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Sulfation in lead acid batteries

Battery sulfation and battery life

Battery sulfation is the #1 cause of car battery failure and the most common cause of sulfation is severe discharge and long period of non-use.

What is battery sulfation?

When a lead-acid battery sits in a discharged state, whether due to leaving lights on all night or leaving your vehicle unused for long periods of time, the lead plates develop a coating of lead sulfate. The longer the battery sits unused in this state, the more the plates accumulate lead sulfate. And, the deeper the discharge, the worse the sulfation becomes. If the battery is left in this state for long periods, the lead sulfate begins to crystallize. The lead sulfate crystals clog the porous plates to the point where the battery will no longer accept or hold a charge.

This sulfation process happens in all lead-acid batteries, whether the battery is used in automotive, standby power, mining, trucking, or marine industries.

Early sulfation is reversible, late crystalization is not reversible

Lead sulfate is a chemical reaction between the lead plates and the sulfuric acid
during the normal discharge of a lead-acid battery. When the battery is recharged, the
lead sulfate dissolves back into the electrolyte. But a small fraction of the sulfate remains on the battery’s plates and hardens the lead sulfate on the battery’s plates. This hard sulfate can not be dissolved back into the electrolyte through ordinary battery

Lead sulfate has very weak electrical conductivity, so it acts as an insulator to prevent recharging and further discharging. In fact, the lead sulfate deposits cover both the negative and positive plates so much that over time they become inactive to the point where the effective capacity of each cell is reduced to around 10% of its normal capacity. The harder the crystal, the more it insulates, preventing recharging and discharging. Studies show that 90% of severely sulfated batteries cannot be recharged and must be replaced.

The chemistry behind battery sulfation

Batteries produce power through a chemical reaction between the lead plates and the electrolyte (battery acid). Car battery acid is a diluted solution of sulfuric acid H2SO4. In simple terms, the battery acid provides the sulfate, hydrogen, and oxygen needed to interact with the plates to produce power.

When a car battery is discharged through use or from self-discharge from non-use, the sulfuric acid in the electrolyte combines with the lead (Pb) to form a coating of lead sulfate (PbSO4). The sulfate crystals reduce the plate’s active/reactive surface area, which, in turn, reduces the battery’s ability to produce power.

Worse yet, the sulfate crystals act as an electrical insulator, increasing the battery’s internal resistance. When fully sulfated, the battery acid has given up most of its sulfate and hydrogen. At that point, all that’s left is water H2O. That’s why discharged car batteries freeze in cold weather.

If caught early enough, recharging can partially reverse the process

During recharging the alternator pushes the sulfate and hydrogen off the plates and forces it back into solution, turns the H2O back into H2SO4. However, that only occurs if the sulfate crystals are soft.

However, if the sulfation is ignored and left in place for weeks or months, large portions of the lead plate surface become permanently unavailable. That dramatically reduces the battery’s ability to produce starting power. It may fail to produce enough power to operate the starter in warm weather and may even fail to provide enough power to light the lights in cold weather.

The longer a battery sits in a discharged state, the more likely the battery is permanently damaged. As a battery sits in the discharged state, the sulfate crystals grow in size and harden. After a certain period of time, they’re impossible to remove, even with recharging. At that point, the battery must be replaced.

What causes car battery sulfation?

• Deep discharge—like leaving your lights on overnight
• Self-discharge—all lead-acid batteries self-discharge about 1-2% per day, depending on the storage temperature (faster in higher heat).
• Long periods of discharge— The longer you leave a battery in a discharged state, the more the sulfation process becomes a problem.
• Faulty charging system—If the alternator isn’t working in your car, you’ll draw more power

Can you desulfate a battery?

If the sulfation is the reversible soft crystal type you can reverse it by recharging. But the recharging process is somewhat tricky. For example, a battery charger that’s equipped with a six-phase battery reconditioning routine may start by recharging process at a low voltage, around 4-volts and 7-amps. That’s called the energizing phase. Why low voltage? Because a sulfated battery has developed high internal resistance. If you hit it with high voltage right off the bat, that high resistance will create HIGH HEAT, and that heat will warp the plates and short out the battery.

Next, the charger boosts the charging current to around 13.5 amps while slowly ramping up the voltage to a peak of around 14.5 in phase 3. In the third or absorption phase, it maintains constant voltage while ramping down the amps. The 4th phase drops the voltage slightly and the amperage a bit further. In the 5th or resting phase, it drops current flow to 0 while maintaining around 12.5 volts. The 6th phase exercises the battery by pulling a load on the battery until it reaches the final restoration phase where it raises amperage and voltage one last time. The key to reconditioning a battery after sulfation is to prevent overcharging and overheating.

If the sulfation crystals have hardened, there’s no way to reverse the process and the battery must be replaced and recycled

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat


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