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.

  1. Bulk phase. This is normally current limited for the purposes of reducing ohmic losses and high battery temperatures, although in principle there need be no limit. Manufacturers may specify a limit of around C/3 (see for example the comprehensive Diamec Technical Manual). This proceeds until the terminal voltage reaches the limit specified by the manufacturer at which gassing begins to occur. High current charging will deliver about 70%-80% of the charge to the battery. This is higher for lower charge rates, typically with C/15 delivering most of the necessary charge.
  2. Absorption phase. This keeps the battery close to the gassing point while additional charge is topped up. It can be stopped when the current drops to about C/50. This phase can take as long as the bulk charge phase due to the lowering current.
  3. Equalization phase.
  4. Float phase. This attempts to keep the battery at a low charging current to replenish self-discharge. For the lead acid battery such discharge is generally very small and the float phase can cause long term overcharging.

The following charging states are usually specified by manufacturers:

  1. Below 70% of full charge the battery is in the bulk charging phase. The current is limited to a specified value and the voltage is allowed to increase. The current limit depends on the battery type and is typically about 0.4C for SLA batteries, where C is the battery capacity in ampere-hours.
  2. Above this point the battery is in the absorption charging phase. At this point the voltage is held at a maximum value, typically 2.5V per cell for SLA batteries (generally lower for other types), and the current is allowed to decrease. Charging stops when the current falls to a suitably low value, typically about 0.05C.

  3. For some batteries a short equalization phase is required where the voltage is raised above the maximum for a short time.

  4. Finally an optional float charge phase maintains the battery charge against internal leakage with a terminal voltage of 2.3V per cell.

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).

The accurate on-line measurement of battery "state of charge" (SoC) would allow a battery charger to determine how to adapt its charging conditions more precisely. This is a goal that has yet to be achieved. SoC could be determined in principle by measuring the open circuit terminal voltage of the battery, but only after it has been disconnected and allowed to settle for 24 hours or longer. This is usually impractical in an 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 conditions.

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.



First created
13 October 2014
Last Modified 13 October 2014
Ken Sarkies 2014