CAUTION!
Sealed or "maintenance-free" batteries need no equalization charging.
Equalizing these batteries can cause fire or explosion.
The equalization process can be visualized as batteries being buckets with electricity being the water filling the buckets. One bucket is one battery cell.
With a few dozen charge cycles every few weeks, the energy flows in and out of the battery many times. It is the same as removing 2 to 5 cups of water from every bucket or cell; that represents the average electrical load consumption each day.
If our PV array generates sufficient energy to put back 3 cups of energy into the cells every day, we would have either a small deficit or extra volume.
If this process is repeated by removing (consuming) different volumes of water with 3 cups added (produced) every day over a couple of months, then the mathematics will show that the batteries are fully charged at some point – the buckets are full.
However, in reality, the water level in every bucket differs. There would be some spillage and removal of uneven amounts of water from individual buckets: the same thing happens with your battery bank. As time goes by, the battery cells will change their charge states. You can determine their current state through a comparison of the cells' specific gravity.
If the difference is not corrected quickly, cells that are low in charge store less energy and will be subject to earlier damage. It is crucial to perform cell-by-cell hydrometer readings regularly to take corrective action. This action, in practical terms, is a periodic, controlled over-charging called the equalization charge.
The equalization charge process is performed early on a sunny or windy morning to tap the renewable energy source. You can also use fossil fuel energy if you do not have a renewable source. Set the charge controller to equalization mode and stages 1 and 2, the normal bulk charge and absorption charge respectively, will be completed. In equalization mode, the controller increases the battery voltage to 15.5V and maintains it for 2-3 hours; the battery voltage is then lowered before the charge controller goes into float mode automatically (Stage 3).
Referring to the bucket example, the equalization effect is like overfilling your buckets deliberately, using a garden hose. When you turn off the garden hose (equalization completed), the buckets are all filled to the rim with water.
Of course, the overflowing buckets have excess water splashing out all over the floor, so what about the excess electricity during equalization? During charging, the excess energy to the batteries will create violent bubbles in the electrolyte where huge amounts of oxygen and hydrogen, heat and water vapor are emitted. It is crucial to monitor the electrolyte levels as well as the battery temperature at this charging stage to make sure that the battery parameters are inside manufacturer ratings.
CAUTION!
If your batteries have hydrogen reformer caps,
remove them during equalization to avoid damage.