Desulfating Lead Acid Batteries with High-Voltage DC Charging: What the Research Shows

ByThe Answerlode Editorial Team

The Science Behind High-Voltage Lead Acid Battery Restoration

Sulfation—the accumulation of lead sulfate crystals on battery plates—is the most common cause of premature lead acid battery failure. Once these crystals harden, they block the chemical reactions needed for charging and discharging. Traditional wisdom held that only expensive pulse desulfators could break them down, but a 2023 study published in the journal Energies challenges this assumption.

Researchers Chee Hiun Lee, Jianhiu Wong, and Yun Seng Lin tested a simpler approach: applying constant overvoltage through a basic DC power supply.

How the Study Worked

The researchers tested 30 discarded 7Ah flooded lead acid UPS batteries using overvoltage charging at 2.6 volts per cell (15.6V for a 12V battery). They selected this voltage carefully—higher voltages damaged the batteries, while 2.67V/cell caused excessive electrolyte loss.

The protocol was straightforward: apply 15.6V for 24 hours, then follow with a normal constant-voltage charge for 48 hours. Current started low and gradually increased, eventually reaching about 0.5A on the small test batteries.

Results were compelling. Many sulfated batteries recovered to 70–80% of original capacity. Microscopic examination showed that the lead sulfate crystals had actually dissolved and reconverted to active lead dioxide and lead on the plates.

Why Voltage Matters for Lead-Calcium Batteries

Modern maintenance-free batteries use lead-calcium alloy grids instead of pure lead. This change reduces self-discharge (to 2–10% per month instead of 15–20%) and gassing, but it makes these batteries more prone to irreversible calcium deposits if charged incorrectly.

The 2.6V/cell threshold appears to work across different lead-calcium formulations, though the exact voltage tolerance varies by manufacturer. This is why contacting battery makers about their specific alloy composition can help refine your approach.

How This Compares to Pulse Desulfators

Commercial pulse desulfators apply high-frequency electrical pulses to break sulfate crystals. The MDPI study suggests that simple, sustained overvoltage may be equally effective—at least for reversible sulfation—without the cost of specialized equipment.

However, pulse desulfators offer one advantage: they work on batteries still in service. Overvoltage charging requires removing the battery and accepting gassing and heat generation during the process.

Real-World Application: Temperature and Safety

The test batteries reached approximately 45°C (113°F) during the 24-hour overvoltage phase. Scaling up to larger batteries (like your 198Ah units) will generate significantly more heat. This is actually a sign that sulfate dissolution is occurring, but exceeding 50–55°C can damage the plates and separators.

Charging in series, as you are doing, distributes current more evenly across batteries. At 10A across four 198Ah batteries in series, you’re applying roughly 0.025A per Ah—similar to the study’s approach of 0.5A per 7Ah.

Monitoring voltage and temperature during the process is essential. If any battery begins to bubble excessively or overheat, reduce current immediately.

What the Study Doesn’t Answer

The research focused on flooded batteries with specific lead-calcium alloys. The exact composition of your batteries matters: different manufacturers use different ratios of calcium, tin, and other hardening elements. The study authors didn’t publish alloy specifications, so contacting them (or the original battery manufacturer) could provide crucial details for optimizing your process.

Also, the study tested small UPS batteries, which have different thermal characteristics than large solar or stationary batteries. Your 198Ah batteries will dissipate heat differently and may tolerate sustained overvoltage differently.

Next Steps After Overvoltage Treatment

Following the study’s protocol: after 24 hours at overvoltage, switch to normal constant-voltage charging at 14.4V (2.4V/cell) for 48 hours. Then test capacity by discharging at the battery’s rated current. A 19Ah recovery from a 1Ah sulfated battery (as you observed on your first test) shows the method works, though results vary based on how deeply the sulfation has progressed.

If the batteries still underperform after the full cycle, a second overvoltage treatment may help—the study notes that some batches required multiple cycles, particularly if sulfation is severe.

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