Lead Acid Battery Three Phase Charging Algorithm
Also known as "Constant Current Constant Voltage" this is the most popular algorithm. However it has some drawbacks due to its tendency to overcharge the battery, and its failure to deal effectively with oxygen recombination in aging VRLA cells. In this algorithm the battery is charged at a high current, limited to prevent overheating of the battery, followed by a constant voltage stage until the current drops to a low level. The high current stage is referred to as the "bulk" charging phase, while the second is the "absorption" phase in which the battery is held at a slightly overcharged point. When the current has dropped to a low level the "float" phase is used to maintain battery charge against self-discharge.
A fourth equalizing phase may follow the absorption phase. This is used with wet cells. The battery is heavily overcharged for a short time to allow the component battery cells, which tend to vary in the amount of charge at the end of the charging cycle, to all come equally to full charge.
The following charging states are
specified by manufacturers:
This charge algorithm is very simple and is the basis of most manufacturers' specifications. It does run into difficulty however when the battery is feeding a load while being charged, as the measurement of battery terminal voltage is distorted by the unknown voltage drop from the highly variable internal characteristics of the battery. The battery is usually modelled by internal resistances, a bulk capacitance representing the major storage component and a surface or diffusion capacitance. The latter models the phenomenon whereby the terminal voltage drops away slowly after a charger is disconnected, sometimes taking several hours to stabilize.
The charger parameters should be adjusted separately for each battery according to the manufacturer's specifications. These may appear to vary only slightly from each other but the differences can have a significant effect on battery life. If specifications are not available, use the lowest values given for the general type of battery (flooded, gel, AGM, etc).
accurate on-line measurement of battery "state of charge"
(SoC) would allow a
battery charger to determine how to adapt its charging
precisely. This is a goal that has yet to be achieved. SoC
determined in principle by measuring the open circuit
of the battery, but only after it has been disconnected
and allowed to
settle for 24 hours or longer. This is usually impractical
operational system. A variety of estimators have been
tried based on integration of measured current flows,
Kalman filters and even fuzzy logic, however each falls
short of producing accurate results under all possible
A stand-alone battery charger is able to make use of the much simpler terminal voltage and current measurements. For more complex systems in the presence of variable loads, a knowledge of the battery capacity coupled with integrated current measurements can provide the basis for determining charge state.