Lead Acid Battery Pulse Charge Algorithm

This is an adaptation of Three Phase, Intermittent Charge and Interrupted Charge Control algorithms in an attempt to improve efficiency for chargers where there are a number of batteries under charge. As with those chargers, the gassing voltage is avoided as far as possible to reduce the effects of overcharging, particularly important for SLA batteries.

The algorithm is aimed at a battery bank in which each battery is charged individually. It keeps batteries always under charge until all have passed to the float stage. An Intermittent Charge mode is used to bring the batteries as close as possible to full charge, with the Three Phase algorithm absorption phase used at the final stage to complete the charging and bring each battery to float phase. Normally the charger is turned off in float phase and reconnected when the battery charge drops to a specified limit.

  The algorithm is described as follows:

  • A selected battery is charged until it reaches the gassing voltage. It then passes to rest phase.
  • Each of the remaining batteries in turn are then charged to the gassing voltage and enter a rest phase.
  • When all batteries are in rest phase, the first battery is placed back on charge if it has been in rest phase for a specified minimum time.
  • If a battery enters rest phase and no other battery is ready to accept charging, the battery continues to be charged at the gassing voltage. This persists for a limited time, sufficient to allow the voltages and currents to settle.
  • The average current over the constant voltage charge cycle is computed and float phase is entered if it falls below the float current limit (or if it persists without falling for a number of hours).
  • A number of minor conditions deal with the case where the charging voltage falls away (eg for a solar power system), and to handle situations where the charging voltage becomes excessive.

(Note that the term "float phase" is used although in reality the charger is removed from the battery until the terminal voltage falls below the fully charged value).

It is not known if this algorithm will fully charge the batteries, any more that any other algorithm can guarantee. The termination condition is similar to that of the three phase algorithm and relies on manufacturers' recommendations regarding the level of gassing that is acceptable at the gassing voltage. The termination condition for this algorithm attempts to draw on that recommendation by measuring the average current over each cycle. The battery is kept at the constant gassing voltage by means of a first order heuristic control that simply pulls the voltage back to the target voltage.

This algorithm will still have periods where the charger is idle, but the efficiency is expected to be better than the other algorithms, including the three phase charging algorithm where the charger is underutilised during the absorption phase.

Initial tests with the algorithm show promising results. For three 50AH car batteries charged from a solar panel the following voltage curves were obtained:

Each of the three lines shows the terminal voltage of a battery placed on charge alternately. The yellow line shows a battery initially on bulk charge. The blue line is a battery nearly at full charge that passes through three absorption cycles until it reaches the float phase. The red and yellow batteries alternate between rest and absorption phases until the red battery reaches float phase. The yellow battery which was at low charge initially continues on in the absorption phase. Note the peaks well above the 14.6V gassing voltage at the start of the absorption phase. These occur because the battery is placed on bulk charge initially to bring it up to the gassing voltage, however because the batteries are nearly fully charged the process is very rapid and an overshoot occurs before the voltage can be dragged back by the constant voltage tracking algorithm. The peaks settle within about 10 seconds as the following trace of a closeup of the early stages shows.

These curves show the batteries swapping between bulk charge and rest phases before moving into absorption phase. The minimum time in rest phase is 30 seconds and in absorption phase 90 seconds.

First created 13 October 2014
Last Modified 28 December 2014
Ken Sarkies 2014