V8 "Please repair" and full tiltback at 50-65% battery.

[Heymelon]

So as title says, I can ride the wheel fine until I get down to those percentages. Then the wheel tilts me back and says please repair. It is at this point unrideable. It usually lets me keep riding if I restart it and go really slow, like 10km/h. But eventually it will repeat this process until I'm not able to start it at all before I give it charge.  When I charge it up it works fine again.

So it seems like my BMS is bugged or a cell or two is not getting fully charged? At least that is what has been explained to me by a friend who has a similar Issue with his V10 that does the same thing at 50%. However his doesn't take full charge while mine does, and euc world typically reads 85V or a little above and 100% at charger green light.

Anyone have any ideas or possible fixes for this that isn't send the wheel to a battery expert for manual balancing? I have no warranty on it, bought it used last year. Checked all the cables once or twice on control board and to the battery. 

 

[Kalman]

Hi Heymelon,

I have very similar issue, although my wheel doesn't say "please repair". Could you fix this since you posted the issue?

Thank you & best: Kálmán

[Heymelon]

Hi, sorry for late reply. No I didn't solve it as such so to this day the battery last to about 60% before this issue happens. But I do think there are solutions in manually balancing the cells with some cheap parts you can buy. And I would have to do that do again to keep the battery working.

So that might be a solution for you if you have the same problem as me. And (I'm pretty sure) that the problem is that the wheel has a crappy BMS system that doesn't do passive balancing well if at all, and on top of that if any of the cells reads below the intended voltage it thinks that the cell is dead and then when you reach that point while driving to draw from that cell it registers this as a battery error and begins to slow down. 

I did also make this post here where I got replies from people:

https://forum.electricunicycle.org/topic/29315-v8-please-repair-and-full-tilt-back-at-50-65-battery/

[mrelwood]

I'm pretty confident that none of the EUCs up to the S22 warn or react to single cell groups getting below any voltage limit. There are several wheels of all makes and models on this forum that have a busted battery that don't charge beyond xx%. That's what reveals a pack with fatally out of balance cell groups.

Does your wheel charge up to 84V?

[Heymelon]

2 hours ago, mrelwood said:

I'm pretty confident that none of the EUCs up to the S22 warn or react to single cell groups getting below any voltage limit.

Yeah I don't know that it does but I think it is simply intepreting unbalanced cells as dead and gives me a battery error "please repair" warning whenever I try to draw from them. But the wheel does indeed fully charge to 84 every time and works like a charm from 100 to about 60 percent. 

[mrelwood]

4 hours ago, Heymelon said:

But the wheel does indeed fully charge to 84 every time

To exactly 84.0V?

[Heymelon]

6 hours ago, mrelwood said:

To exactly 84.0V?

No, why? It goes a little over as it did new. I'm told the wheels readings can vary by a volt or two in these things. It usually hits green light at 85.2 in the charger, but if I unplug and start it up it says 84.1 or 84.2 IIRC. 

[mrelwood]

14 hours ago, Heymelon said:

No, why? It goes a little over as it did new. I'm told the wheels readings can vary by a volt or two in these things. It usually hits green light at 85.2 in the charger, but if I unplug and start it up it says 84.1 or 84.2 IIRC. 

Does the 85.2V read in the charger itself or in the app? Because dropping for a full volt after unplugging would be a sign of a battery that is not full or not at all well balanced.

You can connect to the wheel while charging to see the voltage, so that you'd be checking the voltage from the same source.

[Heymelon]

14 hours ago, mrelwood said:

Does the 85.2V read in the charger itself or in the app? Because dropping for a full volt after unplugging would be a sign of a battery that is not full or not at all well balanced.

You can connect to the wheel while charging to see the voltage, so that you'd be checking the voltage from the same source.

Ok. I have only ever read it from the app, no display on the charger or anything. But I could get a voltmeter and check what it reads. I have semi planned to try and manually balance the cells by suggestion from a guy in the other thread, so I'll start by taking all the necessary measurements then.

[Kalman]

Hello Guys,  

After all I found the reason for this issue and my wheel is working well like before. The solution is this: there are twenty pairs of cells in the battery package – 20x2. One single battery cell of this package was completely dead, and in my case acted as a shortcut. It means, that this one is out of the game, but it didn’t cause any problem other than the mentioned capacity issue, it behaved as it wouldn't even have been there. But it's poor mate piece of cell was trying to work instead of two, which means, it had to produce the power of two cells. Easy to conclude, that it'll be discharged twice faster than normally – which was exactly the symptom in my case. To figure out which cell pair was the guilty, the coverage of the pack needed to be removed and I measured the 20 cell pairs. 19 of them showed ~3.4V, one of them was only 2.9V. That was the cell-pair, which has only one working cell instead of two. I took a new cell and simply soldered it to the appropriate pins of the BMS board. Thus, a new cell has been added to the system. I didn’t even remove the dead cell. And it worked. I can ride over 30km, like earlier, and can ride until the pack is being discharged close to 0%.  

One more thing: it is also logical, that BMS couldn’t recognise the issue, as it could charge the package to the nominal 84V. Remember that the dead cell doesn’t count as it’s mate could also reach 4.2V. But when I was riding, the single cell reached the cut-off ~2.9V twice as fast as the others. The BMS had no choice but to switch off the wheel to protect the weakest cell in the system, although the overall voltage of the system in this case was still about 75V (50-60%) - as all the other cells were still ~3.5V.  

[mrelwood]

 

6 hours ago, Kalman said:

I didn’t even remove the dead cell. And it worked. I can ride over 30km, like earlier, and can ride until the pack is being discharged close to 0%.  

You REALLY should remove the dead cell. It’s a fire hazard if you try to charge it!

You clearly found the cause for the issue, but there are a few holes in your theory.

- The cells are connected in pairs. Each pair is hardwired in parallel. Because of this their voltage is always the same. If one cell fails and goes down to 0V, it acts nearly as a shortcut to the other cell as well, meaning they both go to 0V which destroys the other cell as well. But I don’t think this happened with your wheel. Yet. But it will if you keep the failed cell connected.

- The BMS’s on EUCs don’t have a mechanism to warn about single cell groups’ undervoltages. A cell group can go down to 0V and the BMS is just happy. It doesn’t try to protect the bad cell. It only monitors the cell group voltages for preventing overvoltage.

 

PLEASE disconnect the bad cell from the system as soon as you can! Besides soon becoming a fire risk, it will kill the two other cells as well when it finally fails completely.

[Kalman]

Hi mrelwood,  thank you for your reply, your conclusion is correct, I used  the  wrong word: "shortcut". Although it is an open circuit. So the dead cell behave like it wouldn't even be there.

[mrelwood]

I haven’t heard dead cells ever doing that though.

[Kalman]

So you have heard now. Anyway, like I said, it works well.