I Revived A Car Battery That’s Been Dead For Over A Year, Here’s How It’s Coming Along

A bit over a month ago, I reached new levels of cheapness that I thought wasn’t possible for me. Instead of buying a new car battery to replace a dead one I bought a cheap tool to revive the dead battery. That battery had been dead for over a year, yet my battery desulfator actually brought the hunk of junk back to life. Here’s how that battery has been working in the time since.

If you own a car, chances are there will come a day when you go outside, turn the key, and there’s nothing but dead silence. Nothing lasts forever and that’s especially true for batteries. If you’re lucky, you can jump-start your car or take the battery to an auto parts store and the battery will mostly come back to life. This is something that I have done innumerable times. Charging up dead batteries is also something our resident wrenching hero Stephen Walter Gossin likes to do.

But there will come a time when your dead battery no longer takes a charge or it does take in some juice but fails under load. Common sense would tell most people to replace that battery. What if you’re a serial cheapskate? Can you save a buck and just revive your dead battery?

I ran into that very scenario a few months after I revived my 2008 Smart Fortwo to bring it back into action as a daily driver.

The twist was that I didn’t have a battery for the car. The Everstart battery that was in the car had been dead for over a year. The only other battery that fit in the car’s tiny battery well was a Duralast unit meant for a Toyota Prius and it had been beaten to within an inch of its life being used as a tool battery. Sure enough, the Prius battery gave up on starting my Smart as soon as Illinois temps dropped below zero. Maybe it was finally at peace knowing I couldn’t use it to pull a car onto a trailer again.

At first, I considered rolling down to the local Walmart to buy another battery. But, SWG and a couple of our readers issued a challenge. Instead of just tossing this battery and buying another, why don’t I try reviving it?

As I wrote last time, batteries work through a chemical reaction, and lead acid battery chemistry is not one that enjoys being discharged for very long. Here’s a quick reminder of how lead acid batteries begin to die from the textbook, Batteries In A Portable World:

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.

Eventually, you might end up with a lead and acid brick that can’t start a car but is suspiciously good at holding up a partially collapsed bed frame. Okay, maybe don’t do that. Look, your favorite Autopian writers have a strange relationship with lead-acid batteries.

If you’re a cheap bastardess like me, you might feel enticed to buy a tool known as a battery desulfator. As the textbook passage above notes, you can apply an overcharge to a sulfated battery in hopes of restoring lost capacity and usability. In practice, these devices work by sending quick overcharge pulses to the battery. These devices will do this process for however long you command them to. The idea is that if you do these pulses over and over for at least a day, this is supposed to help break off and dissolve crystalline buildup, restoring function and some performance to the battery.

You can get a desulfator as part of a battery charger or individually. In my case, I bought a $26.99 10-Amp charger from Yonhan with a repair function. Then, I put it right to work, from my first piece on this:

First, I decided to restore the Duralast Prius battery from April 2021. 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.

I let the Prius battery cook for a whole day on the desulfator. When the tool finished, the battery had a standing voltage of 12.8 volts and had no issues starting a car in an Illinois winter. In the month since the battery has returned to its previous role of being a janky tool battery. I’ve even started a Volkswagen Jetta TDI with it in recent times.

Given the success of the Prius battery, I got curious. Could the desulfator revive the Smart’s actual battery? I mean, this thing has been dead for well over a year and it has spent most of that time propping up a broken bed frame. I should have tossed it a long time ago. After all, most sources note that the longer a battery stays dead, the less of a chance you have of getting a working battery when you try to revive it. But hey, I like a challenge.

So, I yanked the Everstart battery from under my bed, finally fixed the bed frame, and put the battery on the desulfator. The Everstart battery was made in March 2021 and I think it last had life sometime in late 2023, maybe? Honestly, I lost track of time. Anyway, here’s how the revival went:

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.

I then spent a whole week swapping the batteries on the desulfator. From here, I perhaps should have taken the batteries to an auto parts store to see how much cranking performance had been retained. Instead, I just plopped the Everstart into my Smart while I immediately started using the Duralast as a tool battery again.

The Smart has been great over the past five weeks. It started reliably every time and hasn’t needed a boost. I’ve even been able to let the car sit for some time. I let the vehicle sit for over a week in freezing temperatures and the charge remained strong. I last drove the car on Friday and accidentally left the automatic wipers and automatic headlights switches on, which are known to cause parasitic drain in Smarts. But hey, the darn battery still hung on.

Yes, I know, these short times don’t really prove anything. But honestly, I was surprised that the battery has even lasted this long since its revival. Every time I’ve gone outside to go for a drive I’ve expected to hear a bunch of sad clicks.

Now, I want to be super clear here. I did not get a “new” battery just by using the desulfator. As noted above, these devices can shake off only so much sulfation. At best, you’ve turned a dead battery into a used battery. Which, hey, that’s better than nothing! At worst, you get good voltage but no performance. Not every situation and not every battery will be the same. I cannot tell you how long your revived battery will last and I cannot tell you what capacity it will even have after the revival.

Because of all of these unknowns, I recommend that most people – you know, people who need reliable daily transportation – just pay the cash to buy a new battery. However, if you’re like David Tracy and I, and you like to haunt junkyards to buy used batteries, maybe this might be a fun experiment for you to try. Since a month ago, I’ve been hooking up the Yonhan to a bunch of different craptacular batteries. It’s also revived the batteries in my VW Touareg V10 TDI and a random motorcycle battery that I used to bring a vintage Vespa back to life. More on that later.

Admittedly, the stakes are pretty low for me. I work from home, so if I’m driving somewhere it’s usually for fun or to get groceries. I also carry a jump pack on me at all times, usually to help other motorists. So a dead battery is usually nothing more than a mild inconvenience to me. I’m usually more upset about the car’s clocks!

If you’re interested in having some fun with a battery desulfator, you have tons of options both cheap and expensive. I still don’t recommend the Yonhan model that I have because its build quality is shockingly cheap. But to the charger’s credit, it hasn’t skipped a beat at all in nearly a month of constant use. Still, I have a feeling this thing will break sooner than I’d want it to. As I said, it’s so cheap it makes Happy Meal toys look well-made. So, I’ll leave that dice roll up to you. Last time, Matt chose a Noco Genius 10 as an alternative. I use a handful of Noco products and they’ve never failed me.

This is not the end of my battery experiments. Soon, I plan on taking a few revived batteries to an auto parts store to see how well each has been brought back. For now, I’m just pleasantly surprised that my Smart still starts.

[Ed note: This piece contains a couple of Amazon links. If you buy a product from Amazon after clicking one of those links we might get a small commission.  – MH]