Deep cycle batteries are expensive and are designed for a long life. If properly charged and looked after, they should last 10 years or more. Their chemistry is quite different from that of car batteries and if you use a charger intended for car batteries, you will definitely not get their maximum capacity.

Furthermore, if deep cycle batteries are consistently under-charged, they will have a short life. By comparison, car batteries are seldom charged above 70% of their capacity but they are designed for "shallow" discharge. If they are subjected to frequent deep discharge, they will have a very short life.

Fig.1: this graph shows the battery voltage during charging. There are three steps to the charging cycle: an initial bulk charge, an absorption phase and then a float charge. An optional equalisation charge phase is also available for deep-cycle batteries.

Deep cycle battery manufacturers specify that their batteries should be charged up to a fixed value called the "cyclic voltage". Once the battery is charged to this level, the voltage must be reduced to the "float" voltage and then it can be left permanently connected to the charger. Continuous charging at the cyclic voltage will damage the battery.

Fig.2: the battery current during charging. The charging current is maintained at 25% of Ah during the bulk charge and then tapers off during the absorption phase. It is then fixed at 5% of Ah during the (optional) equalisation process. When the battery reaches the float voltage, a small charging current maintains it at this level.

The cyclic voltage is usually different for each type of lead acid battery. For example, standard lead acid batteries should be charged to 14.2V and floated at 13.4V, while Gel-Cell (Sealed Lead Acid) batteries should be charged to 14.1V and 13.3V respectively. These voltages are for a battery temperature of 20°C. At higher temperatures, the voltages must be reduced and the amount of compensation is also dependent on battery chemistry. Typically, lead acid batteries require a temperature compensation of -20mV/°C while Gel-Cell batteries require -25mV/°C compensation.

Clearly, a low-cost charger has no means for setting the required cyclic voltage and nor can it provide the float voltage setting or temperature compensation for these voltages.