On larger electric models we usually need to connect two LiPo’s together in Series to give us the required cell count for the model. This might be two 5S packs to make 10S for a Logo 600, or two 6S packs to make 12S for a T-Rex 700, for example.
When using standard single bullet connectors on the batteries and the ESC, this is easy to connect up, simply connect the negative lead from pack 1 to the positive lead of pack 2, and then connect the two remaining wires (one positive from pack 1 and one negative from pack 2) to the ESC, as if it were a single LiPo.
If you use one of the more popular connectors, such as EC5, then each battery has the two connectors inside a shroud. If you don’t want to cut the shroud in half to allow you to connect as if standard bullet connectors are being used, you need a way to connect these packs together in series to power the ESC.
One method is to build a Y lead harness that accepts two batteries, and ends in a single plug that can be connected to the ESC. This works fine, but the downside is more connectors in the system, and more wire being used. This adds a little weight to the model, but more importantly, further potential points of failure to the system, neither of which are a good option!
The solution is to build the Y lead harness directly onto the ESC wires.
Remembering that you need to connect the negative lead from pack 1 to the positive lead of pack 2, you can assemble the wires on the ESC using two EC5 plugs (or whichever your chosen connector type is) with the following connections:
ESC positive wire into EC5 plug 1 positive port.
ESC negative wire into EC5 plug 2 negative port.
Then add a joining wire going from EC5 plug 1 negative port to EC5 plug 2 positive port.
I usually use some 10 AWG or 12 AWG black wire as the joining wire between the two EC5 plugs, and cover half of it in red heat shrink for an easy polarity reference. Of course, it is pretty hard to plug EC5 connectors together backwards, but I think it makes it look neater.
Please refer to the images below to see how the connectors need to be wired together.