Experiencing a dead car battery is almost a rite of passage for vehicle owners. One day your car starts perfectly, and the next, silence when you turn the key. It’s an inevitable automotive hiccup, much like needing gas or rotating your tires. But what if you’re facing a truly dead battery? We’re talking about the kind that refuses to charge, even after hours on a battery charger. Before you rush out to buy a new battery, especially if you’re not equipped with a full garage of tools, there’s a surprisingly simple and affordable solution that can bring many dead batteries back to life. It might sound too good to be true, but I recently put this method to the test and the results were genuinely impressive.
My own adventure with battery revival started when the trusty 12V battery from my wife’s old Toyota Prius, which had been faithfully serving in my 2008 Smart Fortwo, finally gave up the ghost. This little battery had lived a full life, enduring years of heavy Prius driving before I repurposed it as a robust tool battery. It had jumped started trucks, powered up countless project cars, and generally been a reliable workhorse.
Recently, I was using it to solve a space issue in the Smart’s cramped battery compartment. The experiment was going well, until a cold snap seemed to finally push the battery over the edge. Initially, I resigned myself to replacing it. However, encouragement from Stephen Walter Gossin and several readers spurred me to try reviving it. Not only did I succeed with the Prius battery, but I also resurrected another battery that had been lifeless for over a year!
Understanding How Batteries “Die”
In my past role writing for a German automotive site, I had a “Cool Tool” series, showcasing interesting automotive tools. Back then, I didn’t have the resources to test them personally and relied on others’ reviews. Now, my habit of acquiring problematic cars and my less-than-perfect maintenance habits have pushed me to actually use these tools. I’ve used advanced diagnostic scanners, borescopes, and various electrical connectors to troubleshoot and repair vehicles. Recently, I finally got to experience firsthand how a battery desulfator works.
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So, why would you even need a desulfator? Car batteries produce electricity through a chemical reaction. A typical lead-acid battery contains lead alloy plates and sulfuric acid electrolyte. The lead plates function as the anode and cathode. RS Components Ltd. explains the process:
“The lead plates act as an anode and cathode, while the sulphuric acid is an electrolyte that contains hydrogen and sulphate ions. Negatively-charged sulphate and positively-charged hydrogen are attracted to the lead cathode and anode respectively. When the appliance or device attached to the battery is turned on, a reaction occurs on the cathode as sulphate ions give up their negative ions and lead sulphate is created. As levels of lead sulphate increase, the electrolyte becomes weaker and resistance on the plates increases. This triggers the flow of negative electrons into the attached device or appliance.
As these electrons flow back into the battery’s cathode, a chemical reaction occurs between hydrogen in the battery and the lead dioxide in this plate. This produces water, meaning hydrogen ions are continuously supplied alongside the sulphate acid inside the battery, enabling the process to restart repeatedly.
In each type of lead acid battery, this reaction will work slightly differently to make it more appropriate to the particular use case its best suited for.”
As this explanation suggests, lead-acid batteries aren’t fans of deep discharges. This is the opposite of lithium batteries, which prefer not to be charged to their absolute maximum capacity.
This chemical reaction is why, as the textbook Batteries In A Portable World notes, batteries in high-discharge applications like golf carts tend to have shorter lifespans. These deep-cycle batteries are designed for intense charge and discharge cycles with thicker plates and higher capacity, but they still endure significant stress.
Sulfation is a significant issue for lead-acid batteries. Batteries In A Portable World describes it this way:
“What is sulfation? During use, small sulfate crystals form, but these are normal and are not harmful. During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline and deposits on the negative plates. This leads to the development of large crystals that reduce the battery’s active material, which is responsible for the performance.
There are two types of sulfation: reversible (or soft sulfation), and permanent (or hard sulfation). If a battery is serviced early, reversible sulfation can often be corrected by applying an overcharge to an already fully charged battery in the form of a regulated current of about 200mA. The battery terminal voltage is allowed to rise to between 2.50 and 2.66V/cell (15 and 16V on a 12V mono block) for about 24 hours. Increasing the battery temperature to 50–60°C (122–140°F) during the corrective service further helps in dissolving the crystals.
Permanent sulfation sets in when the battery has been in a low state-of-charge for weeks or months. At this stage, no form of restoration seems possible; however, the recovery yield is not fully understood. To everyone’s amazement, new lead acid batteries can often be fully restored after dwelling in a low-voltage condition for many weeks. Other factors may play a role.”
Bringing Your Dead Battery Back to Life (Without Breaking the Bank)
If you encounter a dead battery, a common first step is jump-starting your car or taking the battery to an auto parts store for charging. If you’re lucky, it will recharge successfully. However, like my Prius battery, you might find it only takes a minimal charge, or nothing at all. In the past, aggressive charging attempts have even led to batteries overheating and becoming damaged.
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After unsuccessfully trying to charge the Prius battery for hours, I was ready to replace it. That’s when Stephen Walter Gossin’s challenge came, prompting me to try battery desulfation.
Many devices are available that provide the overcharge described in the textbook, designed to reverse sulfation. These battery desulfators work by delivering overcharge pulses to the battery. Repeated pulses over a day or more can break down and dissolve the sulfate crystals, restoring battery function and capacity.
You can buy a dedicated desulfator unit or find battery chargers with a built-in desulfation feature. Being budget-conscious, I searched for the most affordable charger with a repair function and opted for a 10-Amp charger from Yonhan with a repair mode, costing just $26.99. The positive Amazon reviews suggesting it could revive batteries swayed my decision.
First, I tested it on the Prius battery, which had a resting voltage of only 2.89 volts after a previous charging attempt. After connecting it to the Yonhan charger, it quickly indicated a “full” charge within 10 minutes. I then selected the repair mode.
In repair mode, the Yonhan charger displayed “PUL,” indicating it was sending pulsing charges. Monitoring the voltage in real-time, I observed it briefly spiking to 16 volts before dropping back to around 12 volts, repeating this cycle continuously for a full day.
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Surprisingly, the charger initially showed the battery at 80 percent charge, but after four hours in repair mode, it dropped to 20 percent. I continued the desulfation process throughout the day. Gradually, the charge capacity started to increase. Yonhan’s instructions recommended checking for overheating, and while the battery warmed slightly, it never became excessively hot.
After a day of desulfation, the Prius battery reached a resting voltage of 12.8 volts – a fantastic improvement! But the real test was on an even more challenging case: a severely dead Everstart battery. Initially, the charger couldn’t even detect it.
As Batteries In A Portable World points out, the longer a battery remains dead, the lower the chances of revival. I decided to test this limit.
I had a collection of batteries at home, once maintained on tenders. Neglect led to some being left in parked cars, resulting in complete discharge. Standard chargers failed to revive them.
One of these was the Everstart battery, dead since late 2023 after being left in my project Smart for too long. It had been dead for so long it was literally being used as a bed frame support!
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This very battery was the reason I sought a smaller replacement to fit the Smart’s tight battery compartment:
Could this seemingly hopeless battery be revived? After fixing my bed frame, I put the Everstart on the Yonhan charger. Initially, it measured 0.0 volts on my multimeter – so dead the charger couldn’t even register it. To trick the charger, I connected a jump pack in series with the battery. Again, the Yonhan quickly indicated “full charge” within minutes, despite the battery’s true state.
Just like with the Prius battery, I activated the repair mode. The Everstart took about two days to reach the same charge level as the Prius battery had after one day. Over the following week, I alternated charging both batteries, and both showed progressively higher charge levels and capacities.
Important safety note: Batteries can release toxic gases, as warned by desulfator manufacturers. Always use desulfators in well-ventilated areas, away from pets and children. I performed my desulfation process outdoors.
After a week of desulfation, I removed both batteries and let them rest for a weekend. The Prius battery read 12.7 volts, and the Everstart 12.6 volts. I was astounded, especially with the Everstart. My other chargers had been completely ineffective on it. I was starting to understand the jokes about “Neverstart” batteries.
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Of course, voltage isn’t the only measure of battery health. The real test was performance. I installed the Everstart in my Smart on a cold morning, and it started the car as if it were brand new. The Prius battery, connected via jumper cables, also started the car without issue.
The Catch: Desulfation Isn’t Magic
So, are these batteries fully restored to their original condition? Not at all.
As Batteries In A Portable World explains, batteries suffer permanent damage from sulfation, especially when deeply discharged for extended periods like my Everstart. The extent of damage varies. Desulfation isn’t a magic fix that creates a brand-new battery. Both test batteries were four years old and had endured significant abuse.
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However, desulfation can make a dead battery usable again, preventing it from becoming just a heavy paperweight. While these revived batteries won’t perform like new, they can be functional. I plan to maintain the Everstart with a solar trickle charger to prolong its life.
MotorTrend tested desulfation over two weeks, achieving a 13.1-volt charge, and their battery lasted for months afterward. Mine are currently working, and I’ll continue desulfating to see if further improvement is possible.
My recommendation? If you want to restore batteries, consider a higher-quality charger than my basic Yonhan (maybe this NOCO Genius10). While the Yonhan works, its build quality is questionable. Its flimsiness makes me cautious, hence running it outdoors. Perhaps I’m overreacting, but the build quality is noticeably poor.
Ultimately, if you absolutely depend on a reliable battery, replacement is the safest bet. A desulfated battery is still damaged and might fail when you need it most. However, this inexpensive $27 charger proved the desulfation concept valid. If you’re like me and enjoy experimenting, the next time your battery dies, consider trying to revive it before discarding it!
(Images: Author, this post does contain an Amazon affiliate link, and if you click it and buy something we might get a commission – MH)
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