2 of six boards must have their 3 wires connected to the front hub boards. Without them, a battery connected to the hub boards puts the display into charging mode. (The display lights up automatically and shows the charge symbol and the flashing gauge.) No way to get out of the false charging mode. But...the 6v (5 x 1.2v) power connection to the sense boards is not needed for the bike to run normally. That makes connecting to an external battery, especially a removable battery, much easier.

Below is a picture of my E+. The battery hub boards are in the water proof box behind the seat post. The bike mounts four 18V 10.5ah tool batteries (2s2p). Only 2 are required. Just 2 surpass the WHs and max amps of the original NiMH battery. Using 4 more than double the range since the cells are barely stressed even in a hard start, Level 9 (~1300 watts).