Early charge controllers were crude: generally little more than voltage-actuated on/off relays. Modern charge controllers have substantially improved battery charging and longevity.
The charging process for flooded cells involves four steps: bulk, absorption, float, and equalization. During the bulk phase, which generally fills the cell to around 85% of its capacity, the charge controller (from a PV, wind, or hydro source) or the inverter (from a generator or AC source) allows all available charge current to flow into the cells. As the current is absorbed by the plates in the cell, the cells’ voltage steadily increases. When the voltage reaches the bulk voltage set point (typically about 2.45 V per cell), the controller moves into the absorption stage. During absorption, the voltage is held at the bulk set point, and the charge is regulated to the current necessary to maintain that voltage (plus power any loads that are on).
When any flooded lead-acid battery approaches full-charge voltage, the cells begin to “gas.” The cells are no longer able to absorb all of the energy, and the excess energy separates water in the cells into hydrogen and oxygen gases. Gassing is an important part of the charging process: the process brings weaker cells closer to the charge level of stronger cells, and the bubbling action destratifies the electrolyte.
As the cell approaches 100% SOC, the amount of current necessary to maintain this voltage steadily drops. When either a preset time duration (typically 2 to 4 hours) is reached, or the charge current drops below a set threshold (typically a C/50 rate, or about 2% of a healthy cell’s capacity), the cell is considered fully charged, and the controller moves into float stage. During float, a tiny amount of current holds the cell slightly above its resting voltage, and the charging process is finished until the next charging cycle.
Equalization is the periodic, deliberate overcharge of a full battery. It stirs up the electrolyte, breaks up light sulfation (which is what eventually wears out a battery), and evens out the chemical state of charge in each cell. It generally requires taking the full cell up to about 2.6 V and holding it at or above this threshold for several hours.