Battery Bank Equalization

Routine equalization cycles are vital to the performance and life of a battery — particularly in a solar, wind or micro-hydro power system. During battery discharge, sulfuric acid is consumed and soft lead sulfate crystals form on the plates.

If the battery remains in a partially discharged condition, the soft crystals will turn into hard crystals over time. This process, called “lead sulfation”, causes the crystals to become harder over time and more difficult to convert back to soft active materials. Sulfation from chronic undercharging of the battery is the leading cause of battery failures in solar systems. In addition to reducing the battery capacity, sulfate build-up is the most common cause of buckling plates and cracked grids.

Deep cycle batteries are particularly susceptible to lead sulfation. Normal charging of the battery can convert the sulfate back to the soft active material if the battery is fully recharged. However, a solar battery is seldom completely recharged, so the soft lead sulfate crystals harden over a period of time.


Slowing or preventing lead sulfation – Only a long controlled overcharge, or equalization, at a higher voltage can reverse the hardening sulfate crystals. Normal charging will reduce sulfation, but to remove it, the battery must be equalized.

Balance the individual cell voltages – Over time, individual cell voltages can drift apart due to slight differences in the cells. For example, in a 12 cell (24V) battery, one cell is less efficient in recharging to a final battery voltage of 28.8 volts (2.4 volts per cell). Over time, that cell only reaches 1.85 volts, while the other 11 cells charge to 2.45 volts per cell. The overall battery voltage is 28.8V, but the individual cells are higher or lower due to cell drift. Equalization cycles help make all the cells the same voltage.

Mix the electrolyte – In flooded batteries, especially tall cells, the heavier acid will fall to thebottom of the cell over time. This stratification of the electrolyte causes loss of capacity and corrosion of the lower portion of the plates. Gassing of the electrolyte from a controlled overcharging (equalization) will stir and remix theacid into the battery electrolyte.


The ideal frequency of equalizations depends on the battery type (lead-calcium, lead-antimony, etc.), the depth of discharging, battery age, temperature, and other factors. One very broad guide is to equalize flooded batteries every 1 to 3 months or every 5 to 10 deep discharges. Some batteries, such as the large L-16 type, will need more frequent equalizations. The difference between the highest cell and lowest cell in a battery can also indicate the need for an equalization. Either the specific gravity or the cell voltage can be measured. The battery manufacturer can recommend the specific gravity or voltage values for your particular battery.


NO, NO, NO! Sealed batteries do not tolerate equalization cannot have distilled water replaced by the equalization charge, they are not able to gas (vent) and WILL be damaged if equalized.