Be careful with those balance boards out there...

[chilly81]

I've been balance charging for years, starting with (7) parallel 1.3Ah 4S batteries for my quads. And I'm very anal and overly cautious and hyper scared of burning down my house. And I completely understand all the electrical concepts in play. Especially when balance charging which introduces numerous additional risks.

And as uptight as I am, I ~almost~ f'd up today. Was going to parallel charge (2) 6S 5Ah batteries. Hooked up one, then hooked up the second - checked to make sure cell voltages weren't too far apart. I usually plug in the EC5 connector before the balance, in case there is a bit of a difference. As soon as I did that, it took me less than 10 seconds to figure out what I did. I had a 4S 7Ah battery on the bench I was going to charge on the other channel - but it has the same rough size in the hand, and I picked that up instead. I would have obviously noticed when I went to plug in the balance connector, but didn't get that far. Both were at storage charge - cell voltages matched, but of course the 6S is 7V+ higher voltage. And those battery leads were hot to the touch in 10 seconds, but not melted. I'm hoping I didn't damage either of them - we shall see.

So anyway just a reminder that as easy as it its to mess screw up charging a lipo, it's way easier to do with balance boards.

 

[RedOctobyr]

Oof, thank you for the reminder. And I'm glad this wasn't worse, anyways! Damaging the LiPos is a lot better than the alternatives.

With a 2-channel charger, I guess you could maybe designate 1 channel per voltage. Like cover all but the 6S balance connectors on one, and 4S on the other. I realize that still doesn't fix everything, of course. You could even use different connectors (XT90 vs EC5, for instance) to help further reduce the risk of problems.

Extra cost, but you could do this with a single-channel charger, of course, though it would require buying a second, dedicated balance board. But still, that $30 could be considered worth it.

I try to remember to plug in the main leads first, to give any equalizing-current a larger wire to flow through. But yeah, that still doesn't eliminate all the risks.

Curious: is your balance board fused? Mine (Safe Parallel Boards from ProgressiveRC) are fused on the main and balance leads, but <40A on the main leads would not be enough to pop a fuse. I guess those could be reduced to maybe 15A on each main lead (going to the batteries), or whatever current you would not choose to exceed for a single battery.

I try to be very careful as well, especially when parallel charging, but we're all human. I'm glad this turned out OK, and thank you for sharing the cautionary tale.

 

[Dan B.]

My chargers all bark at you if you select the wrong voltage for the battery. But of course the balance leads have to be plugged in first.

 

[RedOctobyr]

My chargers all bark at you if you select the wrong voltage for the battery. But of course the balance leads have to be plugged in first.

That's interesting, you mean you typically set the cell count manually? I typically leave that as Auto.

Though I suppose if you had, say, 3S 2000mAh, and 6S 5000mAh packs, if you had a profile saved for 3S 2A, that could help avoid accidentally using a 6S 5A charge profile on that pack, and using too much current. The charger would complain that the cell count was incorrect. That could be a helpful "double check".

 

[chilly81]

Curious: is your balance board fused? Mine (Safe Parallel Boards from ProgressiveRC) are fused on the main and balance leads, but <40A on the main leads would not be enough to pop a fuse. I guess those could be reduced to maybe 15A on each main lead (going to the batteries), or whatever current you would not choose to exceed for a single battery.

That's really the best safety against something like this. I have a 4S only board that's fused, but my 6S generic boards aren't. The XT90 connectors are starting to get loose too they are so old, so they are due for replacement - probably get fused ones. The loose connectors are scary too - one falls out and the other battery is getting all the amps, but the charger has no way of knowing that. Not as likely to start a fire if you end up charging a battery at 4C or something, but still a little risky.

Definitely my chargers would have balked once the balance lead went in. At that point the voltage on the mains would have been roughly 1/2 between 6S and 4S as they started equalizing (probably 100A+ hot wires), but the balance leads would still read 6S.... and the charger would know that is bad.
Even my most basic charger (Imax B6AC), although it won't auto detect cells (iirc), it definitely will alert when you start charging with the wrong settings. It would have to be a pretty crappy charger to not be safe with just one battery connected to the channel. Although I'm sure there are cheap ones that probably are like that.... any of the common stuff people recommend won't have any issue with that. I'd just have to run through two or more charging cycles to charge everything if I didn't use balance boards. Or buy two more chargers I suppose.

 

[RedOctobyr]

For the XT90s being loose, can you use a small flat or Philips screwdriver, in the male contacts, to spread them apart slightly, to snug up their fit? They're only taking 5-10A or so, I'd think they're probably less critical than the ones on the battery & ESC. But I've heard using that same trick for any of the connectors (haven't needed to try it yet myself, and haven't installed my XT90s yet).

That does seem like a significant risk if a main connector fell out while charging. Now the balance leads are effectively trying to pass all the charge current to the disconnected pack (maybe 5A per little wire). That can't be good, though hopefully not a fire.

Interesting point, I wonder where the "cutoff" is for the charger to complain. These aren't laboratory-grade, calibrated instruments, so they need to accommodate if the sum of the balance leads shows 12.6V, but the circuit on the main leads measures it as 12.58V. So there needs to be a certain amount of leeway which the charger considers "OK", and will still charge normally. Though in that example (4S + 6S in parallel), I'd obviously be much more worried about the current the batteries themselves are providing to each other, which the charger can't see or control

All my packs are 6S, so thankfully, for now, I can't run into an accidental mix-and-match. But it's an alarming thought, thank you.

 

[chilly81]

For the XT90s being loose, can you use a small flat or Philips screwdriver, in the male contacts, to spread them apart slightly, to snug up their fit? They're only taking 5-10A or so, I'd think they're probably less critical than the ones on the battery & ESC. But I've heard using that same trick for any of the connectors (haven't needed to try it yet myself, and haven't installed my XT90s yet).

D'oh, of course - that worked great!

Though in that example (4S + 6S in parallel), I'd obviously be much more worried about the current the batteries themselves are providing to each other, which the charger can't see or control

Yeah I wonder how much current. Must be 100's - they are both decent SMCs 5k and 7k SMC's. Not sure but it might essentially just like a dead short, but probalby limited by the power delivery of the lower C battery. maybe lol. idk, but it heated up real quick, not hot or melting, but warm like after coming in from a hard drive.

 

[jkflow]

Been there, done that with a 3S and 4S in parallel. My charge leads are of lower diameter and melted. Used smaller gauge for that reason and it paid in the end.
Those dang things look very similar if you are overconfident and don't bother looking at labels.

Charger or anything doesn't help in this case, the battery leads are in parallel, that is where the current will stay. Charger really won't do anything and even if it would throw an error it's meaningless and of no impact as the wires are already glowing at that time.