Improved charge algorithms for valve regulated lead acid batteries

The cycle life obtained from valve-regulated lead-acid (VRLA) batteries is strongly influenced by the manner in which they have been charged over their lifetime. Although VRLA batteries initially behave similarly to their flooded counterparts, that behavior changes as the batteries age and the oxygen generation/recombination cycle begins to dominate at near 100% full charge. This means that an increasing portion of the applied charge is consumed in the recombination cycle and that more and more overcharge must be applied to maintain full capacity. The overall result is that the battery heats up because of increased overcharge and oxygen generation. Conventional charge approaches attempt to deal with rising temperatures by lowering the current during the overcharge phase. However, this approach does not ultimately prevent capacity loss, and a battery charged thusly typically will yield 200-300 cycles to 50% of initial capacity. The main failure mode appears to be undercharging of the negative plate, not positive-plate corrosion. Two approaches, called partial state of recharge (PSOR) and current interrupt (CI) were successful in extending battery life. PSOR uses nine limited recharge cycles followed by a tenth cycle using 120% charge return. The best PSOR cycle life to date is 1160 cycles to 50% and 800 cycles to 80%. CI uses a high current in the overcharge applied discontinuously to control battery temperature. CI effectively maintains negative-plate capacity, with an Optima group 34 deep-cycle battery yielding 415 cycles to 80% initial capacity and 760 cycles to 50%.