Battery Applications: Don’t Overlook the Humble Battery Connector

We talk and worry so much about batteries: their longevity, safety issues, charging/discharging, temperature effects, and so many other issues that it’s easy to forget about an important link in the battery supply chain: the connector. Two events in the past week, one at the extremely low end of the current range and one at the higher end, reminded me that the best batteries are useless without a solid connection.

On the high end, I get in the car, turn the key, and…nothing. The dashboard was broken, the lights were out, and of course the engine wouldn’t rev at all. This is most likely a badly drained battery due to an internal fault or an external short circuit, or a bad connection to the mains power cable. I popped the hood to find that the cable clip on the positive battery post was loose (I replaced the battery a few weeks ago). No problem, I thought; I’d just tighten the clamping screws with whatever tools I had on hand and problem solved.

but it is not the truth. Apparently the battery post on the new battery is a tad smaller than the original, and the minimum size of the clip doesn’t reduce the extra amount I need. Since changing the battery a few weeks ago, I’ve been able to get the car up and running, mostly by luck. Since I’m at home, I figured I could at least temporarily fix this by opening the clip wider and inserting a ring or ring made of thin, high-quality phosphor bronze contact material (which I got from other projects obtained) between the fixture and the post.

Nice idea, but it doesn’t work either. The clamp is designed to only open a little larger than its nominal diameter, and I couldn’t even fit a thin ring into the gap: the clamp has a very limited max/min range. In the end I was able to wedge a small piece of phosphor bronze material on either side of the post, just enough to secure contact while I was looking for another clip (note that working on the +12V/200A contact a few inches from the post has a ground connection to it Nerve; short circuit creates welding sparks and worse!) Luckily, I didn’t have to replace the entire clamp/cable assembly as the clamp itself is screwed onto the fitting at the end of the cable and can be replaced without installing a new cable.

I’m not complaining: I’ve been very lucky in terms of a dead car, location and being able to start again. I’m actually impressed when you do a basic V=IR analysis that the car can start at all. Look at it this way: you have a 12V supply and it takes about 100A to start the car. This means that in the series sum of the cable, connector, battery post and contact resistance, even a resistance of just 0.1 Ω produces an IR drop of 1.2V – and a car starter needs about 6 to 8V to start . There’s not much headroom on the supply rail, that’s for sure.

Around the same time as the car problem, I had a problem with a supposedly “better quality” handheld that used a standard button cell battery. When it died I thought it was the battery, but before I ran out to change it, I took it out, measured the voltage and it was full nominal. While this was a no-load test and thus not entirely indicative of performance under load, it gave me pause to examine the situation further.

Battery Applications: Don’t Overlook the Humble Battery Connector

A quick look showed a small amount of corrosion on the battery holder terminals. I wiped it down gently, put the battery in, and it’s all good. Not only did I save myself the replacement cost and time to get it, but the unit probably still wouldn’t work (unless the act of inserting it scraped off enough corrosion, similar to that on some connectors). In short: I have an otherwise good product whose reliability has been killed by a cheap, shoddy battery holder.

Perhaps because I’m “sensitive” to the situation, I later noticed an ad in a design magazine by one supplier advertising a higher quality coin cell holder, reminding engineers not to lose sight of the vital part of this small component Role – I’m sure there are other providers.

When you’re dealing with so many other hardware and software issues, design lessons are obvious, easy, and easy to overlook. Whether you’re using a primary milliampere coin cell or a 100amp secondary (rechargeable) lead-acid battery: don’t overlook the humble, simple connector.

Have you ever overlooked the understated parts of a design in favor of the more obvious and exciting aspects, only to have intermittent or even “hard” failures? Did these issues arise during the prototype or trial phase, or after months of use of?