Battery needs balancing?

[Zerogain]

Long time lurker, first time poster. Need some advice on battery balancing.

I built an ebike as a fun project to mess around with. When it came to buying the battery, I grabbed a 10Ah 36V Vpower pack from an eBay seller in Hong Kong. Charged it up, and the bike works. So far so good. Then, after a few minutes riding, the motor cuts out. Flat battery, already?

From reading on here, I realise that VPower isn't exactly a quality brand, and buying them from eBay was probably a mistake. What can I say - I wanted something cheap to experiment with. Unfortunately, now that the bike works, I've caught the bug and want to ride some more

The pack is almost new (it was bought a few months ago), and has been run only about 5 or 6 times.

It's never gone further than about 3 miles on a single charge, so I guess the pack is not balanced. At all.
Most cells were still at 3.34V when the motor cut out, but there's one at 1.36V. I know that voltage isn't a good indicator of state of charge, but it's clear something is up.

Charging on a 6A charger takes only 10 minutes, which is nowhere near long enough, so again I suspect the battery isn't balanced, and I'm not getting the full capacity out my battery. When the charger is "finished" the majority of cells are at 3.65V, but this one is down at 3.25V.

Leaving the charger to run overnight doesn't seem to help, either.

First of all, is the battery still good? Is it worth trying to balance it?

If so, is there any way to balance charge without opening up the pack (and unwrapping several miles of duct tape that's holding it together?)

Would replacing the BMS with a balancing BMS help? The one that's on there is as cheap as it gets, and I suspect it doesn't do balancing.

 

[dogman dan]

One cell that much lower than the others is clearly out of balance. The question is, why? Storage for a long time without using it, or recharging it to let the bms do it's work could be one reason. The other could just be they put some poor quality cells in the pack.

Do you have a single cell charger, such as an old no longer used cell phone charger? If you do, then try to charge that one really low cell to 3.5v, or something much closer to the other cells. The bms will balance the pack, but if it's that far out, it could take a month of sitting plugged into the charger. You would attach that small charger by unplugging the bms, and connecting through that pair of bms wires that show the low cell.

If you can get it closer to balance manualy, then you can try finishing the charge normaly, then go ride and see if you get better range. Your bms will balance the pack, but it's slooooow. Too slow for a difference that big. Hopefully, it will not go right back to that far out of balance if you get it balanced by hand.

But unfortunately, there is a definite possibility that a poorly made cell passed a test when brand new, but then failed to last till it was shipped to you. If that is why it's so far off, then nothing short of replacing some cells is going to work. It's possible that the bms is discharging one cell and won't stop when its supposed to. That can be detected if that is the case.

If you have some basic skills, you can make something usable out of the rest of the pack. But I won't call it easy.

 

[dnmun]

do you have a voltmeter?

 

[dogman dan]

Re reading, from 3.25v to 1.36v in three miles does not sound good to me.

But first thing to try to get done, is get the battery balanced somehow. Then see what happens if all cells are at least 3.5v to start with. See if that wonky cell keeps getting lower and lower just sitting there, that would indicate one possibility could be a bms defect. If it keeps self discharging with the bms unplugged, then it's the cell self discharging.

 

[dnmun]

it should never get to that low a voltage either. if you measured it after it came to a stop and you had time to measure, a cell that had cut off at 2.1V would have climbed back up to 2.7-2.9V already. so the cell you discharged went to zero i bet.

so you gotta cut it open and take it apart.

 

[Zerogain]

Thanks for the replies.

All measurements were taken with a basic multimeter.

It's good to know that you can use the BMS monitoring wires to top up the cells. I'll have a go with an old phone charger (being careful not to overcharge!) tomorrow. Here's hoping it's just a lack of battery care that's the problem, and not a bad cell in the pack.

Tearing the pack open is a last resort, but it may be a bit beyond my skill to fix at that point!

 

[dnmun]

there is no way around it, you have to take it apart and remove the dead cell. if you discharge a lifepo4 cell below the 2V level under load it is ruined. there is no way it will ever hold full charge again so all you can do is cut it out and convert your battery to a 15S. the battery will never balance again with the dead cell in there. it is not as though you have a choice now. you had a choice before you bot it. now you have to pay for your decision.

 

[dogman dan]

Quite likely the pack will never perform right with that cell in there. Very likely the bms cannot cope with a bad cell, if that is actually what is going on. The bms can balance a good pack, but it's not designed to handle one that out of whack.

Worth a try though, to manually balance it, watch it, and see what happens before tearing into it.

If you can't fix it, you might be able to sell the remaining good cells to others with the same problem. But not for much money at all.

We also have not addressed another likely issue. Did you buy a big enough pack to run your device? Did a cell die because you have a huge controller? Only the very smallest ebikes and scooters can run on a 10 ah v power pack.

 

[Zerogain]

The bike's fitted with an em3ev MAC motor with 9 Fet controller. The controller's rated for 30A, which would be 3C. VPower claim their cells are good for 2C, so the plan was to use the controller with limited power (there is a switch that selects 50%/75%/100%). As it turns out, 50% power is plenty for a newbie anyway, but maybe the battery disagrees...

Eventually I'll get round to sharing some pictures in the Before & After thread.

The offending cell is on charge at the moment, so I'll let you know how it goes. Could be waiting till tomorrow to get the combination of enough charge and sufficent daylight to go for a long ride.

 

[dogman dan]

Uh, 10 ah at 2c is 20 amps, and you have a 30 amps controller. Even if your cells did like being discharged at the rate they claim they can, it was doomed. Even with the switch at 50%, I bet you can pull a 30 amps spike getting started.

So even if you fix that battery, you are screwed. You would need IMO, 20 ah of that type of cheap lifepo4 cell to run a 30 amps controller, and really, 30 ah would be better.

You need a better battery, for example, the ones EM3ev sells, or any other with a better track record for running a 30 amps controller reliably.

 

[Zerogain]

For what it's worth, the balancing seemed to work. The range was greatly improved - just over 8 miles... I had to go round the short route I planned a few times Not sure if it will last though.

You're probably right about the C-rate being too high for the pack. I'll look into a replacement battery, and in the meantime, try not to murder the one I have...

 

[dnmun]

that pack is already dead on the cell that got over discharged. the BMS is not working so if you use it you can damage even more cells. why go buy another junk battery when you have to fix this one first?

 

[999zip999]

Places like v-power is how E.S. got started helping cheap people on junk ebay junk.
So you have learned a something about matching a battery to your controller.
Em3ev and a few others sell a working battery. Just fully charge your battery and be awear of the probem cells as a damage cell dose not repair itself. But can take down others.

 

[friendly1uk]

Why do people use phone chargers? I have seen it said a few times, but I have never seen a phone charger. Things that call themselves chargers yes, but never one that was a charger. Phones have the charger circuitry built in. The box we call a charger is often an unregulated supply. Now we have about settled on a universal charger rather than phone specific one's a 5vdc supply is common place.

My first nokia's were 9v and they slowly came down to 3.7v but that was a lie if you measured it, 3.7v is not enough to charge a battery to 4.2v, it should be obvious. Now they are 5v. At no time have they been a charger though. Although I have seen suitcases full of them used for a cell each.

I guess I must of missed the one's that were actual chargers, but I'm not using off-brands here, just nokia's. Based on my findings you should never use a phone charger. They have all been unregulated dc supplies of incorrect size.

As a blanket statement that covers the dozen or so I have looked at, you can't use a phone charger. They don't even exist.

 

[recumbent]

Why do people use phone chargers? *** snip *** As a blanket statement that covers the dozen or so I have looked at, you can't use a phone charger. They don't even exist.

Many folks here, including myself use these small phone chargers to top-up a low cell. We do this by pulling out a pin from the "HXS" (typo) plug used extensively on ebikes and RC models, then connect one end to the small phone charger and the pin is inserted into the "low" balance tap then charged carefully for an hour till the runt cell is up to standard. Takes much less time to build than type.

 

[dogman dan]

Hope that battery lasts you long enough to get a better, bigger one. Meanwhile, pedal first.

Did you ever find out if the cell is self discharging? Or the bms discharging it?

 

[friendly1uk]

Why do people use phone chargers? *** snip *** As a blanket statement that covers the dozen or so I have looked at, you can't use a phone charger. They don't even exist.

Many folks here, including myself use these small phone chargers to top-up a low cell. We do this by pulling out a pin from the "HXS" (typo) plug used extensively on ebikes and RC models, then connect one end to the small phone charger and the pin is inserted into the "low" balance tap then charged carefully for an hour till the runt cell is up to standard. Takes much less time to build than type.

Thanks for the reply. I know the plug you mean. It's not the method of connection I'm puzzled by though, It's the existance of a phone charger. I don't think I have ever seen one. I just saw dnmun advising their use, but still... I can't find one. I just know of the common misconception that gets something quite different called a charger.

I have 3 here. 3.7v 7.2v and 9v and any of them will charge my phone. So will 5v from my usb. The charger is in the phone. How are yours different?

 

[Tench]

What is your pack configuration, if there is more than 1 cell in each parallel string it is possible that the offending string has a dead or separated cell in it, that would explain why that string drops voltage faster than the others, because it has lower capacity. it could be easily repaired?
First step would be to bring all the cells up to full voltage, then ride just a couple of miles and check them all again, don't go to LVC! if after a couple of miles they are all the same it was just the balance that was off, if that string is low again you will have to open the pack up to investigate. If there is a broken connection reducing the number of cells in that string repair it and follow the same procedure again. if there is no broken connection or it drops more voltage again after a repair/balance and test then that whole cell string will need to be replaced.

Then you will have a decent functioning pack again.

But even if the pack was restored to its optimal performance you are over stressing it with a 30a controller which will damage the cells. You need a bigger pack!