Dual Battery Mod

[htib]

which is negative leg, a or c?

 

[cheddarman]

which is negative leg, a or c?

Me thinks that if you have to ask that, you should not be mucking about with this! Your multimeter will soon answer this!
Also remember that the battery (with its firmware) is all part of the whole firmware package. An "attached" Lipo will not have this firmware!
Just get a replacement battery!

 

[htib]

so the dji phantom 3 battery has firmware on the battery itself?

 

[cheddarman]

From DJI update pages:​

Battery Firmware Update​

If you use several batteries for a quadcopter, do not forget to update the firmware of each of them.

To do this, after the drone/controller update is completed, change the Phantom 3 battery and start the process again – the Phantom software itself will not be updated again, but the smart battery will receive the current software version. The process is worth repeating with each battery that you have.