Battery Circuit board and how to replace new cells or fuse

[Omegalpha]

I accidentally used a 48V charger on a 36V battery and I think the battery is dead as it reads only 29V and charger stays green when plugged in and doesn’t turn red meaning the battery cannot be charged. Is there a fuse? Are the cells dead? Or can the battery work if I replace the circuit board, which I think is a BMS? Would it be easier or cheaper to just buy a new battery? I know very little about bike batteries and would appreciate some help and info. This is a Sailnovo 36 V bike battery.

 

[E-HP]

I accidentally used a 48V charger on a 36V battery and I think the battery is dead as it reads only 29V and charger stays green when plugged in and doesn’t turn red meaning the battery cannot be charged. Is there a fuse? Are the cells dead? Or can the battery work if I replace the circuit board, which I think is a BMS? Would it be easier or cheaper to just buy a new battery?

How are you measuring the voltage and from which points? (e.g. multimeter? pack output connector?)
Fuses blow due to current, not voltage, so more likely you fried the BMS or at least tripped it. The cells aren't dead, if you are measuring 29V, but there still could be damage. It may work if you replace the BMS, but you should test the individual cell group voltages first to see where you're at.
The harness with the 10 white conductors and 1 black represent each of the 10 groups, so by measuring the voltages between each adjacent pair of conductors (from the battery side of the connector), you can write down the voltages and see if there's an issue.
Lastly, it's not about easier or cheaper, it's about safer or not safe. Buying a new good quality battery will always be safer.

 

[amberwolf]

The most likely thing is that (some of) the cells overcharged, and so the BMS disconnected the charge port (and probably the discharge port too, if it's different) until it's balance circuit (if it has one) can drain down the overcharged cells. That could take hours.

The voltage you see on the output will be lower than ti should be because there is still a "leakage" thru the BMS when ti's off, which makes a "ghost voltage" on the output. If you connect a load to it it'll drop to zero pretty quick.

The BMS is probably just doing the job it is supposed to, protecting you from charging a battery that could be damaged from overcharge, which could cause a fire.

Bypassing it isn't safe, and replacing it (if the cells are still overcharged) still won't work because the new BMS will disable itself too.


If your BMS doesn't have balancers, it'll never drain the overhcarged cells, and never recover; you'd have to manually drain them down to be within whatever the BMS limits are.

 

[Omegalpha]

The 29 V is when I put the probes of the multimeter at the XT60 plug, which I think is the battery output to the bike. I tested the what appears to be 10 groups of cells in order from nearest to the BMS I got these voltages: 3.6, 7.67, 6.9, 10.86, 14.84 Volts. On the other side 29.18, 25.6, 21.5, 22.26, 18.4 volts. I put the red probe at the XT60 plug and another probe at a white wire solder area at the cells.

Is this normal to have ascending values of voltage in these groups?

Each cell says 3.7V 9.62 Wh. I think there are 4 cells per group, 10 groups, so 40 cells total. What is a normal reading?

 

[E-HP]

The voltage of each group should be the voltage difference between two adjacent readings, so the voltages should be ascending. Tha 6.9v and 21.5v readings looks odd. Can you remeasure? The other groups look to be within their normal ranges, but unbalanced.

 

[docw009]

Can you remove the white 11 pin plug. If it's not glued in, do so. It's easier to measure it like this. Black is the negative end of the cells, and the voltage will increase as you approach the end, I like to do it this way, because I don't have to strip the whole battery apart to access the cell terminals,.

You can either measure the increasing voltages and subtract the difference between adjacent pairs or reposition the pins and measure them directly.

You can determine the health of the cells by their voltages, They should be between 3 and 4.2Volt and ideally will all be he same. The BMS is set to (a) shut off if any exceed max or min lebvels, which is about 4.3V and 2.9V. It will inhibit charge if any are below 2.9V. The BMS are set to run the cells between 3.0V and 4.2V,



Right, with 40 cells, that would be 10 groups of 4 cells each,

By the way, a working BMS will allow a 48V charger to be connected to the charging port of a 36V battery. My opinion anyayw. The new UL standard for batteries requires that they withstand 20% over voltage for some time when charging. Let's see what the healt of the cells looks like before blaming the BMS,

,

 

[Omegalpha]

I remeasured it in order at the 11 pin plug. The 21.5 and 6.93 are still a lower the value than the next one. I was not able to pull it out but the underside has pins exposed for me to test.

29.14 25.5 21.5 22.23 18.3 14.34 10.73 6.93 7.68 3.53

 

[amberwolf]

Either those cells with lower values are actually negative, or the BMS is wired wrong to the cells, or the BMS connector is not like most and is not wired sequentially.

In the first case, the cells are damaged and could cause a fire if recharged (the BMS could also be damaged by the reversed voltage across the cells). This kind of thing can happen if the cells are badly mismatched (aging, defects, etc) and those get far overdischarged, and the BMS doesn't detect the fault and shutdown in time.

Wrong wiring means the pack would never work, as the BMS would keep the output shutdown from the fault condition detected (if it's correctly designed and/or not damaged in a way that prevents it from detecting it).

Not being wired sequentially is unlikely, but possible.

I recommend measuring from the + and - of the actual cells themselves, making sure you use red meter lead on + and black on -, and writing down the results on the cell cans with a marker.

Pay close attention to the meter display; if it shows a - in front of hte number, that's a negative voltage. If you have the meter leads on there correctly it means the cell has reversed and is damaged, and it's safer to replace the pack at that point.

 

[Omegalpha]

Did a voltage calculation by subtracting.
3.64 4.0 -0.8 3.9 3.96 3.61 3.8 -0.75 4.15

Seems like some of the cells have failed and have reversed polarity due to using the wrong charger designed for a higher voltage battery. I am not sure but non-sequential wiring seems unlikely. I will probably proceed with checking the individual cells. I see on Amazon or 18650batterystore there is a battery charger and tester. Would that give me information on cell health and what tools are there to test cell health?

How can I know if a BMS is working or not? I have tried to use the correct charger but the light stays green, so does it mean that BMS is working because it has a feature preventing charging if it is lower than 29V ? If I replace the damaged cells with new ones, do you think the battery pack can be saved or will it have balance issues.

I see for my readings besides the negative ones I have the lowest as 3.61V and highest at 4.15V, which I assume is not normal for a balanced battery. What is a normal difference between voltage levels to be considered as balanced vs unbalanced?

Thanks for all your help and information.

 

[E-HP]

If those negative readings are accurate, then the pack is toast. I wouldn’t mess with it and figure out how to dispose of it.

 

[docw009]

I'll take those values as real then. I've never seen cells reverse polarity, Those cells are toast and unsafe to recharge. I don't know how you will dispose of the battery, If a trash truck were to compact the battery, the charged cells would probably ignite,

 

[amberwolf]

Seems like some of the cells have failed and have reversed polarity due to using the wrong charger designed for a higher voltage battery.

They wouldn't reverse because of the charger. It happens because they are lower capacity than the rest, so were already low or empty before riding, then riding drove them below the BMS protect level and then below zero. Why the BMS didn't protect them...could be failed, could be poor design, etc.

But make sure they really are that low first....

I am not sure but non-sequential wiring seems unlikely. I will probably proceed with checking the individual cells. I see on Amazon or 18650batterystore there is a battery charger and tester. Would that give me information on cell health and what tools are there to test cell health?

If they actually went negative, don't bother--they wouldnt' be safe to recover. To go negative, they'd have to have been really low charge-capacity compared to the ones that haven't failed yet...and it's possible others are headed that way.

Repairing it would require getting all the tools to do the interconnects, etc., learning how to do them, getting matching cells for the type that's in there that you know are good, doing all the work to fix it, and that's all going to cost probably as much as a new pack...and it will only be as good as whatever the worst cells still in there are, so if any of *them* are close to aging out, etc., then you have to do taht work again for those, and so on.

To use most of the cell testers you'd have to disassemble the pack pretty completely, and then you need all the tools, parts, and knowhow to reassembled it, essentially building a whole new pack.

How can I know if a BMS is working or not? I have tried to use the correct charger but the light stays green, so does it mean that BMS is working because it has a feature preventing charging if it is lower than 29V ? If I replace the damaged cells with new ones, do you think the battery pack can be saved or will it have balance issues.

Since any new cells you use won't be identical in properties to the other cells, it will always require balancing (even if all the other cells are identical, which they almost certainly aren't). If the BMS has a balancer, then you just leave it on the charger long enough to rebalnce at the end of every charge (until the charger stops turning on and off, usually a few hours or more).

If the BMS doesn't have a balancer, then you'd have to manually balance it.

But it'd probably a good idea to replace the BMS anyway, if it let cells drop below LVC and actually go negative.

What is a normal difference between voltage levels to be considered as balanced vs unbalanced?

Balanced is zero difference between cells (0.0000000000v difference)
Anything else is unbalanced.

Some BMS will even prevent pack use or charge if the difference is greater than 0.1v. (some are programmable)

AC

 

[eMark]

Balanced is zero difference between cells (0.0000000000v difference)
Anything else is unbalanced.

(notice the wink) thus ...

Balancing your 10s groups within 0.005V of each other is close enuf to be considered balanced assuming you have a DMM that reads 3 decimal places ($30-$40)

Rechargeable TRMS 9999 Count multimeter,Measures AC/DC Voltmeter,Current,Resistance,Capacitance,NCV,Diode,Voltage Tester，Automatic DMM
$26.99 via Amazon.

ANENG Digital Multimeter Tester Smart Touch TRMS Auto-Ranging 9999Counts Rechargeable anti-burning Ohm Amp VoltMeter With NCV,DC/AC Current,Voltage,Resistance Diodes,Continuity,Capacitance,Temperature
$35.00 via Amazon

Actually balancing 10s cell groups within 0.100v (100mv) is generally considered the borderline of safe with an older battery with no name name cells and BMS with both still in usable condition.