Upgrading batteries with intention to extend driving range, speed, and torque
Pros: By placing batteries in parallel you can increase motor torque, and driving range; and very minimal speed.
Cons: Voltage drop per battery is halved. Li batteries (which should never get completely empty), can undervolt beyond repair.
Pros: When connecting a low voltage, high amp battery, or cell, with the vehicle's battery in series, you can increase torque and speed, and in small ways driving range.
Cons:Possibility of overvolting onboard electric/electronic material, including light bulbs, controller, and meters.