How I Jump Started My Car Using A Power Drill Battery

What do you do when your car battery dies and you don’t have a jump pack on hand? You could call AAA, or reach out to a fellow motorist to help. But, if you’re like me, and you hate talking to other people, you might prefer to solve the problem on your own. Here’s how a power drill can get you out of trouble.

Rewind back to 2016. I’d just moved to a new state to take on a an engineering role with a major automaker. I was out shopping for my new home, and when I returned to my car, I realized I’d made a mistake. I turned the key and got nothing. I’d left the lights on, and the battery was dead.

I had jumper cables, but no friends in town, and my fellow shoppers looked unfriendly and mean. I also had no road service, so I was on my own. Rustling around in the back of my Daihatsu netted me a solution, though. I found my trusty Ryobi power drill. Surely, this thing could get me going again, right?

See, here’s the thing about modern cordless drills. The vast majority of them use a small lithium-ion battery pack. Inside the drill battery, the lithium cells inside are often the same 18650 cells that you find in laptops and even some EVs like the Tesla Model S.  In the case of my Ryobi drill, it had a string of five cells in series, adding up to a nominal voltage around 18 volts or so. That’s higher than a car battery, but we’ll get back to that in a minute.

I’ve always been tinkering with electronics, and I was well aware these cells could happily deliver 20 amps or more without really breaking a sweat. I suspected they could theoretically hold up to maybe ten times that in a short-duration burst, such as cranking over a small engine. All I had to do is find a way to hook it up to the car with my jumper leads.

But wait! At this point, you’re probably hearing alarm bells. We’re told that most cars use 12-volt electrical systems, and here I am talking about jump-starting with an 18 volt battery. That’s six more volts, right? Surely that’s too many!

Here’s the thing, though. Cars do run on 12-volt electrical systems, but that’s a nominal rating. We call it 12 volts, but reality is more complex. A car battery actually sits closer to 12.8 volts when it’s fully charged. Meanwhile, an alternator outputs anywhere from 13.8 to 14.4 volts in normal operation. It’s the same for my Ryobi drill battery. It’s nominally 18 volts, but it ranges up as high as 21 volts when fully charged.

The Ryobi battery did sit at a higher voltage than my little Daihatsu Feroza was built for, it’s true. But there was really only a few volts in it. I figured it was close enough that I was unlikely to damage the simple automotive-grade electronics in my early 1990s car. I also knew that when I hooked it up to the dead car battery, the Ryobi battery’s voltage would sag significantly. This is because the dead car battery was trying to charge itself back up off the Ryobi pack, and that was a significant load on the small drill battery.

The hardest part was actually hooking everything up. My jumper cables were short, so I had to carefully place the Ryobi pack on the front bumper. I then hooked up the the red jumper lead to the positive terminal of the car battery, and then wriggled its jaws on to the positive contact on the Ryobi battery. This was hard to do, as the drill battery contacts were recessed and not easy to access. I then repeated the same with the black jumper lead.

This was an incredibly delicate operation with minimal room for error. The jumper cables were just millimeters apart, and the slightest slip would have seen a nasty short circuit potentially ending in a small fire. I tried to set it up so that if the Ryobi battery fell off the bumper, the jumper cables would slide off the car battery side and hopefully fall clear.

With the batteries hooked up, I gently climbed into the cabin. I noticed the Daihatsu’s volt meter was right in the healthy range, sitting nicely above 12 volts. Even though my knowledge told me this was an okay thing to do, I was nervous.

The moment of truth had come. I feared smoke or flames, but as I turned the key to the ON position, there was none—just the usual friendly lights on the dash. I cranked the starter, and the car burst into life with precisely zero drama. Success! I quickly disconnected the drill battery to stop the alternator back-charging it at the wrong voltage, and drove on home.

I was psyched, because I was now able to drive home and I hadn’t needed to talk to any randoms for help. It might sound stupid, as I’m pretty extroverted, but people in mall carparks are generally pretty uncomfortable about random people approaching them. Plus, I’d pulled off a nifty hack and hadn’t blown up my car to do it. It’s easy to sound confident about this after the fact, but I had been sweating right up until it worked. Even better—I hadn’t killed the drill battery! I plugged it back into my drill and it ran just fine.

It’s worth noting that this worked for a very obvious reason—the difference between my drill battery and an off-the-shelf jump pack was minimal. Both pack a bunch of lithium-ion cells into a small plastic case. The only difference was mine was less convenient to hook up to a car battery, and the voltage was a touch higher. As my success proved, that wasn’t a big deal in my specific case. Indeed, four years later, I’d do the same hack with a DeWalt battery and my 1992 Mazda Miata, and it worked just as well then as it did here.

I would warn you, though, not to try this at home unless you really know what you’re doing. While my early 1990s cars were perfectly able to handle a little extra voltage, your car might not be so robust. I’d be particularly reluctant to try this on more modern vehicles with more complicated and fussier electronics. I’d steer well clear of doing it on a modern BMW, for example. In contrast, though, you could absolutely do this to a Tesla Cybertruck, as it has an advanced low-voltage electrical system designed for jumping from 12-volt, 16-volt, and 48-volt power.

In any case, I found this to be a useful technique for getting myself out of trouble. There are just a few key points to note when doing this. It’s important to take extreme care when hooking up your batteries. Drill batteries often have very small, recessed contacts that are incredibly difficult to connect with automotive jumper leads. You want to avoid short-circuiting your positive and negative leads at all costs, lest you cause a fire with the drill battery or your car battery—or both. At times, I’ve used creative solutions to hook up to a drill battery, by using bits of metal to create larger, more accessible contacts, for example.

You should also accept that you risk wrecking your drill battery in doing this. The excess current draw might over-drain your batteries to the point of failure, particularly if you have a large engine with a big starter motor. Using the largest drill battery you have will also help—bigger batteries can deliver more current. There is also another way to avoid this—you can simply hook up your drill battery to your car battery, and leave it sitting for half an hour or so without cranking the engine over. The idea is that you’re using the drill battery to effectively recharge your car battery. You then disconnect the drill battery before you try turning the engine over.

Hopefully, your car battery is always full, and you never have need to do this. However, if you’re ever stuck far from help, and you absolutely need to get some juice into your vehicle, this might just save your butt one day. It’s a good idea to keep in the back of your mind, if nothing more.

Image credits: Lewin Day