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How to Jump Start Your Ride, Cordless Tool Battery

Jump Start Your Car with a Cordless Tool Battery: An Enthusiast’s Guide

Have you ever found yourself with a dead car battery and no traditional jumpstarter in sight? What if I told you that you could potentially get your car running using a cordless tool battery? It might not be the recommended approach, but it’s certainly possible, and I’ve done it myself. Here’s a detailed breakdown of how this unconventional method can come to your rescue and what you need to consider to do it safely.

The Unlikely Solution

This week, when I tried to start my car, all I heard was the clickety-click of relays, but the engine didn’t turn over. The battery was too weak to start the car. I faced three possible issues: a failing four-year-old battery, an underperforming alternator, or a possible short draining of the battery.

Without many options and recalling a heated debate I saw on social media about using a tool battery for a car jump start, I decided to give it a try, despite some commenters insisting it wouldn’t work.

Technical Considerations

Here’s the science behind why it can work: Most cordless tools use a lithium-ion battery that can put out a significant voltage and current. The typical configuration for these batteries (like those found in power drills) might range around 18 volts. Assuming a series configuration of five cells at 3.7 volts each gives us about 18.5 volts total.

The resistance per cell might be about 0.008 Ohms, adding up to 0.04 Ohms for the entire pack. Using Ohm’s Law (I=V/R), where I is current and V is voltage, we find that the battery can deliver a current of 462.5 amps at maximum. However, such a high current at 18.5 volts wouldn’t be sustained without significant voltage drop, calculated as follows: Vt=Vic-I*R, leading to a practical output of nearly 0 volts under maximum load. However, if we adjust to maintain about 9 volts output (sufficient to crank a weakened engine), the current output would be about 237.5 amps.

Making the Connection

To connect the tool battery to your car, you’ll need a way to securely attach the battery terminals to your vehicle’s battery terminals. This can be risky without the right setup, as makeshift connections can lead to poor contact and potential sparks.

  1. Safety First: Ensure the tool battery is in good condition and that you’re wearing protective gloves and eyewear.
  2. Connection: Use automotive-grade clamps to connect the positive terminal of the tool battery to the positive terminal of the car battery, and do the same with the negative terminals.
  3. Start Your Engine: With the connections secure, try starting your car. If it starts, disconnect the tool battery immediately to avoid overloading the tool’s battery cells.

Cautions and Considerations

  • Voltage Concerns: Using an 18-volt battery on a system designed for 12 volts could potentially harm your vehicle’s electronic systems. In an ideal setup, a buffer (like the car’s main battery) would mitigate excessive voltage.
  • Amperage and Heat: If the battery outputs high current, the wires and the battery itself can get very hot, posing a risk of burns or even a fire.
  • Battery Health: Not all tool batteries are designed to deliver high currents for cranking engines, which can degrade the battery rapidly.


While using a cordless tool battery to jump-start your car is not a standard procedure and comes with risks, it’s a handy hack if you understand the technicalities and take the necessary precautions. Modern battery technology showcases versatility and power, but reserve its use for emergencies and acknowledge potential risks.

In the world of automotive troubleshooting, where necessity often breeds innovation, this method is a fascinating example of cross-utilizing technology. However, it’s crucial to prioritize safety and consider investing in a dedicated jump starter to keep in your vehicle for emergencies. This tool-battery method is more of a last resort than a go-to solution, suitable for those moments when you’re in a pinch and need a creative solution to get back on the road.

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