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A motor connected directly to a battery will pull current whilst driving, and automatically charge the battery while "undriving" or going down hill. that presumes you don't have a switch disconnecting the battery from the motor.You can only get current to flow through a motor when a load is presented to the motor shaft. If you "drive" the motor shaft mechanically harder than it wants to go, you reverse the current flow, and the battery starts to charge.Remember that putting 100watts into a battery only gets about 80 watts stored, and demanding 80 watts from a battery requires around 100 watts.Charge and discharge always has a cost of about 20%. You only get a portion out from what you put in - The loss is heat on the battery.