What is an acceptable max voltage for a used 84v wheel?

[GottaWay]

My MCM5V2 800wh does not charge to 84v. Never has. Has about 1600km on it and charges to 82.4v no matter how long you leave it on a charger(tried cell balancing a few times). Charger output is verified d to be outputting 84.6v.

 

I have not checked the voltage at the packs as a disassembly doesn't seem necessary at this point. 

 

I believe the math adds up to 20 cells in this wheel. At my 82.4v, I'm still averaging 4.12v per cell, which seems on the low side of tolerances. It doesn't seem to be off by a whole cell yet.

 

 At point should I be worried? Eventually I'll end up tearing the wheel down, but until then...

 

I guess I'm just worried I have a lazy cell rather cells on the low end of the acceptable voltage. Thoughts?

 

 

Edited February 13, 2023 by GottaWay

[Tawpie]

I would open it up and check the pack voltages with a multimeter. The voltmeters installed in the wheel are... functional but are known to have errors. Cost cutting has a downside.

82.4V is right at my personal "concerned" trigger. If there's a single weak cell that's responsible for the pack stopping 1.6V below expectations (19 cells at 4.2 plus 1 at 3.6), that would mean that the weak cell is only 1.1V from the "do not ever go below this voltage" threshold of 2.5V. When you run the pack down such that every cell loses 1V (and the state of charge will be near 0%), then the 19 good cells will be 3.2V. But that weak cell will be 2.6V and every time you pull current from it it'll drop to or below the 2.5V level where damage rapidly accumulates. The damage can and does result in catastrophic failure, sometimes leading to fire.

But, if you never let the pack get below say, 40% and you are a very gentle rider, you may not be pushing a weak cell below the 2.5V limit and you might be ok. But a weak cell will weaken faster than the rest, and you don't have much room to play with.

Personally, I would have the packs looked at. ASAP.

Edited February 13, 2023 by Tawpie

[Denny Paul]

…I personally wouldn’t bat an eye until below 82 volts. It’s just so common that the internal voltage meter is off by 1-2 volts.

It’s still a good idea to test the packs for peace of mind when you open the wheel up next. If there’s a voltage difference between the packs, do not reconnect them. 

[GottaWay]

So, in theory, if I had a bad cell, one of the two packs would be off in voltage from the other pack since the bad cell would be drained more so than the other cells in the same pack. Correct?

So if both packs measure identical and are over 82.4v, then it's just the internal voltage meter reading wrong and no actual issue. Correct?

[RagingGrandpa]

It's common for the controller to measure voltage inaccurately. Especially old controllers. 

Prove it by recharging fully, disconnect the charger, and measure the voltage at the charging port. (Carefully, with insulated probe tips!) 
Compare your voltage measurement with the voltage reported by the controller in the app.

If you measure 84 but the controller reports 82, that's a controller measurement offset and not hazardous  
If you measure 82... something's wrong with the pack  

[Chriull]

3 hours ago, GottaWay said:

So, in theory, if I had a bad cell, one of the two packs would be off in voltage from the other pack since the bad cell would be drained more so than the other cells in the same pack. Correct?

Everything connected in parallel has the same voltage.

As through everything connected in series the same current flows.

If there are bad cells they often have lower capacity and hence get charged full fastest. So the single cell overvoltage threshold is reached before the other cells can reach ~4.2V.

The overall pack voltage is then below 84V once the bms cuts off 

3 hours ago, GottaWay said:

So if both packs measure identical and are over 82.4v, then it's just the internal voltage meter reading wrong and no actual issue. Correct?

Maybe. Maybe not.

[GottaWay]

So the charger is outputting 84.8v. Apps read 82.4v. The charge port on the wheel reads 84.2v. I'm guessing the internal voltage meter in the wheel is off. I'll ride it and check voltage at the charger port to see if the misreading stays linear(as in the offset remains -1.8v off at all times).

Edited February 15, 2023 by GottaWay

[Chriull]

18 minutes ago, GottaWay said:

I'll ride it and check voltage at the charger port to see if the misreading stays linear

Most likely - inaccuracy with the voltage dividing for the μC.

18 minutes ago, GottaWay said:

(as in the offset remains -1.8v off at all times).

Imo that's a constant offset.

But not too much difference in this case.

Edited February 15, 2023 by Chriull

[alcatraz]

Next time you have a side panel off measure the pack voltage with a multimeter. Then turn on the wheel and note the reported voltage. (It's best to do it after a full charge)

Reported voltage X = actual voltage Y.

You can simply remember how much the values differ and apply to any reading in the app.