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How did batteries with a charge indicator work and why are they not more widespread?

It will have happened to everyone to find some batteries forgotten in some drawer or inside some household appliance that is no longer used disposable. In these cases it is difficult to understand whether throw them away or keep thembecause without the appropriate tools it is impossible to know if they contain enough energy that they can still be used, perhaps in less energy-hungry devices such as wall clocks or remote controls. Already in 1996 one had been invented for this problem quick, convenient solution and always at hand: two competing companies, Duracell and Energizer, had in fact launched batteries on the market which, with the simple pressing two keysthey could indicate it battery status thanks to the use of special thermal inks. Today these batteries are no longer used, it is not clear why, but we can assume that it is simply not worth it from a production cost point of view.

What are alkaline batteries and what characteristics do they have

Common disposable batteries work thanks to chemical reactions: in the case of batteries alkalinethe most widespread on the market, at the two poles we find metallic zinc (Zn) and manganese dioxide (MnO2) separated by an alkaline compound that gives them their name, thepotassium hydroxide (KOH)which allows the passage of ions from one side to the other.

A battery has two main characteristics: thecurrent intensity which is measured in amperes (A) e potential difference which is measured in volts (V). Both quantities can be measured with electronic instruments such as multimeters or special testersoften sold by the battery companies themselves.

How the battery charge level was measured: a circuit and ink

However, every measuring tool has a problem: you never have it with you when you need it. At least that’s what they must have thought Burroughs and O’Kainthe inventors of a miniaturized system to insert on the label to test the batteries. In the 1996 both Duracell and Energizer, although competitors, proposed very similar battery strength measurement systems: in one case a simple indicator, capable of coloring and make the word “good” appear contrasting, while in the other the test area was composed of one long stripe that could be colored partially or completely yellow based on the remaining power, like a “loading bar”.

In more recent versions, the colored stripe has been changed and divided into 4 sectorswith a color scale from red to greento make the battery health status even more intuitive.

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The battery test in action: by pressing on the indicated points, the band on the label turns partially coloured, indicating the approximate amount of charge left. Those who have used it will remember the pain in their fingers.

But how exactly does this indicator work? The label has a colored area with thermochromic inkthat is, capable of change color as the temperature increases. Below the label, there are several layers. The first is a metal foil, which is placed with the pressure of the fingers in contact with the battery poles: the positive pole is a single piece with the body of the battery, so one of the two points to press is on the side, while the negative pole is a bottom (that little button on the back) separate from the main body.

The remaining layers, an insulating film and a thin piece of paper, serve to maintain the foil detached from the battery: in fact, every time the test is carried out, a small amount of charge is consumed, therefore it is important that the two parts do not touch each other if not the few seconds needed to read, to avoid draining the battery unnecessarily. When we press with our fingers, the foil comes into contact with the battery poles allowing the passage of current.

Stack powercheck elements

As can be seen from the photos, the plate is narrower at the negative pole And expands towards the positive: it is this difference that allows us to estimate the amount of residual power. And here’s why. A conductive material crossed by a current, even as small as that of a battery, it heats up because it offers a certain resistence to the passage of electrons. This resistance depends both on the material used and by its size (or section): in the narrowest part the lamina has greater resistance and it heats up with a lower voltage, so as long as the battery has little charge. While to make the sectors that we would see green change color, which correspond to the maximum charge you will need the 1.5 V from a new battery, because below these the plate is wider and it has one lower resistance.

When we press on the two contacts, due to the current flowing through it, the foil heats up and the thermochromic ink on the label colours! This nature thermochromic of the ink, however, also makes the reading dependent on the ambient temperature: in fact the label suggests perform tests at 21°C to get a correct reading. If we heat the battery with a hairdryer, for example, we can notice how the test bar colors even without having pressed the buttons, as you can see in the video that accompanies this article.

Because the battery indicator has disappeared

How come such a simple to use and convenient function it did not spread to all batteries and today, in fact, it is rarely used? In reality it is not clear to define a reason why they have not spread. We can though hypothesize that, simply, the game is not worth the candle. As simple as it is, theadding the test translates to a higher cost for battery for sale: not surprisingly, today this technology is found only in some “premium” batteriesalso to dyears since the expiration of the two patents.

Definitely there “patent war” between Duracell and Energizer, it did not help the spread of this system. The first to be filed was that of Burroughs/O’Kain, filed in 1991 and licensed to Energizer, but, it is thought due to the carelessness of the patent office, a few months later in 1991 the Kodak However, he managed to record his version of the test, which was later used by Duracell. In 1996 both companies they decided to launch the products on the market, despite the risk of legal consequences due to similarities between patents. Legal consequences were not long in arriving and, as we learn from the Wall Street Journal, the two inventors Borroughs/O’Kain who had first filed the patent prevailed.