Deep cell batteries are more commonly referred to as deep-cycle batteries. They are usually flooded-cell lead-acid batteries, but innovative battery technology has introduced gel and absorbed glass mat (AGM) lead batteries. These batteries are considered safer because acid can’t leak from the battery if the casing gets damaged. Batteries wired in parallel increases endurance but it has no effect on the output voltage, so charging parallel batteries is relatively simple.
Check the label on the parallel deep-cycle batteries to determine the chemical composition. You need to know this to correctly charge them, as the method varies by type. The label will say flooded or wet cell lead-acid, AGM or Gel. If the battery label says either of the first two, you can use a regular lead-acid battery charger, but if it says AGM or Gel, use a specifically designed Gel charger, or a regular charger with variable charge settings.
Attach the battery charger's cable clamps to the terminals of one of the batteries connected in parallel. It’s important the clamps are attached to the two terminals on the same battery. The red handled clamp attaches to the “Pos” or “+” terminal and the black handled clamp attaches to the “Neg” or "-" terminal.
Select the charge setting, if it’s variable. You can usually select a fast charge or trickle charge. Charge time varies widely with the size and power of the battery but a trickle charge takes twice to three times as long, in general. If using a regular lead-acid battery charger to charge a Gel battery, set it to trickle charge or the battery will be seriously damaged.
Charge the batteries. The more batteries connected in parallel the longer the charge will take. For example, if fast charging one battery usually takes about an hour or two, than two batteries connected in parallel takes two to four hours. The same applies to charging a Gel battery, but a specifically designed Gel charger automatically detects when the batteries are charged to stop overcharging. If you are trickle charging Gel batteries using a regular charger, then charge two batteries for about 18 hours and three for 24 hours. Don’t leave them any longer and regularly touch the side of the batteries to ensure they don’t get hot. If they do, turn off the charger and let them cool before continuing to charge.
Turn off the charger after the prescribed time. Disconnect the cable clamps.