Charging 18650s - Series or parallel? Reverse polarity protection?

Thread Starter

GenericUsername

Joined Feb 5, 2022
4
So, I have a few things which use 18650 cells.
I also have a balance charger for hobby RC aircraft, an IMax B6AC to be precise.
Instead of buying an 18650 charger I went for the cheaper option of using the balance charger that I already own.
It comes with several outputs, one being two female bananna plugs. I've used this with crocodile clips (clipping onto the positive terminal and then using blu tack to attach a wire to the negative terminal for charging one cell at a time) this is obviously unreliable and I could only charge 1 cell at a time.
So a bought a dual 18650 cell holder and soldered some wire on so it (which I then insulated) could charge in parallel, it works fine but is still very much a prototype as I just clip the positive lead to the positive croc clip and the same for the negative lead. Yes I know there's a risk of shorting two 18650s out if the leads or clips touch during charging. I plan on putting a 3A or 5A fuse in my final design. The cells are rated to output 25A each so the fuse should blow before the cells do in the event of a short.

My question is, was wiring it up in parallel and just charging two cells as if they were one big cell the best choice? I've read online that charging in parallel can cause one cell to drain the other leading to overdischarge in one cell. However as there is no balancing feedback would series charging really be safer than parallel?

Also, should I build in any reverse polarity protection or just be careful when I insert cells?

Thanks
 

dl324

Joined Mar 30, 2015
16,914
Welcome to AAC!
My question is, was wiring it up in parallel and just charging two cells as if they were one big cell the best choice?
That's just about the worst thing you could do.
However as there is no balancing feedback would series charging really be safer than parallel?
Series is almost as bad as parallel.
Also, should I build in any reverse polarity protection or just be careful when I insert cells?
I vote for just being careful.
 

bassbindevil

Joined Jan 23, 2014
828
If the cells are matched and discharged to the same level, parallel is absolutely fine. That's what millions of laptop and e-bike battery packs do. But if you're using loose cells, they're unlikely to be discharged to the same level or perfectly matched. Series with a balance connection to the B6 should be OK.
As for polarity protection, I'm sure the B6 won't attempt to charge a backwards cell - it won't even charge an unusually low voltage cell. (You have to charge those as nickel-chemistry cells first to get them up to a reasonable voltage, I've heard.)
But, the simplest solution is to pick up a dedicated 18650 charger; I've found a couple of decent ones at thrift stores. Or get something like a Liitokala or Maha multicell charger.
 

LowQCab

Joined Nov 6, 2012
4,067
If You want to build your own,
here's a Schematic for a very High-Current Charger/Dis-Charger.
You can of course, scale the specs of the parts to suit your needs.
.
.
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35-Amp 3S Li-Po Charger 1 Flat 1-MB  .png
 

dl324

Joined Mar 30, 2015
16,914
If the cells are matched and discharged to the same level, parallel is absolutely fine.
Cells in battery packs may start out matched, but they don't stay that way. If the OP is using cells that were purchased individually or salvaged from battery packs, they won't be matched and he won't be able to match them.
 

Audioguru again

Joined Oct 21, 2019
6,690
You have a balance charger so why don't you connect it to up to 6 battery cells to properly balance-charge them?

But the charger has many buttons and many outputs that can easily be set wrongly that could cause the battery cells to explode/catch on fire.
 

Thread Starter

GenericUsername

Joined Feb 5, 2022
4
You have a balance charger so why don't you connect it to up to 6 battery cells to properly balance-charge them?

But the charger has many buttons and many outputs that can easily be set wrongly that could cause the battery cells to explode/catch on fire.
I've a balancing board which has a bunch different connectors for RC cell charging. I think 6 or 8 cells balance charged.
At the moment I'm using two bananna plug to croc clip outputs running directly back to the charger rather than the balance sockets on the charger which will ensure balanced charging. I assume the banana plugs to croc clips are for charging of Pb batteries, which is an option on the charger.
I also have a bench power supply but I've only charged LiFePO3 batteries on it, except for a Li-ion 18650 which arrived overdischarged to about 2V even when bought new online, I used the PSU to ressurect that until I could get another, after which the resurrected 18650 was discharged and recycled.

I've seen the schematics above and assume that's how to build a balance charger from scratch. Since I have a pre-built one I just need to know the best set-up for balancing cells.
A schematic or design for a balance charger connection for 2-6 18650s would be good. I'll probably look it up myself too and if I can find a good schematic/design I'll ask your opinion. That way I could build it using female dupont connectors to fit to the charger (or a correctly sized plug some wire and a and crimper) to build it.

I'll look into some schematics of simple 2 cell lipo balance chargers, see where the three wires

I've not got a huge amount of experience. In building balance chargers. I'd rather just use what I already have as well as learning.
 

Audioguru again

Joined Oct 21, 2019
6,690
I have E-flite two-cell Li-PO batteries and I use an E-flite charger for them that safely balance-charges them in series and does not charge more cells of many different chemistries like your dangerous charger does.
 
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