Charging 2 x 3S Lipo's in series: please look over my schematic

Hi all,

I made some balancing harnesses today to charge 2 x 3S packs in series on my 6S charger. But when I connect them I get sparks. For the life of me, I cannot find what I’m doing wrong. Can somebody please look over my schematic, have I gotten something wrong ?


the schematic may be right but are connectors 1 and 2 correctly connected?

Thanks, you were right, I focussed so much on the balancing leads I didn’t pay attention to the way the charge connectors were wired. I almost toasted my charger.

you are lucky you didn’t burn out the balancing resistors on the charger output…

oh and for what its worth you don’t need to join the red and black balance leads in the middle.

Yeah I opened up my charger to check and the resistors had obviously gotten hot but still working but time will tell for how log they will last.

I’m afraid I don’t understand your comment on the middle balance leads. If I don’t join the red one, how will my charger know the voltage of the third cell in pack #1 ?

Thanks, Tom

The orange wire in your diagram is connected to the black lead which is connected between the +ve and -ve of the cells exactly the same as every other balance lead. When you connect the cells in series as you have the positive lead of the first pack becomes the negative of the second pack.

@trbt555 don’t forget to re-post your schematic once you get it all worked out if you have any revisions. This is very good information and i’m sure a lot of people are interested in it. I definitely am.

@longhairedboy Here’s my revised schematic which I wired and tested today. No smoke. No sparks. :smile:

Lesson learned: the wiring of the charging leads and the wiring of the balance leads should match each other otherwise you’ll have fireworks. Luckily the only thing I damaged were the pins of the balance lead connectors.


Excellent diagram. This is the kind of content people come here for. This, a few connectors, a DIY spot welder and some cells is basically all you need to make your own packs :smile:

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so if you look at this edited diagram:

the green orange and blue lines are all the same node. the long green link that you have made with your Y cable is logically the same as the short green line I have drawn. you can see this now matches the links between each of all the other cells. each cell “link” has 1 balance lead going to the JST connector. in this case I have drawn it in orange. the red balance lead (I have drawn in blue) is superfluous.

also by removing the red wire from you balance lead helps remove confusion about plugging which in where and generating shorts. the balance lead with the +ve red wire simply connects to the pack with the thick red charge wire.

Thanks for that useful insight.

Hi, What software did you use to create this schematic diagram?

Microsoft Visio. It isn’t specifically geared towards this kind of stuff but very flexible.

Which other programs do you recommend?

MS Word and MS Excel :wink:

Google Sketchup.

AutoCad LT.

If you wire the charge connector directly across the red and black wires on the 6s balance connector, you don’t need to connect the power connectors on the packs at all and there’s no chance of frying anything.

Of course, this means you are supplying the charge current through the balance wires, but they can easily handle 5A,and it’s unlikely you’ll be charging at a higher current than this for 3S packs.

(You will need to join the red & black leads on the 3S connectors though, as per @trbt555’s original schematic).

Hahaha this diagram is actually what I needed a month ago before I made some crazy fireworks and damage my connectors as well :smiley:

Nowadays I always check with multimeter the way I plugged in my batteries and use permanent marker to mark them so I didn’t plug them wrong. But I understand the idea that positive end of the battery must go on the positive side of the balance lead and the negative end of the battery has to go to the negative side of the balance lead!

How do you know that the balance wires can handle 5a? I’m considering trying this myself

Look up ampacity for copper conductors. The max amps you can safely put through a certain cross section of copper wire depends largely on how hot it can get (how hot the insulation can get without melting). I don’t know the cross section of your balance wires but I suspect max allowable current will be well above 5A.

If you’re really worried you can try to charge one of our lipo’s (without balancing) through two wires with the same cross section as your balance wires. If they don’t get hot, you’re good to go.