When two 12-volt automobile batteries are wired together in parallel, the unit maintains the same voltage as either individual battery, but the capacity of the unit is double that of either battery. This means that it will take twice as long to charge the unit as it would take to charge one of the batteries separately, but you can shorten this time by increasing the amperage of the charger. Make sure your charger has a controller so that it will switch off the charging current when the batteries are charged, or you will damage them.
Charge batteries on temperate or warm days, or charge them indoors. Ideal charging temperatures are those higher than 15.6 degrees Celsius. Never try to charge frozen batteries.
Connect the positive terminals of the batteries to each other with a battery cable then connect the negative terminals in the same way. It is best if you use cables with a lug that will fit tightly over the terminals since you will also have to clip on the charger to one of the batteries. Make sure both cables are the same thickness and length.
Attach the cables of the charger to the terminals of one of the batteries, making sure the red cable is connected to the positive terminal and the black cable to the negative.
Set the charger voltage to 12 volts and the amperage to the lowest setting. Plug the charger in; when the batteries begin charging, increase the amperage to 10 to 20 amps. Let the batteries charge for six to eight hours or until the charging meter indicates that they are fully charged.
Disconnect the batteries from the charger when they are fully charged.
It is always a good idea to check the condition of the batteries with a hydrometer before charging them. Take the opportunity to check the water level when you do this and add distilled water if needed.
Batteries release harmful and flammable gases while they are charging, so make sure you charge them in a well-ventilated location.