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Lead-Acid Battery Zapper

This simple circuit is designed to extend the working life of liquid-electrolyte lead-acid batteries, by dissolving the lead-sulphate crystals which form on their plates. It's powered by the battery itself (or by a charger) and "zaps" the battery with a series of high-voltage pulses.

By Jim Rowe

LEAD-ACID BATTERIES have been around for over 170 years now – ever since Gaston Plante built the first one back in 1834. They are used in huge numbers all around the world, mainly in the automotive industry. There’s at least one in virtually every car, truck and bus to start the engine and power ancillary equipment, while multiple lead-acid batteries are also used in many electric vehicles to provide the motive power.

They’re also used in large numbers for energy storage in solar and wind power plants. And by the way, we’re talking about "wet" or liquid electrolyte batteries here (also called "flooded" lead-acid batteries).

The lead-sulphate effect

Although we’d now be lost without them, lead-acid batteries are not without their faults. Probably their main drawback is that they have a relatively short working life, typically no more than about three or four years.

Why is this? Well, every time energy is drawn from a lead-acid battery, lead and sulphate ions from the electrolyte combine and are deposited on the plates in the form of soft lead-sulphate crystals. Then when the battery is recharged, these crystals dissolve again in the sulphuric acid electrolyte.

More accurately, MOST of them re-dissolve – but not all. Even if the battery is never over-discharged and is always recharged promptly after it has been discharged, a small proportion of the lead sulphate remains on the plates. These then harden into "hard" lead-sulphate crystals which are much less soluble and less conductive than before.

In practice, the formation of these hard lead-sulphate crystals grad-
ually reduces the energy storage capacity of the battery. It does this both by masking the active areas on the plates and also by reducing the concentration of lead and sulphate ions in the electrolyte.

This "sulphation" effect has been understood for many years. It’s also well known that the effect occurs much faster if a battery is over-discharged, left in a discharged state for more than a few hours, or frequently under charged. In fact, batteries mistreated in any of these ways tend to have a very short working life indeed.

For a long time, sulphation was regarded as non-reversible and batteries that had lost too much capacity due to this effect were simply discarded. This was not only wasteful but was also an environmental problem, because both lead and sulphuric acid are highly toxic materials.

Around the middle of last century, though, people in rural areas discovered that they could "resuscitate" sulphated batteries by zapping them with high-voltage pulses from their electric fence controllers. They didn’t exactly understand why this method worked but kept using it because it did.

Subsequently, in 1976, the US Patent Office granted a patent to William H. Clark of Salt Lake City, Utah, for a method of charging lead-acid batteries by means of narrow high-current pulses. This was claimed to more effectively dissolve the lead sulphate crystals and hence prolong battery life. Since then a number of designs for pulse-type battery rejuvenators or "zappers" have appeared in electronics magazines, including one published in SILICON CHIP (Circuit Notebook) in February 2003 .

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