is pretty mild sag.

Yes, to make 10s. 5s packs were to thick for my enclosure.

Funny enough I can’t replicate this cut off now. I will try later on when board “rests” for a bit :slight_smile: From what I have seen before, voltage would drop half a volt and then BMS would cut off. It did not sag more than this.

How can I reduce sag? 12S? Add one more battery?

Maybe sag is not the issue here. Maybe you have a loose connection or faulty solder joint that causes intermittent lose of connection and throws the bms into protection mode. This happened to me once when the last pack in my setup developed an interment disconnect where the main positive wire connects to the solder tab inside the pack. I was cruising along and the bms shut down. When I got home a took a closer look I found that if I moved the wire around it would connect and disconnect randomly.

I think it’s because that your cells gets out of ballance to each other, because you are using two different batteries. With most likely different C ratings.

Well I don’t think having different cells matters. All are the same brand, same C rating and capacity. Only difference is S count. Also I’m positive it does not make them discharge unevenly. Even when you have same batteries in series they will discharge/charge unevenly to a degree.

1 Like

I’ll check wiring. Maybe thats the cause.

1 Like

But something is triggering the BMS

@Namasaki nailed it!

My board is now doing the same exact thing, with Zippy Compact 40C 6.2Ah batteries. My pack is perfectly balanced at resting voltage, but under load, some of the cells rapidly drop to unsafe levels triggering the BMS cutoff. I don’t have my log book right now, but now 3 of the cells will drop from 3.85V resting to 3.2V with just 1A direct loading; while the others might only drop to 3.7V under the same exact loading. So under higher load, I am definitely hitting the 2.9V BMS cutoff with at least one cell

Took me some time to diagnose the problem; however, I built a device to measure individual cell voltage with a constant load. It has a calibrated voltage meter and can select 0A, 1A, or 2A loading. What I learned from all this is even if your pack is “balanced” while resting, it could still be very unbalanced under load

TLDR - Don’t bypass your BMS. Especially with lipos! I would have been unknowingly wrecking havoc on my batteries