Cell on its way out

[stinkyface]

How long do you think this pack has before it's done....

This was after an overnight balance charge and a shortish 7km ride.

[Lia]

@stinkyface That second to last cell looks very out of place.

What does it look like when it's "finished" charging right after the green light comes on? You'll want to wait till it goes green then take it off charge and see what the cells read, this way you can see where each got to by the time one hit the full charge mark.

On a side note balance charging can take forever if a pack is really out so might be worth leaving it a bit longer. If memory serves me correctly most balance circuits can only handle 50mAh per cell bank so anything really out of whack leads to needing much longer balance cycles.

[biell]

Leave it plugged-in whenever you aren't riding it for the next week. You can ride as much as you want, just make sure to plug it in right after and leave it charging until your next ride. Get back to us after that.

[stinkyface]

@biell @Lia thanks, will follow the advice and charge charge charge and monitor.

I suspect it's worse as was not having any issues last week. I have a downhill near the start of my daily, I have to go slow to avoid
a regen trip, I usually watch the voltage on the way down and can hold it at 63.0v. today, after a longer than normal charge and with a green light after the overnight charge. I was at 58v at the top of the hill where usually I'm at 61.7v . Something is different since Friday last week.

the board tripped at 59% remaining with that bad cell at the expected 2.8v after 12km total travelled. Usually I'm up over 24km before it shuts down. Will take the charger to work tomorrow and report back with whatever develops.

Thanks again.

[Lia]

@stinkyface Let us know if the issue resolves itself after a longer charge, if it's still not working I think you may have a broken weld. The situation doesn't sound to me like a dead cell, I've never seen them go bad like that considering the circumstances.

A sudden sharp vibration can break the welds if they aren't done well. This would make sense as to why you've seen nearly a complete halving of your range suddenly.
To add to confusion the issue may spontaneously come and go as the weld makes contact randomly from further vibrations. This isn't ideal as it can make contact when there is a voltage imbalance that would cause them to charge/discharge rapidly where the BMS cannot protect them.

Below is an illustration of what the issue might be as described above. Hopefully this helps better show what I mean.

When this happens you halve the capacity of the cell pair and in turn the rest of the pack because the BMS steps in.

Doing surgery on the Onewheel battery isn't easy though, it's wrapped in tape, covered in balance wires and the welds hold the pack together pretty securely. I wouldn't recommend the type of repair to many since battery welding requires kit to do it properly which puts it out of the realm of possibility for most.

You may have luck with an eBike battery repair places, they're more than familiar with weird pack layouts and would probably be easy for them to re-weld whichever cell came loose. They may even be able to quote you for a rebuild if you feel your cells are on their way out anyway.

[stinkyface]

@Lia the broken tab makes sense based on what we know. Good call.

Rode in again today and got a couple of screenshots post overnight charge. This one is while still on charge. Still holding 63v overall:

Then 30 seconds after turning off the charger, cells stable but overall voltage down

Then after a ride...cells look like they are singing a similar song again.

[stinkyface]

Gives a good indication of how the charger works. I was actually watching the cells voltages when the charger flicked to green. They all dropped straight away.

Looks like the balance function was bringing the other cells down to match the low cell. I'd always assumed it would pump power into the low one to bring it up.

I've sourced a 2nd hand battery to have as a backup, pickup this weekend for a few hundred bucks. This scare has been good for me to recognise that this is my daily transport and needs spares to stay reliable.

[Lia]

@stinkyface That's looking better!
Good to hear you managed to get a spare. I should probably look to make or get one just in case too seeing how old mine is getting.

[stinkyface]

Just to close this out. Ride home tonight after more balancing at the office today.

These puppies are licking the same bowl! 😎🐶👌