Generic EUC battery problem

[DebboR]

Hello!

Recently, I've bought a second-hand generic EUC. It runs nice, but it has a problem with the battery. When charging, the battery only charges continuously for about 5-10 minutes before starting to pulse on and off. It fully stops charging at about 20-30 minutes.

When I turn it on, the battery indicator only shows 3 out of 4 bars, and after driving for about 20 meters, it goes back to 2 bars. The full range is around 800m - 1km before it starts beeping and is completely empty.

Being somewhat of an electronics hobbyist, I opened the battery pack and measured the cell voltages after a complete charge and about 100-200m of driving (showing solid 2 bars, and dipping to 1 bar when accelerating). These are the values I measured:

Cell 1: 3.715V
Cell 2: 3.644V
Cell 3: 3.797V
Cell 4: 3.889V
Cell 5: 3.845V
Cell 6: 3.751V
Cell 7: 4.080V
Cell 8: 3.961V
Cell 9: 3.760V
Cell 10: 3.707V
Cell 11: 3.608V
Cell 12: 3.694V
Cell 13: 3.762V
Cell 14: 3.584V
Cell 15: 3.823V
Cell 16: 3.801V

These values seem to me to not be good at all, with a maximum difference in voltages between cells of half a volt!

I'm not sure if this is because of a defective BMS, or is there one/multiple faulty cells? I did order some replacement cells with the same mAh-rating, but I'm still waiting for them to arrive.
I can most definitely rule out the charger, as I have two, and they don't seem to produce different results.

A picture of the BMS:

What do you think? Maybe I should drain/charge the battery completely and see how the voltages change?

Thanks!

 

 

[Tom]

It could easily be the charger, they are apparently quite prone to breaking.

[DebboR]

I don't think it's the charger, as both my chargers give the same result, and the cell voltages are so different. I suspect the cell with the highest voltage is the culprit. Would it be possible that when this cell reaches it's maximum voltage, the BMS just cuts the power from the charger?

Also, if anyone has a schematic/good explanation of what the BMS is doing exactly, and how it balances, that would be very welcome. I can't seem to find much on the internet.

EDIT: I just checked to make sure, the charger gives a nice 67.7V with no load.

[DebboR]

A little update:

While charging, it seems like cell 7 is the one responsible for the BMS/charger shutting down. It fluctuates between 4.19V (while the BMS has shut down) and 4.37V (while charging).

Would it be safe for me to say that this cell is bad? It went from the 4.08V from before to the 4.19V in about a minute on the charger.

[Volker Behrends]

It is likely the bad cell. You could try discharging the 4V cell to match the others. But I guess it will only increase range slighty after charging again..

[esaj]

A little update:

While charging, it seems like cell 7 is the one responsible for the BMS/charger shutting down. It fluctuates between 4.19V (while the BMS has shut down) and 4.37V (while charging).

Would it be safe for me to say that this cell is bad? It went from the 4.08V from before to the 4.19V in about a minute on the charger.

Sounds like that one cell could maybe trigger the overcharge protection once it hits "too high" voltage. Does it also go to lower voltage faster than the others when discharging? That would probably hint that the capacity of that one cell is much lower than the others (charges up faster, discharges faster).

[DebboR]

After riding it completely empty, I checked the cells again:

Cell 1: 3.790V
Cell 2: 3.612V
Cell 3: 3.698V
Cell 4: 3.785V
Cell 5: 3.732V
Cell 6: 3.666V
Cell 7: 3.960V
Cell 8: 3.849V
Cell 9: 3.672V
Cell 10: 3.646V
Cell 11: 3.545V
Cell 12: 3.643V
Cell 13: 3.672V
Cell 14: 3.482V
Cell 15: 3.716V
Cell 16: 3.697V

The 7th cell still read high, just like before, so I think this is a balancing problem? I discharged the cell partially, and started charging again. As expected, the charging now took way longer, and now most cells check out at around 4.17V. Now, I'm charging the other cells separately by hand using my lab power supply (yes, a decent Agilent one, set to 4.2V CV and 1.5A CC) until all cells are mostly balanced.

I suppose this will make the battery last a lot longer, as more cells are charged correctly to their maximum capacity?

It's also weird I think that the BMS doesn't take care of the balancing. Why is that? Is it defective, or are they just designed to protect each cell from over/under voltage without actually balancing the charging process?

 

[esaj]

Hard to say whether it's a problem with balancing or just the BMS not "working correctly" in general (ie. does it overcharge that one cell itself or is there something weird with the cell). I don't know enough about electronics to tell just by looking at that BMS-picture, but it would seem there's at least some sort of balancing-related circuitry, with all those surface-mounted resistors and transistors there, and the tabs soldered to the BMS from all the cells. Maybe it's just defective balancing/charging circuitry.

Also, summing those voltages together, the battery pack is still at 59,165V (if I calculated correctly ), which is at the top-end of the nominal voltage (depending on cell-chemistry, typically 3.6-3.7V, 3.6V * 16 = 57.6V, 3.7V * 16 = 59.2V), most wheels let you ride the 16S-packs down to something like 54V (around 3.375V per cell) or even below (Gotways go to 50V or 52V? Newer King Songs also?). Of course the lower the voltage is, the more danger there is of undesired overdischarge/undervoltage cut-off during acceleration or hill climbing. And those wheels also usually have more than one pack of batteries (2-4 packs), so the voltage sag isn't as deep as with a single pack.

Edit: If it's comparable, my 14" generic with 174Wh pack (2900mAh cells) could be ridden around 10-12km on one charge, maybe somewhat more, as I usually stopped riding it once it started dipping to 1 light, and the reaction of the wheel started to feel more "sluggish".

[DebboR]

...

Also, summing those voltages together, the battery pack is still at 59,165V (if I calculated correctly ), which is at the top-end of the nominal voltage (depending on cell-chemistry, typically 3.6-3.7V, 3.6V * 16 = 57.6V, 3.7V * 16 = 59.2V), most wheels let you ride the 16S-packs down to something like 54V (around 3.375V per cell) or even below (Gotways go to 50V or 52V? Newer King Songs also?). Of course the lower the voltage is, the more danger there is of undesired overdischarge/undervoltage cut-off during acceleration or hill climbing. And those wheels also usually have more than one pack of batteries (2-4 packs), so the voltage sag isn't as deep as with a single pack.

Edit: If it's comparable, my 14" generic with 174Wh pack (2900mAh cells) could be ridden around 10-12km on one charge, maybe somewhat more, as I usually stopped riding it once it started dipping to 1 light, and the reaction of the wheel started to feel more "sluggish".

My wheel gets nowhere near the 3.375V per cell when discharged. The voltages I posted were measured after I ran the wheel until it started beeping rapidly, indicating that it was completely empty. This was also not even close to 8km (which I should be able to get with my 2200mAh cells), more like 1-2km maximum.

Maybe my mainboard could also be reading the pack voltage wrong? I am atm also ordering a 500W motor with 30km/h controller and bluetooth from microworks, which should eliminate this potential problem.

I'm still bringing individual cells up to 4.2V, I'll keep this thread updated on any updates :-)

[jayjay23]

I have the same BMS than on your above photo and I used the wheel a little bit (about 1km) from fully charged, the battery pack was laying around for about 1 day and for curiosity I measured all the cell voltages and they are all(!) at 4,05V, there is not even one that is 10mV off, so maybe this is a indication that balancing is somehow not working.

Btw, one funny difference is that my C+ cable is connected to the P+ plate, making the row of diodes basically useless on mine (should not have any other impacts I guess).

The revision of by BMS PCB is exactly the same, but my batteries are not soldered they are also spot welded (there is some additional small plate between the PCB solder area and the metal stripe that comes from the batteries), anyway this should not make a difference to your problem but I find it interesting that the same stuff is sold to different vendors that are using it differently.

[Maxime]

From Hobby16, I learnt that even after the charger passes green and stops fan, it continues to load at a small rate, which equalizes the batteries. What is more, the wheel itself, at rest, equalizes the batteries at a slow rate too (even smaller ?). So, if you let your wheel rest, you will find automatically all batteries at same voltage. I don't know what happens if one battery is importantly defective.

[jayjay23]

What I read just today is that some chargers do not balance until a certain higher voltage is reached, this was probably in the context of rc were the battery pack has a balancing connector takes the job of balancing, but who knows ... maybe the BMS also has such a implementation?

[DebboR]

On 25-10-2015 19:20:33, jayjay23 said:

I have the same BMS than on your above photo and I used the wheel a little bit (about 1km) from fully charged, the battery pack was laying around for about 1 day and for curiosity I measured all the cell voltages and they are all(!) at 4,05V, there is not even one that is 10mV off, so maybe this is a indication that balancing is somehow not working.

Btw, one funny difference is that my C+ cable is connected to the P+ plate, making the row of diodes basically useless on mine (should not have any other impacts I guess).

The revision of by BMS PCB is exactly the same, but my batteries are not soldered they are also spot welded (there is some additional small plate between the PCB solder area and the metal stripe that comes from the batteries), anyway this should not make a difference to your problem but I find it interesting that the same stuff is sold to different vendors that are using it differently.

It's nice to hear that someone else has the same BMS, I looked around on the internet, and did not find any references / pictures of this particular BMS. Also, good to know that the circuitry actually balances, and isn't just for over- and undervoltage protection during charging. I'm assuming there's additional circuitry on the back side of the BMS, since there are some via's that I can't seem to trace out to any other place on the top side of the board. Maybe i'll get the soldering iron out this weekend to see if this is correct.

My theory about the diodes is that they are only there to prevent the charging voltage being applied in reverse accidentaly.

On 26-10-2015 19:20:33, Maxime said:

From Hobby16, I learnt that even after the charger passes green and stops fan, it continues to load at a small rate, which equalizes the batteries. What is more, the wheel itself, at rest, equalizes the batteries at a slow rate too (even smaller ?). So, if you let your wheel rest, you will find automatically all batteries at same voltage. I don't know what happens if one battery is importantly defective.

Interesting theory. Do you have any kind of thoughts about what balancing rate to expect? I suppose it should indeed be pretty small, considering the small SMT packages on the BMS. I have written down the individual cell voltages from after I manually balanced them until they were pretty close (all between 4.05 and 4.15V, so the capacity stored in each of them should be pretty much the same). I'm only at home during the weekends, so I'll see if they got even closer by using & charging the wheel all week.

On 27-10-2015 19:06:53, jayjay23 said:

What I read just today is that some chargers do not balance until a certain higher voltage is reached, this was probably in the context of rc were the battery pack has a balancing connector takes the job of balancing, but who knows ... maybe the BMS also has such a implementation?

If this is the case, this could explain why it wasn't self-balancing.


Also, since I balanced it, the range I get is at least quadrupled. I'll test the range using GPS-tracking more accurately as soon as I have the time...

Thanks for all the response!