Low volts on one block of parallel cells. BMS or cell(s)?

Hi, I've just got back from my first ride on my first e-bike, which was curtailed because the BMS cut power from the battery after 2 or 3 miles. The battery is a 5P 10S I built using a Vruzend kit. Disconnecting and reconnecting power to the controller got me going again but the same thing happened several times. I'm hoping the expertise here will get me back in the saddle.
I measured the pack voltage as 39.4 and all the parallel cell groups, except one, at between 3.96 and 4.08. The exception was the last one, the negative terminal next to the BMS, which was just 3.34. So presumably that was why the BMS cut the power?
I disassembled the faulty parallel block to measure the cells individually and all 5 were 3.34. (I admit I was expecting the volts to vary, pointing to a bad cell or cells).
After putting everything back together I charged the battery, getting a pack voltage of 40, 4.03-4.18 for the other parallel cell groups and 3.41 for the faulty one once I noticed the green light was on the charger. After another hour the only difference is the two which were 4.18 are now 4.17.
So now I'm puzzled but hoping the answer is obvious to those with more knowledge.
Do I have one or more bad cells and the BMS isn't allowing the others in the group to go past that voltage. If so, is there a quick way of identifying the bad ones?
Or is it a bad BMS?

I would disconnect the BMS and try charging the one low group to get it up with the others. It sounds like a parasitic drain, but could be from the BMS or inside a bad cell. If you charge the low cells to 4.17 like the others and let them sit at least 24 hours, you can tell if there is an internal drain by measuring the voltage.

To charge a single group, you need a single cell charger or a variable power supply. Ghetto way is to use a 5v phone charger, resistor, and a voltmeter to watch the voltage. This method is dangerous as you need to be around when the voltage reaches the target to turn it off, otherwise there is a risk of fire.

If they hold voltage for a day, then try reconnecting the BMS and watch the voltage on that group. If it starts dropping, I would assume a bad BMS.

The most common BMS failure I've seen is one or more balancing shunts blows and gets stuck on, draining the cell attached to it. The drain is fairly low, so it may not be obvious right away.

If a cell gets drain below about 3v, it could be damaged by charging at full current.

fechter said:

To charge a single group, you need a single cell charger or a variable power supply. Ghetto way is to use a 5v phone charger, resistor, and a voltmeter to watch the voltage. This method is dangerous as you need to be around when the voltage reaches the target to turn it off, otherwise there is a risk of fire.

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Thanks for the reply. I'm fairly hopeless with electronics but figured the resistor is to reduce the 5v to 4.2v? But I didn't understand how it would not just reduce the current. So I found an adjustable step-down module cheap on ebay and have ordered that. I don't have a voltmeter as such but plan to adjust the output voltage to 4.2 using my multimeter, then use the multimeter to monitor the cell pack until it reaches 4.17. Hopefully that sounds like a plan?
Cheers

Best way to safely single cell charge is using a fairly inexpensive RC, 6 cell charger and power supply, like the imax b6 or similar.

Handy thing to have around, it will charge lead batteries like your car, as well as up to 6 cells at a time lithium, NiCad, etc.

You can set it to whatever you need, like 1 cell 4.2v, and not risk an overcharged cell group. Discharge a high group too, if that is your need.

I think your battery might have simply been not fully balanced on that first ride. Balance it somehow, and try again.

Another ghetto method to balance, is discharge one group at a time, till all are at the same, then put back on charge with the bms to top balance it. you can discharge a 4.2v group using a 12v car turn signal bulb.

smugmonster said:

Thanks for the reply. I'm fairly hopeless with electronics but figured the resistor is to reduce the 5v to 4.2v? But I didn't understand how it would not just reduce the current. So I found an adjustable step-down module cheap on ebay and have ordered that. I don't have a voltmeter as such but plan to adjust the output voltage to 4.2 using my multimeter, then use the multimeter to monitor the cell pack until it reaches 4.17. Hopefully that sounds like a plan?
Cheers

Click to expand...

I have a bunch of those little dc-dc stepdown boards and they work great for this. Do invest in a decent voltmeter. Accurate measurements are vital.
Yes, a simple resistor will only limit the current and not prevent the cell from getting over voltage. That's why you would need to constantly monitor it. With a stepdown converter, you can just set it for 4.2v and not worry.

Are you using a charger designed for lithium 18650? A 10S battery will have a full charge of 42V, and each of your 5P groups will be at 4.20 volts. The charger's LED should still be showing "charge mode" if you measured 40.2 volts.

The group that was at 3.40 volts was only 20-30% charged. DId you also get the cells and BMS from Vruzend? I would expect them to sell good quality components.

If you have non-brand name cells, maybe the 3.40V group and others are sagging under load and tripping the LVC in your motor controller.

I've little experience in the BMS, but note that some of mine have no balance circuits. If you were using one of mine, my expectation is that a 3.4 volt group could ever get up to max charge without serious overcharging the others already at 4.2 volts.

docw009 said:

Are you using a charger designed for lithium 18650? A 10S battery will have a full charge of 42V, and each of your 5P groups will be at 4.20 volts. The charger's LED should still be showing "charge mode" if you measured 40.2 volts.

Click to expand...

Yes, it's a basic 42V 2A li-ion charger.

docw009 said:

DId you also get the cells and BMS from Vruzend? I would expect them to sell good quality components.

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I got the Samsung cells and Vruzend kit from nkon.nl, who have a good reputation this side of the pond, and the BMS, which does have balance wires, from Amazon.

I plan to charge and monitor the low group as fechter suggests once the stepdown board arrives and will report back then. Thanks for everyone's input.

It was a faulty BMS. For some reason the ghetto charger didn't work (I adjusted the pot on the stepdown board to get 4v with no load but once connected to the cells it didn't seem to be charging) so I ordered an Imax B6. By the time that arrived a few days had elapsed and the low group were all down to 1.7!
I had some spare cells so I used the B6 to bring each parallel group to around 4.1 and fitted a new BMS. Since then I've managed a 35 mile round-trip with no issues and after three charges with the 42v pack charger all cell groups are within 0.07.
I wasn't sure what to do with the five cells left over that were down to 1.7 but by the time I got around to doing anything they had recovered to 2.7 all by themselves. So I used the B6 in storage mode to bring them back to 3.7 and they have all stuck around there.
Cheers

I know this is an old post. But I'm having s similar issue... sort of..

I have one parallel group running higher voltage than all of the others.

All groups are within 0.09v of each other, except the last 14th group, which is also the battery positive.
The difference kept increasing, as I was charging the battery. It looked like it was only the 14th group that was taking charge beyond a certain point.

It clocked in at almost 0,1v higher than the lowest group.

What could be the cause of the issue? Also a bad BMS?

I know with my oldest batteries, when I measure the IR of all the p-groups, then partially discharge, then stop - the p-groups that discharged the most were the healthy ones with the lowest resistance. So maybe you have a cell dying in the higher voltage p-group causing higher resistance. I wouldn't measure during discharge, though, since the dying cells might have more voltage sag.

Regardless, you'd usually balance the battery once by hooking a light up to the high p-group, getting them all equal, then see how things go. Many BMS can't balance. Others only balance at full charge by directing the highest cell to a resistor and only achieve like 50mA balance current. So not that weird that a cell that was ahead stayed ahead during charging. Didn't sound like they were all full voltage yet anyway, so a top balancing function in the BMS wouldn't have activated.