The Rise and Fall of Built-In Battery Testers: A Nostalgic Journey
If you were a kid sometime before, say, 2010, there was a phrase your parents probably learned to be quite irritated by: batteries not included. Our gadgets and gizmos used to be quite reliant on the good ol’ AA battery or, if designers wanted to be annoying, the AAA battery. Look, there’s not a huge difference in size between the AA and the AAA, and they usually cost the same per battery. So whenever some engineer out there decides that a thing which could clearly fit longer-lasting AA batteries in it will go with just as expensive but shorter-lived AAAs instead, I get a wee bit angry. I mean, just look at this remote control. It could totally fit AAs in there, just look at how much extra room th – wait is that a Duracell PowerCheck? Indeed it is!
The Advent of Duracell PowerCheck
If you were buying batteries in the late ‘90s, you probably remember these. The idea was that rather than need to use some sort of battery tester to see if a battery still had any charge left in it, you could build that tester into the battery. And that’s what Duracell did – as well as some of their competitors. As a matter of fact, this version of the idea came from Kodak, and there’s a whole fascinating story around patent disputes which I won’t be getting into (long story short, James R. Burroughs had a similar but technically different idea slightly earlier than Kodak but both patents got approved which caused some kerfuffles – I’ve linked to a great article down below which explains all that). Here, I just want to focus on how the Duracell version worked, because it’s quite clever! But also much simpler than you might have imagined.
How the Duracell PowerCheck Worked
If you want to test a battery like this, you’re going to want to measure its voltage. Like most battery chemistries, ordinary alkaline manganese dioxide cells experience a drop in voltage as they discharge. When fresh, each cell has a voltage around 1.5, maybe 1.6 volts (and the standard AA, AAA, C, and D batteries are composed of just a single cell, like an amoeba!). But that voltage steadily diminishes as the battery is used up. When you’re halfway through, it’ll be down to around 1.2 volts, and when it’s near empty, it’ll struggle to produce even a single volt.
So, a quick check of a battery can be done with a multimeter like this one. If I use it and probe across this battery here, it’s reading 1.57 volts, so this is either brand new or hardly used. This one, though, it’s only reading 1.06, so it’s pretty dead. This test isn’t perfect because we’re not checking the batteries under a load, but it’s still fairly reliable. Trouble is, you need a multimeter to do this as well as the knowledge of how much voltage drop is acceptable before it’s dead.
The Intricacies of the Built-In Tester
So lots of inexpensive battery testers were made which are really just analog voltmeters with a scale that reads good and bad. But what if you could build a voltmeter into the battery itself? Well, that’s ultimately what Duracell did with these batteries, but the way they work is a little… odd. First, you’ll notice that the battery says to test it at 70 degrees Fahrenheit or 21 degrees Celsius. That probably means the tester’s functionality has something to do with temperature, and indeed it does. But before I get ahead of myself, let me just show you how these were used.
It says “press dots to test” and, well, that’s all you had to do. Except… if you remember these, you’ll know that you had to press quite hard. Dig your nails in there a little bit. Once you did it though, the little bar below the power gauge would suddenly turn yellow, filling up from right to left. If the whole bar turned yellow, the battery was full. And less and less of it would turn yellow as the battery wore down.
The Science Behind the Test
Now, with a use-by date of March 2003, it probably won’t surprise you to learn that these batteries are in fact very dead. I got the tester to turn yellow with a little movie magic involving a foot switch and a hair dryer. Because heat is what turns that little bar yellow. Behind that little window is a small amount of thermochromic material, which as the name implies changes color based on temperature. It’s the same sort of stuff that goes in a mood ring or one of those thermometers you might stick on an aquarium. The specific chemical in here is simpler – it merely indicates it’s above a certain temperature threshold, but it’s the same general idea.
What makes it heat up, though? Well, you may have noticed that the testing window is perfectly in-line with the two dots you’re supposed to press to test the battery. The tester is built into the battery’s label, which I’ve removed from this one. In case you’ve never seen an unwrapped alkaline battery before, well this is what they look like. The positive terminal is actually part of the battery’s shell, and it extends almost all the way down to the negative terminal. A small insulator separates the negative tab from the shell.
The Mechanics of the Built-In Tester
And if we compare with one of the still-wrapped batteries, we’ll see that one of the testing dots rests on the negative terminal and the other rests on the battery’s shell (which, remember is the positive terminal of the battery). Looking now at the backside of the label, we’ll see that those testing dots are above electrically conductive pads. The pads on the negative side are very difficult to see, they’re these three grey patches which don’t look all that impressive, but the positive contact patch is quite obviously a metal foil of some sort. And the contact patch sits behind a round hole in a thin piece of beige insulating material. That insulating layer keeps the pad from touching the battery shell unless you actively press on the label and force it through.
If you exert enough force on both dots for their respective contact patches to touch both ends of the battery, you complete a circuit. To what, you ask? A tiny little heater. That heater is in contact with the thermochromic material, so if the battery has sufficient charge, when you press in those dots and complete the circuit, the heater will warm up the indicator and turn it yellow. But it’s not quite that simple. While you could just use a very thin wire heating element, that would only allow for a simple pass/fail test. If the battery had enough oomph left to produce enough heat to make the indicator yellow, great! But you wouldn’t necessarily know how much oomph it had left.
The Tapered Heater Element
So instead, the heater has a varying cross-sectional area to produce a gradient of heat output across its length. If I remove that paper insulator, we can actually see the heater. Kind of, anyway. I’m not sure exactly what it’s made of – it might actually just be the same foil that makes up the contact patch. Regardless, it is visible, and you’ll notice that it has a slight taper, becoming narrower towards the negative end of the battery. That means its resistance actually varies across its length, which also means the voltage drop across each part of this heater when it’s operating is different.
If the battery is strong enough, its voltage will be sufficient to heat the entire thing, so all of the indicator turns yellow. But if the battery is getting weak and voltage begins to fall, then only the skinny, high-resistance part of the heater will get warm. Most of the voltage drop occurs in that part of the heater, and the battery doesn’t have any more volts for the rest of it, so only that part gets warm enough to turn the indicator yellow. And luckily I can show you this with a DC power supply. When I provide this with just 1 volt, you’ll see that only the right side of the graphic turns yellow.
The Demise of PowerCheck
To clarify, the indicator always turns yellow at the same temperature, and the entire bar is filled with the same indicator. But low voltages (caused by a weak battery) can only get the skinny part of the heater warm enough to actually change the indicator’s color. But when I crank the voltage up, now all of it gets warm and so the whole bar turns yellow. And of course, the bar is aligned with the zero to 100% scale to give a reasonably accurate impression of the battery’s useful life remaining. It’s a very elegant solution using tech so simple that it can be printed on a label.
Oh, and remember that beige insulator? Turns out it’s not just electrically insulating, it’s also thermally insulating. That slot cut into it is there to maintain an air gap between the heater and thermochromic material and the battery shell. The battery shell is made of thermally-conductive material, and it’s rather massive, so if the tester were in physical contact with the battery shell, it would suck all the heat right out of it and it would never turn the indicator yellow. Keeping it away from the battery is necessary for the indicator to function, so the same piece of what’s essentially just paper is functioning as both a crude power switch for the tester and thermal isolation for the indicator. Neat.
The Legacy of Disposable Batteries
You might have realized that since the battery is powering a heater to test itself, that means each time you test it you’re using up a little bit of its capacity. And this is true – though it also means the test is slightly more accurate since you’re doing it under a load. It’s not like the actual scale here is all that accurate but testing under load is critical. These 25-year-old batteries test at about 1.3 volts on the multimeter, which would suggest they’ve got about half their capacity remaining. But their ability to produce any current flow is completely shot.
If I manage to activate the tester while also probing the battery, you’ll see that the voltage plummets to not even half a volt as the heater is placed in-circuit. So, sure enough, these batteries are dead, and their inbuilt tester does actually reflect that. Yet the multimeter doesn’t. So then… why did it go away?
Duracell PowerCheck was a pretty short blip on the landscape of disposable batteries yet it’s clearly a useful, functional feature. Well, the patent disputes and licensing shenanigans are one clear reason, but I honestly think there’s a simpler one: not that many people were actually testing their batteries on a regular basis!
Yes, we all know about the loose batteries in the junk drawer that may or may not save the TV remote in a trying time, but aside from random situations like that, I think it’s fair to say most people used and continue to use batteries in a pretty straightforward way: they simply stay new in the package until they get put into a thing which needs batteries, and then they stay in that thing until they die, at which point they’re gotten rid of and another set of fresh batteries replaces them.
The Shift to Rechargeable Batteries
I’d argue it’s pretty uncommon to take partially-used batteries out of a thing and then put them in the junk drawer for later use. I’m not saying I think that never happens – I myself have a few tools which need batteries but I don’t keep batteries in them since I rarely use those tools. But I do think in the grand scheme, keeping half-used batteries around is infrequent. Certainly infrequent enough to question whether it actually makes sense to spend the money putting a battery tester on every single battery. These are commodity items, and while there surely is some brand loyalty out there for the most part they’re competing on cost, and so if you need a pack of batteries and you use them like I do, a built-in battery tester is just a weird gimmick which makes them more expensive (and makes me less likely to buy it).
And as I already said, if you remember these, they weren’t easy to use! It was actually quite frustrating to jam those dots in there and hope it turned yellow. And if I were the type of person who kept partially-used batteries in random places, I would probably just buy a battery tester and keep it around. Or, ya know, just try putting them in whatever needs batteries and see if they work. That was always an option.
I think that in hindsight, this is a pretty clear case of a really clever, and very neat idea which was unfortunately born from a very particular frustration. A frustration that the inventors thought was more common than it actually was.
Duracell used to include inexpensive battery testers in the packages of their batteries, and they worked very similarly to the PowerCheck feature here. Some people found them hard to use, or were bothered by the fact that they were easy to lose, and that could be seen as a legitimate problem to solve. Which Duracell did by incorporating those testers into the battery! But I don’t think anyone asked how often those testers were actually getting used in the first place. Because if they had, I’m not sure this would have ever happened.
Of course these days, most of our devices which need batteries are using rechargeable batteries, so we’re just not buying or using disposable batteries as much as we used to. Which I think is unequivocally a good thing! It’s kinda nuts how normal it used to be to burn through these and then just throw ‘em away. I had a Game Boy and that one device must have gone through dozens of these. And then there was the Walkman, my camera, the light for the game boy, and who knows how many other toys I had which needed constant battery replenishment. Thank goodness we’re past that. Mostly.
The Future of Battery Use
Uh, but even for the devices on sale today which need AA batteries? They often tell you how much charge is left, so putting a tester on the battery itself really just isn’t necessary anymore. Alright, well I think we’re done. I want to extend a thank you to Luis Solano for suggesting this video topic. It’s been on the docket for a while but, uh, well these haven’t been in production for quite some time, so it took me a while to get my hands on them. And I have a second thank you for Ernie Smith – this is at least the second time my research began and immediately I was presented with a thorough Tedium article.
I would have gone more deeply here were it not for the fact that this video was originally slated for November, but production issues on a different topic moved it up to now, so if you’re interested in those patent details and some of the backstory, be sure to check out the link in the description. Normally I try to put something clever here but I think I’m all out of juice.
♫ exhaustedly smooth jazz ♫ like most battery chemistries, ordinary alkaline magaz— magazine? Are you subscribed to Alkaline Magazine? Lots of inexpensive battery testers were made which are really just analog voltmeters with a scale that ride— DANG IT Kind of. I’m not exactly… oh I don’t like that delivery …but there ability to produce any curr…mmmmnaAAGH …plummets to not even half a volt as the plater… mehah huhhh It was actually kind of frustrating to push these dots in and wait for it to light up. And if I were the type of person who kept part- yep, I can’t say “light up” I’ll get pedanted for that. If I were the type of person who kept randomly used partially used aaaaahggghhh! So apparently these were made for much longer in some markets and also hung around for a while here in the US on some of the high-end batteries Duracell sold. TBH I had no idea but mostly because I’m a value-obsessed Midwesterner who mostly buys generics (or might splurge for Rayovac if it’s on-sale). Does this count as an end-captions gag? You decide!
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