Charging 7S with 8S Charger

ESK8 Electronics
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1

Hey guys,

I have just recently removed a dead cell from my 8S pack, as described in this thread..

New problem is that I need to trick my 8S charger into charging my 7S pack. I have made my own charging port using a D-Sub15 pin plug (essentially a big VGA plug) and now I just cant figure out how to re-wire it to make it work.

This here is my charger:

https://tomtop_shop.guphotos.com/i/w?u=/images/R/RM3758EU/RM3758EU-4-dd67.jpg

If someone could draw me a quick sketch, basically explaining how a 8S to 7S JST adapter would look like, id be forever greatful :slight_smile:

Thanks guys!

2

um is the busted cell still wired in series? if it is not, you just need to make a 7s balance plug…

3

Hey @Cobber, no I cut it out and re-wired it to be a 3S pack. So I have a 7S battery that is made of a 4S and a 3S Lipo.

Thats exactly what I dont know how to do :slight_smile:

4

You have 9 pin connector and 9 cables. Remove first or last cable. Use 8 pin connectors. Done. Seriously, not sure what’s bugging you, it’s the simplest thing :slight_smile:

5

putting un-balanced cells together in a single battery aside… the cells would all need to be in series with the 7 balance wires in a single balance lead… be careful. you will need to watch charge and discharge voltage constantly. you don’t want a fire

6

The cells were all balanced to ±0.02 difference in voltage :slight_smile:

For sake of simplicity, lets just assume i had a 7S battery with a single 7S balance lead. How do I connect that to my 8S balance lead on the charger? Thats ultimately my question

7

plug the 7s plug in to the 1-7 cells on your chargers balance port?

8

Remove the extra wire that’s not connected to any cells…

9

Does your charger have setting for charging 7s pack, is it configurable in any way?

10

Well on the manual it says its capable of charging 2-8S batteries and it also originally came with a balance board (its long gone), so i am assuming it can

11

And do you have 9pin JST coming out of your pack?

12

along the lines of what sceppy said, what does your balance lead look like? 7 cell balance wires?

13

No i am hooking up the balance leads into my vga port, so that i have a plug’n’play solution without bms, so i am not actually using the jst plugs on my lipo

14

Alright, but you do have pin 0 (gnd) - pin 8 (v+) and getting around 29,4V across them?

15

I have 8 balance leads:

1st pack is 3S:

  • and + first cell,
  • second cell,
  • third cell = 4 balance wires

2nd pack is 4S:

  • first cell (leaving - disconnected as its redundant),
  • second cell,
  • third cell,
  • fourth cell, = 4 balance wires

= 8 balance wires in total

16

yeah i do, voltages seem ok

17

Alright, yo need to make a cable that will connect (-) from the charger to the pin 0 on the VGA, following through pin 1 (4,2V), 2 (8,4V)… up to 8 (29,4V). Pin 8 will finish at marking 7 on the charger. 8 on the charger will be free.

1 Like
18

Thank you, i think i got it, i will try in a few hours when i am at home!! :slight_smile:

19

No problem. If your charger doesn’t have auto detection and selection of number of cells, make sure you switch to the 7S mode. BTW, where did you get the charger? :slight_smile:

20

I got it from tomtop.com but now its also available on amazon