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ASK THE EXPERT

Can specifying a larger charge controller result in faster charging/recharging for lead acid batteries?

Collins: No, and here’s why. A common misconception regarding VRLA (Valve Regulated Lead Acid) is that they recharge quickly. While it is true VRLA can provide substantial discharge currents, on the order of 1 Amp of discharge current for every Amp-hour of capacity, charge currents must be limited to a value much less than this. 

There are two basic reactions that can take place when charging a VRLA battery. One action results in the battery acquiring charge, which is a good thing. The other reaction results in electrolysis, the conversion of water into hydrogen and oxygen, and its not a good thing. The idea is to avoid electrolysis when possible. 

Most VRLA batteries contain a catalyst that combines the hydrogen and oxygen back into water. But this process creates heat and pressure that may result in venting and some loss of water and gas. The loss of electrolyte leads to loss of capacity and eventual battery failure.

How do you avoid an electrolysis reaction? 

Keep the charging rates at a low value -- a value of C/10 is good number. So, for a battery with a 100 Amp-hour capacity, “C”, keep the charge rate at 10 Amps (100/10) or less. The next factor is to employ a precise charging algorithm. Quality solar chargers often employ multistage charge routines designed to minimize overcharging while maximizing charge efficiency. AC powered chargers often employ a “Constant Voltage” scheme to minimize battery wear. In either case, be sure some form of charge tapering takes place as the battery approaches full charge. Quality solar chargers and AC powered float chargers will both accomplish this.

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Rob Collins is a Research and Development Engineer at Solarcraft, Inc.

"Ask the Expert" is a monthly series on power solutions inquiries. If you have a question you would liked answer, please CONTACT US today. 

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