Charging 12 volt batteries wired in parallel requires a 12 volt battery charger. Two 6 volt 45 ah batteries wired in parallel are capable of providing 6 volt 9 amp hours 45 ah 45 ah.

How Configure Battery Bank Web
Charging deep cycle batteries in parallel. Batteries wired in parallel increases endurance but it has no effect on the output voltage so charging parallel batteries is relatively simple. Its common in many rvs to make use of pairs of 6v deep cycle batteries wired in series. Make a parallel connection of the deep cycle batteries by connecting all the positive terminals together. Attach the battery post connectors to each top post on the deep cycle batteries. Use the 12 inch end wrench to tighten the clamps bolt. Check the label on the parallel deep cycle batteries to determine the chemical composition.
Two 12 volt batteries wired in parallel continue to produce 12 volts but the capacity doubles. Batteries may be wired in parallel or in series. What this means is that as the batteries discharge voltage on the one with lower capacity will fall faster. One approach is charging the battery in a place where the charging source is fixed and another approach is using a portable charger. Multiply the time anticipated to charge one battery by the total number of batteries. The basic concept is that when connecting in parallel you add the amp hour ratings of the batteries together but the voltage remains the same.
You need to know this to correctly charge them as the method varies by type. Do not over tighten the bolt but make it tight enough so the clamp cannot move on the battery post. 20 gauge excelene welding cable has high flexibility and is rated for 600 volts. Two 12 volt batteries wired in series produce 24 volts but the capacity remains the same as one for a 12 volt battery. Connecting batteries in series parallel to increase battery system capacity. Just connect the clamps according to the directions and its auto sensing circuitry monitors the environment and the deep cycle marine battery to provide a fast charge without damaging your battery.
Not to repeat the best way to charge a deep cycle batteryis to pick up a portable charger what is eligible for this task. When connection 6 volt batteries together it is recommended to use thick diameter cable. In a pair of 6v batteries in series the voltages of each are not guaranteed to be the same as they are when wired in parallel. You can charge the deep cycle battery in two methods. For example if charging one battery normally takes three hours and you have five batteries linked in parallel the charging time will be five times three hours or 15 hours. The best way to charge a deep cycle marine battery is to get a charger designed for the task.