# Automatically connecting LiPo batteries in series (for charging)

I have a battery charger capable of balance charging up to 10 cells in series (10S), but most of the batteries I use are somewhere between 1 and 4 cells. I have plenty of experience charging the batteries in parallel, but I'd rather charge them in series. Doing so allows my charger to report out to me details on the health of each individual battery, rather than some aggregate across them all.

Connecting the batteries in series is pretty trivial: connect + on one battery to - on another, make a balance lead tap between each battery and at the ends. The net result looks something like

Balance:   |  |   |   |...|
Batteries: -C1+-C2+-C3+...+
Charge:    -              +


The problem is that charging / discharging batteries doesn't actually happen over the balance lead but instead over the charge wires. As a result, I either hard wire the plugs to always charge the same number of batteries or I have to manually adjust which battery the + charge lead goes to.

I've tested this out with a 5x 2S battery rig, but I'd rather have a solution that didn't require I adjust the + terminal each time.

Is there some way I could rig up a simple circuit to detect the presence of a cell and use that to switch where the + lead is hooked up to? For example, in the below diagram, could I use some form of a transistor in a SPDT setup for each of the |---| lines below?

-C1+-C2+-C3+-C4+
|  |   |   |   |
|  |       |---|
|      |-----|
|  |------|
-     +

• You could use relays to do the switching... – Majenko Dec 6 '16 at 23:28
• Can you put an extra connector-pin on the battery connectors, or use a mechanical switch for battery sensing for that matter? – ThreePhaseEel Dec 6 '16 at 23:40
• My worry about relays is that they'd be too slow - I think. When I plug in C4, I'd need to minimize the time that + is hooked up to C3+. They'd also draw too much power, as I don't want to be discharging the batteries just to support this. – Jeff Wight Dec 7 '16 at 22:54
• As far as the extra connector, that's what I do today, but I'd like to eliminate the manual part of that process, as it can still result in me hooking up 3 batteries but leaving the pin / switch connected to C2+ (when it should be C3+). – Jeff Wight Dec 7 '16 at 22:55