Deep cycle batteries are designed for extended load times and with proper maintenance can last for years before requiring replacement or repair. Most deep cycle batteries are lead-acid, zinc plate designs that are similar in make-up to a standard car battery. The long draw time, or being able to cycle the charging and loads "deeply," gives these...
M/F Test the battery acid level with a dropper tester by drawing up some of the sulfuric acid through the battery's top access holes. The holes are capped with individual plugs, or a three-hole plastic plate. The colored balls in the dropper will rise or fall depending on the acid ratio of the fluid. The most efficient acid level is between 30 and 40 percent sulfuric acid in 60 to 70 percent water. When this level goes up or down, the battery could become damaged and not charge correctly.
M/F Align the batteries into "banks" to raise the effective load time so that the total charge never drops below 80 percent. When deep cycle batteries are discharged to less than 50 percent of their total charge capacity, they wear out more quickly and lose the ability to stay charged. Discharging below 20 percent will damage the battery permanently and shorten its lifespan considerably. Placing multiple batteries into a parallel configuration gives more amp hours and can take surges and long-term loads without discharging below the damage threshold. Connecting them in series will boost the voltage, not the amps.
M/F Remove "bad" batteries from a bank quickly, as they can damage the other deep cycle cells. If a battery fails the acid test, remove it from the bank to add acid or water. Battery construction can be at fault, and sometimes the plates (at the core of the battery) can have chemical, corrosion or electrolyte issues that prevent them from charging, regardless of the acid level. Taking these damaged batteries out of a bank will prevent the other batteries from developing problems.
M/F Keep the deep cycle battery off of a grounded surface with a riser or box. Very rarely, batteries can "ground out" to a concrete or soil surface and lose their capacitance, or develop corrosion quickly due to contaminates. Building a small rise out of wood or plastic can keep a battery from reacting with the ground.
M/F Prevent the deep cycle batteries from surging for more than a couple of seconds. Starting a car off of a deep cycle battery is not recommended, unless it is a "hybrid" or marine cell. Heavy, short-term loads can quickly destroy a deep cycle's ability to store energy.