One of the many calculations designers make when sizing battery capacity for an IoT-type device is the current drain on the battery in various modes. This metric applies to devices whether the battery is a primary, non-rechargeable type, or a secondary, rechargeable one.
Determining battery life in a system using that data seems like a straightforward calculation, but that’s often not the case. Part of the reason is the variability of the load: back in those ancient days when “off” really meant “off” for most products, a mechanical switch cut off the current flow from the battery to the load, and there was no load-induced drain on the battery when the product was off, period, end of story.
Instead, you now have to distinguish among and get numbers for current drain in various “off-like” states including quiescent, sleep, hibernate, standby, and shutdown modes — and just defining these can be confusing. In general, the quiescent current is determined in the circuit’s quiet mode when the source isn’t driving the load. It is not the same as shutdown current, which refers to a device that’s asleep. With quiescent current, the system is idle but ready to wake at any time to take some action, which is generally what users want. Designers usually use quiescent current to assess dissipation of a supply at light loads, while they use shutdown current to calculate battery lifetime when the device is powered off, but its battery is connected to the regulator.
…Then there’s self-discharge
It may seem that the battery-capacity requirements can be worked out once these numbers are determined along with anticipated device-use cycles, but that’s not the case. The “elephant in the corner” is battery-self-discharge, which occurs even if the battery is not connected to any load at all. It’s easy to ignore at first, while battery-usage neophytes may be unfamiliar with it, yet for low-power IoT-type applications, it can be as detrimental to useful battery life as actual load drain.
Self-discharge is not due to a manufacturing defect but an inherent, unavoidable battery characteristic, although poor fabrication practices and improper handling can increase the problem. It’s a complicated function of basic battery chemistry, manufacturing, quality, storage, temperature, and even state of charge. For these reasons, it’s not possible to provide a single number for a given battery chemistry.
For lithium-chemistry batteries, this self-discharge is on the order of 1% per month. The qualifying phrase here is “on the other of” since it varies significantly between vendors, battery types, and even apparently similar batteries from a single vendor. A basic internet search shows the many different numbers for the same nominal battery. Figure 1 shows one assessment, but you’ll find surprisingly different numbers from various reputable sources due to the inherent nature of the situation.
Note that self-discharge is not easy to measure accurately; however, as it is a very important battery specification, there are DIY arrangements as well as sophisticated instrumentation packages which do it, Figure 2.
The problem is that self-discharge is analogous to interest in your savings compounding over time, except in the opposite direction. A difference of a percentage point or even a fractional point may not seem like much at first, but the cumulative effect is significant. For example, a high-end bobbin-type LiSOCl2 cell can have an annual self-discharge rate as low as 0.7%, and thus retain over 70% of its original capacity after 40 years. By contrast, lower-quality cells using that same chemistry can have a self-discharge rate of up to 3% per year and lose 30% of their capacity within a decade. Some cells have even higher nominal self-discharge percentages, even under optimal conditions.
For ultra-low-power IoT devices, self-discharge loss in battery capacity may be comparable to or greater than the loss due to load drain even in various “off” modes. That’s a harsh reality that is easy to overlook when doing those “back of the envelope” calculations on battery capacity and run time, Figure 3.
My eternal battery?
But here’s what has me really puzzled about self-discharge: I have a small kitchen scale, made in 1994 (actually, I bought two of them at the same, intending to give one away as a gift, but that didn’t happen; one is used daily for a few minutes while the other one is rarely used), Figure 4.
They were sold by Measurement Specialties, Inc., which was acquired by TE Connectivity in 2014. By that time, MSI had moved on from mainstream, lower-end consumer products to instead focus on sensor technologies related to pressure, vibration, force, temperature, humidity, ultrasonics, position, and fluids, primarily for industrial/commercial applications and industries.
What has me mystified is that both scales still work and show high-contrast digits on their LCD displays, (digit fading is a sign of battery weakening). Even if the scale usage cycle is low, that’s still a remarkable battery life considering self-discharge and the soft on-off switch which means some circuitry is always consuming current to monitor that switch. I doubt if this product, which cost about $20 when purchased, had one of those higher-end batteries with self-discharge under 1%, so I can’t figure out how this is possible, even with the favorable ambient temperature in which it is stored.
I tried to get some insight by doing a non-destructive investigation on the less-used scale but was only able to do a partial tear down due to the mechanical arrangement of its strain-gage cantilever. I could see the small battery, but it was encased in what looked to be yellow shrink-wrap tubing and had no visible markings (the scale’s bottom-side product tag does use have a minor clue, the word “lithium”). I really want to know how this relatively inexpensive scale can still work after nearly 30 years, considering both self-discharge and sleep-mode drain.
Have you ever had a design’s operating life falls short due to failure to adequately characterize the impact of self-discharge? Or did someone inadvertently or innocently substitute a lower-performance battery in place of the vendor and model you specified on the bill of materials? Perhaps you’ve had counterfeit or mislabeled batteries worked their way into your supply chain (after all, functioning batteries are relatively easy to fake).
References (there are many more, of course)
Battery University, “BU-802b: What does Elevated Self-discharge Do?”
MicroBattery, “Battery storage: expiration, self-discharge, and shelf life”
Battery Technology, “Measuring Lithium-ion Cell Self-Discharge”
Keysight Technologies, “Self-Discharge Measurement Solutions”
Apogeeweb Semiconductor Electronic, “What is the Self-discharge of Batteries?”