I went to start my car (a 2006 Toyota Camry) and when I turned the key in the ignition switch, NOTHING happened. The car was utterly inert. The radio wouldn’t play, the passenger cabin ceiling light was dark and I wasn’t going anywhere, at least not in that vehicle.
I guessed that the car’s battery had failed, but being an engineer (you know the type.) I just had to measure the battery’s terminal voltage. I went and got my trusty Sears digital multimeter, raised the hood of the car to expose the battery and touched one of the multimeter probes to the positive post of the battery at the exact spot you see in the image below. That one probe slipped into a small crevice between the battery’s positive post and that post’s clamp and when it did, there was a spark right there at the spot where I’d stuck my probe. Nothing was connected (yet) to the other multimeter probe which left me wondering “Why did I see that spark?”
Figure 1 The battery that was probed and the location where the unexpected spark occurred.
This was in May of 2025 which meant that battery was two years old having been installed in May of 2023. The battery had not been touched at all during those two years which led to the
problem at hand. Gradually, the two post clamps had worked themselves loose. The clamp serving the positive post had actually lost its electrical connection to that post. When my multimeter probe got involved, the spark arose from the battery making connection again via the metal of the probe tip to all of the stuff the battery was normally called upon to feed.
Now that I knew what was wrong, I set about making repairs by tightening the two post clamps, BUT there was a very specific safety issue at hand to which I want to draw your most alert attention.
My car uses an internal combustion engine, which incorporates a 12-V lead-acid battery whose negative post is grounded to the frame of the car. This has been a conventional design approach for many years. I think that pre-1950 or so cars with 6-V lead-acid batteries had their positive posts grounded to the car frames, but that’s a whole other thing.
When you are going to do any work on a car such as my own, where that work involves the car’s battery, you MUST, MUST, MUST first disconnect the clamp that connects to the battery’s negative post. If you fail to do so and you accidentally happen to make a connection with some tool (a socket wrench, maybe) from the battery’s positive post to the car frame, that accidental connection will short-circuit the battery. There will be a flash, and according to something I once read, the battery might even explode.
You do not want to risk having that happen.
I disconnected the clamp from the negative post. I then disconnected the clamp from the positive post and scoured both post surfaces and their clamp surfaces. Next, I reattached the positive post’s clamp, I reattached the negative post’s clamp (in that order) and I started the car.
Everything worked. Everything was back to normal. I drove it to the grocery store and back again. We needed some milk.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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