According to Happy Living, a car battery should be recharged if it has been in storage for a long period of time, if it has been accidentally drained or if the battery is ageing and simply needs a boost back to heath. Indoor charging is not much different from charging the battery outdoors; however, take precautions when charging a battery indoors as sparks can easily ignite if battery recharging is done improperly.
Wear safety glasses. Protect your eyes from possible diluted sulphuric acid solution that can spark in reaction to the battery's flammable hydrogen gas.
Place the battery charger on a concrete, flat surface away from flammable materials and wood. Use the charger in a garage or basement where it is away from household furnishings.
Place the volt setting of the battery charger on 12 volts and choose a low amp setting that will charge the battery slowly. Slowly charging the car battery will desulfate the crystallisation of lead sulphates, providing a higher amperage output return.
Select the correct battery-type setting on the charger if applicable. Most modern car batteries use a conventional/low-maintenance battery. Use a compatible AWG gauge extension cord if necessary to reach the garage electrical socket.
Apply the red positive alligator clip to the positive terminal end of the battery first. Place the black negative alligator clip onto the non-painted metal bracket. Activate the charger. Allow the battery to charge for six to eight hours. Never charge the battery over 8 hours (unless directed by battery-charger manual), as it can damage the internal components.
Test the battery's voltage with a voltmeter and set its dials on the 0-50 V range. Disconnect the battery charger from the battery. Connect the voltmeter's positive red lead to the battery's positive terminal. Connect the black lead to the negative terminal. Check the voltage level; if the battery has a range of 12.6 to 12.8, the battery is charged. Keep charging the battery if it has a voltage under 10 V.
When charging the battery indoors, keep the battery as far away from the charger as its clamp wires will allow to prevent sparks. Maximise charging by cleaning the rust and debris off the battery's terminals before charging.