Hard Shutdown letting off throttle or applying brake

Hello,

I've got one of those 1500w Voilamart kits, a home made 58.8V 23AH battery with BMS. I've ridden this bike around 1K miles and recently started exhibiting this odd behavior. I'm getting a complete system shutdown (have to disconnect the battery from the BMS) in order to get power back.

If I am going full speed(~35mph) and let off the throttle where it slaps back to 0, the system dies. If I am going full speed and hit the brake lever, triggering the motor cut sensor, the system goes offline again.

I've hit voltage sag shutdown before and I do not believe this is it. If I do half speed mode, (~20mph) I do not get this symptom. After my ride, I was registering 54V which is about normal.

There is something about cutting power quickly that is making the system flip out. Any ideas?

Check the main (larger) capacitors in the controller. If they are failing, then voltage spikes from motor voltage collapse wouldn't be smoothed and could cause controller shutdown, or even BMS shutdown.


Usually you'll see bulging or swelling when they're beginning to fail, and sometimes they'll actually explode leaving confetti and metal everywhere.

Sometimes there is no visible sign but they can still be bad.

Does your controller have regen? Regen could be causing the voltage to go too high during braking. If there is no regen, I can't think of a reason it would cut off when you let off the throttle. There may be a way to disable regen in the programming but I am not familiar with that particular model.

It sounds like it is the bms that turns off, because you need to disconnect the battery to get it working again.
You could try to trigger the problem close to home, and check if you get an output from the bms (with a load, measure with the controller still connected and give throttle for example)
Check your cell group voltages on the battery, maybe you have a battery problem that makes the bms turn off at regen.

amberwolf said:

Check the main (larger) capacitors in the controller. If they are failing, then voltage spikes from motor voltage collapse wouldn't be smoothed and could cause controller shutdown, or even BMS shutdown.


Usually you'll see bulging or swelling when they're beginning to fail, and sometimes they'll actually explode leaving confetti and metal everywhere.

Sometimes there is no visible sign but they can still be bad.

Click to expand...

I'll crack it open soon. Honestly, I'd love to buy a new controller because I want regen. I'm blowing through brake pads. Unfortunately, I think it's the battery. I'll post below.

fechter said:

Does your controller have regen? Regen could be causing the voltage to go too high during braking. If there is no regen, I can't think of a reason it would cut off when you let off the throttle. There may be a way to disable regen in the programming but I am not familiar with that particular model.

Click to expand...

No regen, I'd love to have that someday.

j bjork said:

It sounds like it is the bms that turns off, because you need to disconnect the battery to get it working again.
You could try to trigger the problem close to home, and check if you get an output from the bms (with a load, measure with the controller still connected and give throttle for example)
Check your cell group voltages on the battery, maybe you have a battery problem that makes the bms turn off at regen.

Click to expand...

I did some high speed runs with the battery off the ground and not supporting my weight. I don't think it drew enough current. I took my battery apart and checked the groups at the BMS lead connector:

Grouping Series Voltage Parallel Voltage
1 3.9 3.9
2 7.8 3.9
3 11.7 3.9
4 15.7 4
5 19.7 4
6 23.8 4.1
7 27.9 4.1
8 32 4.1
9 36 4
10 40.2 4.2
11 44.3 4.1
12 48.5 4.2
13 52.6 4.1
14 56.6 4

I've got a luna charger that's supposed to go to 58.8V but the BMS is cutting around 56.6 which makes the luna charger light go green, as if it's done so it cut.

I don't think my battery is balancing. For Now, I took the leads off as that is supposed to let the BMS reset. Hopefully that fixes the issue.

You may need to leave it plugged in longer, maybe overnight, for it to balance. The highest vs lowest cells are pretty far out of whack.

As far as "taking the leads off" goes, if you mean the balance leads, you don't want to use or charge the pack that way, as you can overdischarge or overcharge the cells, and damage them, and create a fire risk.

A balance problem is not likely to be caused by the BMS needing to be reset, it's caused by cell or construction problems in the pack itself. See below.

thetailwind said:

I've got a luna charger that's supposed to go to 58.8V but the BMS is cutting around 56.6 which makes the luna charger light go green, as if it's done so it cut.

Click to expand...

That's because the BMS shuts off when that highest cell group reaches 4.2v. Then the others don't get any more charge (and so are not filled up) until that group gets drained down to the point the BMS can allow charging again, which takes a while (minutes to hours). Then the BMS will restart charging, if you've left the charger plugged in.

This cycle repeats until all cells are equal voltage (which doesn't mean equal condition or capacity), and for a large imbalance can take days to weeks to correct because the BMS is only meant to handle a very small imbalance. Yours is pretty far unbalanced, but not as bad as some, so probably a minimum of days.

This kind of imbalance happens because it's rare for battery makers to match the cells to each other before building a pack. Some of them use new cells taht are already close, but if they're not identical, then different groups will have different internal resistance and capacity, so each time the pack is discharged and then charged, their voltage levels change relative to each other, the weaker cells getting lower, and the better cells getting higher, until you get what you see.

If everyone would leave their packs on the charger until it was balanced each time (when the charger stops turning on and off), this would be a much less apparent problem until the pack ages enough to become a problem on every ride, which is sooner for cheaper packs.

It's even worse with cheap packs because they may be made of very mismatched cells, even different models/brands, and often enough are just literally recycled garbage cells. Those get like this even on first use.


The problem you're experiencing is not the LVC issue most people have with unbalanced packs, because that would happen when you *apply* hard throttle. In your case it's the HVC (almost certainly), which if it's happening at any time other than when you have a full battery already, means you must have a pretty high resistance cell group, if it's caused by the battery and not the controller caps. That is usually going to be the lowest voltage group under load--whatever sags the most in voltage from it's resting state. It might have bad cells in it, or it might have a broken interconnect between some of the cells in the group, so it is "smaller" than all the other groups, less capacity, higher resistance.


But I'd still check those controller caps.

If the BMS was cutting off I'd expect that to happen when the current is the highest, like full throttle from a dead stop, not when applying the brakes or letting off the throttle. That's just bizarre.

If the BMS cuts off, the controller display would go dark.

how did you build the battery? solder it?

goatman said:

how did you build the battery? solder it?

Click to expand...

I have a battery welder. welded like the 6 side of a dice (die?).

fechter said:

If the BMS was cutting off I'd expect that to happen when the current is the highest, like full throttle from a dead stop, not when applying the brakes or letting off the throttle. That's just bizarre.

If the BMS cuts off, the controller display would go dark.

Click to expand...

Agreed. When it cuts, the display goes dark.

I believe I have arrived at a conclusion. Hopefully this will help someone if they stumble across

I have one of those cheap chinese BMS, less than 20 bucks.

I have not ridden my bike regularly and I believe one of my parallel groups(PG) had fallen out of balance. I noticed PG 1-3 was at 3.9V and there was a few other PG which were 4.2. It only charged the battery to 56.6v

Notice, this is observation, research and opinion ahead not fact, as I am not an electrical engineer:


The cheap BMS's logic is to stop allowing a charge when ANY of the PG reach 4.2v then take the voltage from the 4.2v PG to distribute to the lower PGs.

In my scenario PG 7 hit 4.2 while PG 1-3 were at 3.9 when charging stopped. I left the charger on overnight even though it was green (not charging). While I slept, I the PG who was at 4.2 redistributed charge to the lower PGs. When I woke up, it was at 58.4, much higher than the 56.6 earlier. My theory is, when the balancing was occurring, the BMS left the "stop charging when any PG is 4.2" and resumed charging per normal.

I took the bike out this morning and went a hard 6 miles and the symptom did not return. I'm still puzzled by the let-off condition being what freaked the bms out rather than the high load. I'm going to do a few more %100 charges then quick ride to get it back into storage voltage ~%80 charge. I'm hoping to see a full 58.8v charge.