Arduino mini, Nidec 24H motors, MPU6050, 3S 500 mAh LiPo battery.
Balancing controller can be tuned remotely over bluetooth.
Example (change K1):
Send p+ (or p+p+p+p+p+p+p+) for increase K1.
Send p- (or p-p-p-p-p-p-p-) for decrease K1.
About schematic:
Battery: 3S1P LiPo (11.1V).
Buzzer: any 5V active buzzer.
More about this:
This video may also help: