Don't Check Batteries!

What’s the first thing you check when something doesn’t work? Power! The classic “is it plugged in” question, though hated by everyone, can pretty often solve the problem in seconds. What about equipment with batteries? Well you can change them, but some equipment has expensive or difficult to change batteries and you might pull out a multimeter to check the voltage. Next time you are ready to do that, stop and remember this cautionary tale!

We had a piece of down hole well measurement equipment that we’d designed come into the shop for some software upgrades and a small bit of troubleshooting. The tool had been dropped from a great height during a deployment and we were supposed to check out the systems while we had it. The software update went smoothly and after a successful day of testing we went home feeling great. The next morning we started up the tool and ran another test to ensure things were in good working order before packing and shipping it back to our client, but we noticed that the time in the file was off by many hours. Not sure if it was correct the day before, we reset the clock and did our tests, but decided to hold on to the tool to see if the same thing happened again.

The next day, the exact same scenario happened – the clock was off, but this time we calculated by how much. About 15 hours. That’s about how long we were gone overnight. Instruments like this (or your computer, or TV, or really any appliance that remembers time when powered off) have a small battery that keeps the time running when the device is not under power. What puzzled us most was the date was right, so it was not forgetting the time, but simply time was not running when power was removed. Suspecting a software issue, the team spent several hours running over the code and testing things to no avail.

Running out of options, we hooked a volt-meter up to the coin cell battery and it read 2.7V. It should have been about 3V, but the clock can run down to 2V, so we moved on. The battery is soldered onto the board and was only about a year old, so nothing seemed out of place. Yes, putting the battery under load is the right way to test it, but the clock pulls virtually no power, hence why these coin cells last years when used as clock backups.
After more investigation, software prodding, and thinking we were running low on options, so we tried unsoldering the battery to replace it. That’s when we immediately started to get suspicious. There was a build up of something under one lead of the battery. Though we couldn’t see a leak, it was certainly something to note. After removing the battery, we hooked it up to a volt-meter and applied a load. A “decade box” of resistors lets us simulate various loads on devices to see how they perform. That’s when we all vowed to never trust a coin cell battery under no-load test conditions again.

With virtually any load applied at all (even tiny ones like a clock), the battery voltage dropped like a rock. As it turns out, the voltage was barely enough for the clock to store the current time, but not enough for it to run and keep ticking without external power applied, so the clock was frozen in time.

Corrosion under battery leads

The original design called for a soldered battery to make sure the battery didn’t get knocked out during use, but we opted to replace the battery with a very secure clip in battery holder. This holder should keep the battery safe and make it easy to change so that in the field a battery change is an effortless job and this problem won’t ever occur again!

Moral of the story: never check a battery under no-load, even if the load is tiny! Most field technicians would have scrapped the board and installed a new one (a nearly $1500 proposition) after checking the battery with a multimeter. This is just one example of our team taking the time to really understand the root cause of a problem and making sure that the same problem never happens again for our clients.  So, what problem are you trying to solve? Tell us what you’re working on and how we can help – we’d love to hear from you!

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