E-Bike periodically shuts down when going faster

[ewokg]

I have an e-bike that's battery is 52v. I haven't had this problem for years but the issue is starting to occur now. My e-bike will automatically turn off sometimes when I ride at greater speeds. If I take out the battery and plug it back into the bike, it'll last for a good 4 minutes before turning off again and eventually not working at all. I figured that it was a BMS issue from my research but not sure if the battery cells are fried because if they are I might have to get a new battery which would cost me a hefty 500 USD. If it is a BMS issue, what can I do to resolve it? Thanks.

 

[JackFlorey]

If it is a BMS issue, what can I do to resolve it? Thanks.

If you have access to the cells themselves (by taking off the cover or something) check each bank of cells. If one is dramatically different - like all of them are 3.6V but one is 2.7V - then that's your problem. The pack is unbalanced and is not recovering. To fix this temporarily, charge that cell to the same voltage all the others are charged to with an external power supply.

If they are all OK check the current shunt for the current sensor. If it's damaged it can read high, which will cause an overcurrent fault at lower currents.

 

[ewokg]

If you have access to the cells themselves (by taking off the cover or something) check each bank of cells. If one is dramatically different - like all of them are 3.6V but one is 2.7V - then that's your problem. The pack is unbalanced and is not recovering. To fix this temporarily, charge that cell to the same voltage all the others are charged to with an external power supply.

If they are all OK check the current shunt for the current sensor. If it's damaged it can read high, which will cause an overcurrent fault at lower currents

So if I do find out that one of the cells has a discrepancy in the volts that it produces, how would I exactly charge a particular cell to the same voltage as the others? Is this a temporary solution because that discrepancy will eventually arise again?

 

[JackFlorey]

So if I do find out that one of the cells has a discrepancy in the volts that it produces, how would I exactly charge a particular cell to the same voltage as the others? Is this a temporary solution because that discrepancy will eventually arise again?

You will have to place your meter probes on either side of the cell to test it. The charge leads from the bench supply will go to the same points. Use grabbers or alligator clips so you can let it charge overnight.

Something unbalanced it in the first place (bad cell? BMS leakage?) so it will likely happen again. But I've gotten another 6 months of life out of a li-ion battery by doing this.

 

[ewokg]

You will have to place your meter probes on either side of the cell to test it. The charge leads from the bench supply will go to the same points. Use grabbers or alligator clips so you can let it charge overnight.

Something unbalanced it in the first place (bad cell? BMS leakage?) so it will likely happen again. But I've gotten another 6 months of life out of a li-ion battery by doing this.

So I would charge specifically that one cell overnight until it reaches its max battery capacity like the other cells. To determine if the cell even has a discrepancy in voltage I'll need a multimeter to read the individual outputs from each cell and determine which one, if there is one, is damaged. Also, after those 6 months, did you just charge the cell back up again?

 

[amberwolf]

I figured that it was a BMS issue from my research but not sure if the battery cells are fried because if they are I might have to get a new battery which would cost me a hefty 500 USD. If it is a BMS issue, what can I do to resolve it? Thanks.

It's almost certainly not a BMS issue. The BMS is probably just turning off to prevent damage to cells that are dropping in voltage below it's LVC.

A years-old battery probably has worn-out cells that just can't handle what the bike needs to run now, so they drop under load so far that the BMS does it's job and protects them from damage that could lead to a fire.

In most cases, replacing cells in an old battery isn't worth it, unless you are replacing all of them at the same time and already have all the equipment and know-how to do this (it usually costs as much or more to build one as to buy a good one).



Most BMS issues are sudden complete failure at high load or during connect of battery to system, from an overcurrent that blows up the FETs. Sometimes it's from overheating of an undersized BMS inside a heat-retaining package, or trying to parallel or series battery packs in ways that cause overvoltage or overcurrent situations in the FETs.

If a system recovers it's almost never a BMS failure.

Occasionally a BMS will have a stuck-on balancer that continuoulsy drains one group of cells. If there is a voltage across the shunt for any balancer, and that group is not over HVC / balancer start voltage, then that balancer is probalby stuck on. But it's fairly rare.

 

[amberwolf]

So I would charge specifically that one cell overnight until it reaches its max battery capacity like the other cells.

Well, it wont' be at same capacity as all the other cells, just the same voltage. You can't fix low capacity except by replacing the cell(s).

But the easiest way to fix an imbalance, if your battery has a balancing BMS (some do, some don't) is to just leave it on the charger until the charger stops turning on and off. That can take hours, days, or longer depending on the difference between the problem cell(s) and the rest of the pack.

The fastest way to fix it is to use a single cell charger to manually recharge just that cell or group. If you are using a charger made for single cells charging then you might be able to leave it unattneded, but if it's antying else, don't leave it, stay there and watch it the whole time so you don't accidentally overcharge the cell and cause damage that can lead to a fire (or have one immediately).

Also, after those 6 months, did you just charge the cell back up again?

If you have a problem with a pakc like this, then as ong as it has a balancing bms you simply always leave it on the charger till the charger stops cycling on and off, which means balancing is done and all the clels are the same voltage.

Balancing doesn't "fix" anything, it just lets you keep using a failing battery's remaining capacity more completely until you can do something else.

 

[JackFlorey]

So I would charge specifically that one cell overnight until it reaches its max battery capacity like the other cells.

Well, you would charge it until it matches the VOLTAGE of the other cells. If that is 3.6 volts, then set your supply to 3.6 volts and let it sit overnight. You are basically manually balancing your pack, to replace the balancer in the pack that may be broken or not strong enough to balance it.

Note that if the cell group itself is bad this won't do much; it will just temporarily balance the pack, and the cell will just droop again once you put it under a heavy load.

Also, after those 6 months, did you just charge the cell back up again?

No, it just gave me time to look for a deal on a new battery.