A lead-acid battery left discharged grows hard lead sulfate crystals that ordinary charging cannot convert, and the best-supported cure is a very slow charge at one fiftieth of its ampere-hour rating for one to three days. Fresh dilute acid, voltage pulses and an EDTA soak rest on thinner evidence, and the corpus itself says results vary and that severe sulfation may not be reversible, so confirm the diagnosis first and measure capacity afterwards.
Part of Battery Building and Maintenance
Diagnosing and trying to reverse sulfation in a lead-acid battery, with the evidence for each method.
Sulfation Is Lead Sulfate That No Longer Dissolves
Every discharge makes soft lead sulfate on both plates, and charging converts it back (Lead-Acid Chemistry). If the battery stays discharged, the sulfate recrystallizes into hard crystals that resist normal charging. The carrier names the cause (left discharged too long), the energy-storage article the symptom (the battery charges quickly and dies quickly) and Rechargeable Batteries the trigger (discharging below 1.75 V per cell). Sulfation becomes permanent within weeks, per the energy-storage article.
Not every dead battery is sulfated. A cell that reads near zero volts with a short between plates, a cell with shed paste on the floor of the case, and a worn-out grid all look similar from outside. Check each by cell voltage, specific gravity and case condition before spending days on a recovery charge (Maintenance and Reconditioning).
The Corpus Offers Four Methods With Unequal Evidence
- Slow charge. The carrier gives C/50 for 48–72 hours, the lead-acid article C/50 for 24–48 hours, and the energy-storage article 1–2 A for 24–48 hours. Three sources agree on the idea, differing on time.
- Fresh acid. Drain the old electrolyte, fill with new acid at specific gravity 1.100, charge slowly, then adjust the strength (carrier only).
- Pulse charging. High-voltage pulses of 50–100 V and microsecond length to break up crystals (carrier). The energy-storage article says only that some people report success with pulsing, and that results vary.
- EDTA. One tablespoon of EDTA (ethylenediaminetetraacetic acid) per cell, left 24 hours, then slow-charged (carrier only).
The carrier’s success rate is about 60% for mild sulfation and about 20% for severe. The lead-acid article states that severe sulfation is not reversible. These disagree, and no source gives a method for telling mild from severe other than trying.
What You Need
- A low-current adjustable charger — Constant Current Charging
- A hydrometer, a voltmeter, an ammeter — to track the recovery
- Distilled water and fresh electrolyte — Electrolyte Preparation
- A load for the capacity test — Handmade Resistors
- A glass container with a lid — for spent electrolyte
Method: Recover A Sulfated Battery
Step 1: Test the starting point. Record resting voltage, specific gravity of each cell, and, if it will take any charge, a short discharge test.
Step 2: Slow charge. Set the current to C/50 and hold it for 48 to 72 hours, with the case cool to the touch. A 50 Ah battery at C/50 draws 1 A.
Step 3: Re-test. Read specific gravity and voltage; discharge-test to 1.75 V per cell.
Step 4: If the gravity has not risen, try the fresh-acid method, keeping the old acid in a labelled glass container.
Step 5: Treat the other two as last resorts and keep notes of each outcome.
Worked Example: What The Slow Charge Delivers
A 50 Ah battery at C/50: 50 ÷ 50 = 1 A. For 48 to 72 hours the charge is 1 × 48 = 48 Ah to 1 × 72 = 72 Ah, between 48 ÷ 50 = 0.96 and 72 ÷ 50 = 1.44 times the rated capacity. A slow charge at this rate will gas little, and at 0.42 L of hydrogen per ampere-hour (Water Electrolysis) a battery taking 72 Ah of mostly gassing current would release at most 72 × 0.42 = 30 L of hydrogen. Pulse charging adds a risk of its own: 50 to 100 V is in the 50–120 V range that the lethal-current article calls potentially serious.
Common Mistakes
- Treating a shorted cell as sulfated — no charge repairs a short
- Charging a frozen or very low battery at a high rate — heat and gas without recovery
- Trusting a success rate that comes from a single source — measure your own result
- Pouring spent electrolyte out carelessly — after an EDTA soak it contains dissolved lead (general knowledge, not from the other articles); keep it in a labelled glass container and keep it away from drains, soil and water supplies
Safety
Recovery charges are long gassing events, and pulsers are high-voltage
A recovery charge keeps a battery gassing for a day or more, and the hydrogen it releases accumulates at the ceiling and in any box until a spark ignites it at 4% to 75% in air. Work with top ventilation and no switches nearby (Hydrogen Gas Hazard). A pulse unit at 50 to 100 V can give a dangerous shock through wet hands (Lethal Current). Drained acid burns skin and eyes, so use goggles and gloves, and never pour water into it.
What’s Next
- Learn next: Maintenance and Reconditioning — avoiding sulfation in the first place
- Combine with: Equalization Charging — breaking up light sulfation before it hardens