Leisure batteries, otherwise known as marine batteries or deep cycle batteries are commonly used for boating, motor-home, solar power storage mobility vehicles and powered golf trolleys.
They are substantially different from your conventional auto battery. Although similar in size, weight and output voltage, they are specifically designed to deliver moderate levels of current over a sustained period. Your auto alternative, on the other hand, is designed to deliver a very high current for a short period of time. Leisure batteries are also designed to be discharged to around 50% of their capacity, whereas your auto version is seldom discharged below 90% capacity; as it is always being trickle charged when your car is running.
Using an auto battery under leisure battery conditions will result in a very short battery life. The fundamental difference in behaviour and performance is due to the construction of the battery plates and the separators between the plates. Leisure batteries have thicker plates and separators and therefore lose less active materials during the charge-discharge cycle.
Types of Battery
The most common type of leisure battery is of a lead-acid construction. These come in two main variants:
Flooded batteries, as the name suggests, have the acid freely immersing the battery plates. They require a degree of maintenance; a hydrometer to ensure that the electrolyte strength is maintained within limits and they may also need topped up from time-to-time with distilled water.
Sealed variants require no topping up, and minimal maintenance - other than charging. The acid is contained in a glass-fiber mat surrounding the battery plates. One further variation of sealed battery is the gel battery. Here the acid is combined with micro-silica to form a gel. Gel batteries can withstand higher temperatures, vibration and shock and are well suited to harsher environments such as maritime activities. Sealed variants cannot spill acid when inverted. They also vent less gas and are more suited to enclosed spaces.
Whichever type of leisure battery that you select it is important to realise that these batteries should never be completely discharged or left in a low state of charge for any significant time. Either of states can cause irreversible damage to the battery due to two main causes:
Sulphation is the result of an internal chemical reaction, resulting in Lead Sulphate being deposited on the battery plates, showing as a whitish-gray deposit. In severe cases sulphation is irreversible. In minor cases some leisure battery chargers provide the battery with high current, high frequency pulses to try to break these deposits down.
The majority of 12 Volt batteries comprise 6 cells. If one of these cells becomes significantly more discharged than the others, then it becomes part of the load and the other cells will start to pass current through it, in a direction to cause it to become even more discharged. This causes irreversible damage to the cell and battery and is more likely to occur when the battery is in a low state of charge.
The lifetime of your leisure battery is in inverse proportion to how low you allow the charge level to become. For optimal life you should never allow your battery to get below 50% of its maximum charge. This has a direct effect on what rating of battery you should choose for your needs. Over time your battery will eventually become less efficient, so you can discount 20% of the capacity as compared to the manufacturers rating. Once you have discounted this you should estimate that you have only 50% of the remaining capacity available to you. So a battery rated at 100Ah can be discounted to 80Ah maximum capacity with 40Ah usable capacity.
To calculate how much this will power you must know the power consumption of the appliances being powered, either in Watts (W) or Amps(A).
Watts(W)=Volts(V) x Amps(A)
So an appliance rated at 12W will draw 1 Amp from a 12 Volt battery. The battery rating is in Ah (Amps x hours), so based on our usable capacity of 40AH we could run this appliance for just over 3 hours before having to recharge the battery.