How To Charge A Car Battery With A Portable Charger. Make sure the charger is off. Generally, there are three ways to charge a portable charger:
If it's a stationary charger system, plug it in before turning it on. Attach the charger's red clamp to the battery's positive terminal and the black clamp to the negative terminal. Select the type and size of the car battery that you have and then press the charge button.