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Equalizing deep cycle batteries

by Tom Jones

Have you noticed your deep cycle batteries need recharging more often? Before replacing them, try equalizing them.

If a deep cycle battery isn't discharged and charged very often, lead sulphate crystals can form on the plates, effectively reducing the plate area and thereby reducing the effective capacity of the battery. The user will notice that the batteries don't last as long before recharging is required. Another thing that can happen when the battery sits in one place for long periods is that the specific gravity of the electrolyte stratifies i.e. its value varies at different heights within a battery cell. The sulphate crystals can be removed and the specific gravity de-stratified by a process known as equalization.

If you have a hard-wired inverter/charger it's probable that it has a built-in equalization feature, but you'll need to initialize it manually through the control panel.

The equalization process, which should only be done on flooded wet cell batteries, involves applying a controlled over-voltage (14-17V) to the batteries while limiting the current. The batteries should be fully charged before initiating the equalization process.

One caveat - disconnect any sensitive DC loads before intiating the equalization process because they may be damaged by the over-voltage.

To help prevent the loss of battery capacity, you should perform equalization periodically. The "correct" interval is a matter of debate, but can be anywhere between one and four times a year.

The following conditions of use can reduce or even eliminate the need for equalization:

  • Never (or rarely) allowing batteries to discharge below 50% of their capacity.
  • Maintaining a small charge on the batteries when not being used will help reduce the accumulation of sulphates.
  • Normal driving will continually mix the electrolyte, thereby preventing stratification of the specific gravity.

The following conditions of use can increase the need for equalization:

  • Allowing batteries to sit unused and without any form of charge for extended periods allows sulphate to accumulate on the plates.
  • An RV that is seldomly driven will mean that the electrolyte is not continually mixed, resulting in stratification of the specific gravity.

Equalization should only be necessary for wet-cell (flooded) lead-acid batteries, although there are conflicting reports of AGM batteries requiring and being capable of equalization. Check your battery manufacturer's instructions and your inverter/charger manufacturer's instructions for the correct procedure in your situation.