Product longevity sometimes defies expectations

Battery self-discharge can limit product life, but sometimes there’s an inexplicable outlier.

We’re all familiar with products that fail before too soon. I’m not talking about those that suddenly become obsolete when there’s an incompatible mandated software upgrade or similar problem. Instead, I mean a product that is no longer usable due to an issue, such as a failed switch, a display that fades out, or a power-supply capacitor that goes bad after a few years (I’ve replaced many of those). Then there are those cases where the product is fine, except that the battery has died and is not replaceable (I’m looking at you, Black & Decker Dustbuster!).

However, the opposite can also be the case. My small, low-end kitchen scale, Figure 1, gave up after 30 years of faithful service; I have the still-legible purchase receipt, and the date code on the back says 1994. Its final gesture was to display just the letter “L” on the display, presumably saying the battery was low.

battery — Figure 1. A little before it displayed the letter “L,” the scale went to scale heaven. (Image: Bill Schweber)

It was made by Measurement Specialties, Inc., which was acquired by TE Connectivity in 2014. By then, MSI had moved on from mainstream, lower-end consumer products to focus on sensor technologies related to pressure, vibration, force, temperature, humidity, ultrasonics, position, and fluids, primarily for industrial/commercial applications and industries.

While this unit did not owe me anything, I was curious as to what sort of power source kept it going for so many years. Even though the actual usage is quite low — maybe one or two times each day for perhaps a minute — that use is not what would drain the battery.

Instead, it’s self-discharge. Even a quality battery has self-discharge, and the rate is a function of the battery chemistry, packaging, and storage conditions. For the vast majority of battery types, it ranges from 1% per month to as high as 5%, although batteries with a fraction of a percent are available at a much higher cost. Going through the numbers, a good first-level estimate is that the battery will have a maximum useful life due to self-discharge of between two and ten years, again as a function of chemistry and other factors.

My first inclination was just to get rid of the scale, but then I realized that would be technically disrespectful to the unit, which had served faithfully for so long. I really needed to find out how this humble scale continued to work for so long. I speculated that perhaps instead of a conventional battery, it had a micro-sized version of the plutonium-power thermoelectric generator (TEG) used in spacecraft (still powering Voyager 1 and Voyager 2 spacecraft on their 40+ year voyage) as well as some early pacemakers? Or was the simple ON button connected to some sort of piezoelectric energy-harvesting scheme, where every button push gated a jolt of current that recharged a capacitor or battery?

I was able to open the unit with a little work (some screws were hidden under the bottom-side label) and was totally surprised, Figure 2. The battery was a standard Panasonic CR2032 lithium-manganese dioxide (LiMn0₂) coin-cell battery with no special markings. Somehow, in defiance of the accepted data from reputable sources, this was the little battery that could, as it chugged away for 30 years.

The battery edge was encased in a thin yellow plastic film, similar to heat-shrink tuning but much thinner (Figure 3). I wondered if this seals the battery and lessens self-discharge. It’s unlikely since the battery is already sealed, but we’ll never know.

Sometimes, you have to acknowledge that there are technical anomalies and scenarios that just don’t make sense. I also realized that it’s not just the battery I should be saluting but also the engineer of the scale’s electronics. Somewhere out there, there’s an unknown circuit master who designed and fabricated scale electronics with a quiescent drain so low that it did not exhaust the battery over time and did so with 30-year-old process technology. Note that the scale has a soft ON button rather than a hard ON/OFF switch, so the circuit must always be looking for the button push, even when it was apparently off.

Did I replace the battery? How could I resist that temptation, even though it required micro-surgery? While the battery was fully visible, it unfortunately was not in a holder or socket. Instead, it had very thin connector tabs that were spot-welded to either side of the battery case. I was eventually able to slice those tabs off using an X-acto knife with a #11 blade, but those tabs were cracking from metal fatigue and ready to break. At that point, I returned to reality, and, in the words of the princesses from Frozen, I decided to “let it go.”

Now, my big decision is how to give this “family member” a decent send-off. Do I just toss it in the trash? Bring it to a recycling center (and what will they actually do with it)? Or do I bury it in the backyard?

I’m still wrestling with that one. It still has its place in a kitchen cabinet as a tangible testament to longevity.