I Revived A Car Battery That’s Been Dead For Over A Year With This Cheap Tool

Pretty much everyone will experience a dead car battery at least once in life. A battery that fails to crank your car one day is just one of those unavoidable eventualities, like taxes or a visit to a toilet after consuming Taco Bell. But what happens if the battery you have on hand is super dead? I’m talking about a battery you’ve put on a charger for days and it still doesn’t work. There’s a cheap tool out there that can bring batteries back from the dead. It sounds like snake oil, but I just tested one and I’m blown away.

Last month, I wrote about how the slim Toyota Prius 12V battery that was in my 2008 Smart Fortwo finally bought the farm. This battery has been a little trooper. It first lived in my wife’s Prius, where it was subjected to 40,000 miles per year of driving. Then, once it displayed signs of age, I yanked it out and decided to use it as a giant tool battery. This battery’s done a bit of everything from helping me jump-start big trucks to powering my beaters when I need to move them back home to sell them.

Lately, I’ve been experimenting with using the battery as a solution to the Smart’s problem of having a battery compartment that’s just too darn small. The experiment went well, aside from the fact that the battery gave up on life after a recent cold snap. I originally decided to give up on the battery, but the wonderful Stephen Walter Gossin and a number of readers challenged me to bring it back. I did just that, then I brought back another battery that’s been dead for over a year.

How Batteries Die

Back when I wrote for the German Lighting Site, I used to have a series called Cool Tool, where I highlighted awesome automotive tools that you might not know about. At the time I didn’t really have the resources or need to test the tools I wrote about, so the series relied on the reviews of others.

I’m delighted to say that my addiction to collecting bad cars and then my poor stewardship of some of them have driven me toward using the tools I used to write about. I’ve used a technician-level diagnostic scanner to help me find issues in a variety of cars, a borescope camera has been helpful in bringing old vehicles back to life, I’ve experimented with different electrical connectors, and now I finally get to see how a battery desulfator works with my own two eyes.

Let’s start with why you would need a desulfator in the first place. Batteries generate electricity through a chemical reaction. Your typical lead acid battery is filled with lead alloy plates and sulphuric acid, which acts as an electrolyte. The lead plates act as the anode and cathode. RS Components Ltd., a battery supplier, explains what happens:

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 you can probably guess from this explanation, lead acid batteries are not fond of being discharged. This is technically the inverse of the chemistry involved in lithium batteries, which don’t particularly enjoy being charged too high.

The chemical reaction is why, as the Batteries In A Portable World textbook purports, batteries in golf carts and other high-discharge applications tend to have shorter lives than other lead acid batteries. These vehicles tend to have deep-cycle batteries, which have thicker plates, a higher capacity, and are designed for high-charge cycles. But they’re still put under tons of stress for a battery.

Sulfation is a pretty big deal for a battery, from the textbook:

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 A Battery Back

If you experience a dead battery, you can jump-start your car or take the battery to an auto parts store and have the battery put onto its charger. If you’re lucky, the battery will come back to life. If you’re unlucky, like I was with the Prius battery, the battery might get a charge of a few volts but that’s it. In the past, I made a battery boil by trying to bring it back with force.

I tried charging the Prius battery for hours and nothing worked. Each charge from my charger would either complete with the battery not actually making any progress or the charge would just fail. I was ready to give up and buy a new battery, but then Stephen Walter Gossin issued that challenge.

There are lots of devices that you can buy which are designed to provide the overcharge as described by the textbook passage above. In essence, these chargers work by sending overcharge pulses to the battery being restored. Doing these pulses over and over for at least a day is supposed to help break off and dissolve crystalline buildup, restoring function and capacity to the battery. 

You can buy a desulfator on its own or you can get this feature as part of a battery charger. I’m a cheapskate, so I looked for the cheapest charger with a battery repair feature. The one I bought was a 10-Amp charger from Yonhan with a repair function. I paid $26.99 for the unit based on the reviews on Amazon that said the device successfully restored a battery.

First, I decided to restore the Prius battery. This battery had a standing voltage of 2.89 volts after sitting on another charger for about a day. I hooked it up to the Yonhan charger and it read a “full” charge after only 10 minutes. Then I hit the selector until the charger landed on the repair option.

When you activate the repair function on my specific unit, “PUL” shows up on the screen indicating that it’s ready to start shooting pulsing charges at the battery. When I measured the pulsing system in real-time, I watched as the voltage climbed as high as 16 volts, but only for a second before dropping back down to about 12 volts. The device did this over and over for an entire day. 

To my surprise, the charger started off by claiming the battery was at 80 percent, but after about four hours on the repair setting the charge indicator now claimed a 20 percent charge. I let the charger eat at the battery for the rest of the day. Little by little, the capacity crept back up. Yonhan warned to frequently check the battery to make sure it wasn’t overheating. The battery got warm, but never too hot.

After a whole day on the desulfator setting, the Prius battery achieved a standing charge of 12.8 volts, which is great! We’ll return to the Prius battery in a moment. Next, we’ll see what happened with a more challenging battery. Check out the photo below where the charger wasn’t even seeing the super dead Everstart battery:

As the Batteries In A Portable World book notes, the longer a battery stays dead, the less of a chance you might have to bring it back from the dead. I decided to put that to the test, too.

I used to have a wall of batteries at home. These batteries used to sit on tenders to stay nice and topped up for when I needed them. Well, I got a bit sloppy, left some of these batteries in cars that were parked outside. They died and my regular Deltran battery chargers couldn’t bring them back.

One of those batteries was the pictured Everstart unit from March 2021. This battery died somewhere toward the end of 2023 after I left it in my project Smart for too long. In other words, it’s been dead for at least a year. This battery has been dead for so long that it has served as a structural component of my bed at home after a piece of my bed frame snapped.

This is the OEM-sized battery that motivated me to try to find a smaller one that works. See how tight it fits in there:

But I wondered: Could this battery be brought back? I finally got around to doing a proper fix on the bed frame and then put the Everstart on the charger, too. The Everstart was initially so dead that it read a fat 0.0 volts on my multimeter. It was so dead that my charger couldn’t even detect it. So, I first tricked the Yonhan charger by hooking up a jump pack in-line with the battery. Once again, the Yonhan claimed a full charge within ten minutes even though the battery was super dead.

Just like with the Prius battery I selected the repair function and then let the Yonhan eat. This battery took about two days to reach the level the Prius battery got to. Throughout the rest of the week, I began swapping the batteries on the Yonhan charger and little by little, both reported higher and higher charges with the charger claiming higher capacities.

Before I continue, I should note that batteries can release some toxic gases. The manufacturers of these desulfating devices warn of such in their booklets. Because of this, I would avoid using a desulfator indoors nearby pets and children. I put mine on the deck outside when I wasn’t bringing the batteries inside to check on them.

Finally, after a full week, I took both batteries off and then let them sit for a whole weekend. As of this morning, the Prius battery was reading about 12.7 volts while the Everstart was showing about 12.6 volts. I was blown away by this, especially with the Everstart. None of my other chargers were able to do much of anything with the Everstart. I was beginning to see why some enthusiasts call them Neverstart batteries.

Of course, voltage isn’t everything. How does it actually work as, you know, a battery? Well, I first dropped the Everstart into my Smart and even though it was a cold winter morning in the Midwest the Everstart cranked the Smart into life like it was a brand-new battery.

As for the Prius battery, I hooked that up to the Smart through jumper cables–the same process I use to move a car a very short distance–and it also had no problems starting up the car.

There’s A Catch

So, does that mean that my batteries are fully restored? Not even remotely.

As Batteries In A Portable World notes, batteries will get permanently damaged from sulfation, especially if they’ve been dead for a while like my Everstart. The extent of the damage will vary between each battery and each situation. As awesome as a desulfator is, this device isn’t giving you a brand-new battery. Both of my test batteries are still four years old and both of them have been abused for far longer than they were designed for.

However, the desulfator should at least bring your battery back enough that it’s no longer a lead and acid brick holding up your bed. I have no doubt that both of these batteries aren’t nearly as great as they used to be. But hey, so long as I can keep that Everstart alive using a solar trickle charger that’ll be good enough for me.

The car fiends over at MotorTrend put a battery on a desulfator for two whole weeks and theirs got back to a charge of 13.1 volts. Their battery kept on going for months after the restoration was complete. Mine is working fine for now, but I’m going to put my batteries back onto the desulfator to see how much further I can restore them.

My advice? If you want to restore batteries, buy a charger that’s better than the one I have (maybe this one). While the Yonhan seems to do the job, it’s ridiculously cheap. The build quality makes a McDonald’s Happy Meal toy seem top-shelf. It’s so cheap that I make it run outside in case something goes wrong. Am I being a bit much? Maybe, but the build quality is that bad.

At the same time, if you absolutely need your battery to work, I’d also just buy a new one. A restored battery is still a damaged battery and it can still let you down when you need it the most. However, the little $27 charger taught me that the concept is sound. If you’re about as silly as I am, the next time your battery dies, don’t throw it into the proverbial ocean, try bringing it back from the dead!

(Images: Author, this post does contain an Amazon affiliate link, and if you click it and buy something we might get a commission – MH)