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Blog: What's in a number?
How our lives are dominated by numbers yet we somehow get it wrong

8. How long does a battery last?

A sample of batteries in common use.

When we buy a battery, we trust the make and type that it will deliver the performance we need. It should not have to be a matter of trust. In other words, just like any other product we buy, it should be labelled with the appropriate information. After all, when we go into a supermarket to buy some apples, then we know what we are getting either the number is specified or usually the weight. So a kilogramme of apples will weigh exactly that. Unfortunately, with batteries it is rather different.

A battery is a device for converting chemical energy into electricity. It is a convenient, often small, package for delivering a reliable voltage and current, for example to run a portable radio or a torch. Eventually, the chemical energy becomes depleted and the battery needs to be discarded in favour of a new one.

In designing the battery compartment in equipment, the designers bear in mind the current and voltage needed and the likely time of use to determine an optimal combination of batteries. For example, AAA batteries are much smaller than AA batteries and over their lifetime deliver less electricity, and are therefore used in equipment which uses relatively little electricity.

But how much electricity is delivered and why aren't we being told? Instead, we have to rely on experience with a piece of equipment to know how long a fresh set of batteries is likely to last. We also presumably have reports by Consumers Association and so on which compare batteries from different manufacturers. Although such information might be available in principle, very few consumers have probably looked at it, but instead rely on advertising. For example adverts on the television boasting how a particular brand of batteries lasts longer than the competition by running a simple toy. It's very effective advertising in that it gets the company's product to your attention, but it is quite uninformative.

Another thing is that over the decades battery technology has improved enormously, although because the batteries are not quantitative, we don't get to see this. Instead, over time we have been subjected to a diet of superlatives. The last few versions have been Heavy Duty, Super Heavy Duty, and now Plus Power, none of these have any quantitative meaning to the consumer that I'm aware of. The last is even misleading, as it is labelled as "40% more power". This confuses me, because if I buy a battery it is used on fixed equipment which draws a steady power, say a hundred milliwatts. So rather than getting more power from a battery, I would normally like it to supply more charge during its lifetime, so that the battery lasts longer.

For some strange reason, rechargeable batteries are much better labelled. I have just checked on the internet, and you can buy rechargeable AA which have a capacity of 2850 mAh, compared with about 850 mAh for AAA. These numbers are written on the battery casings. I bought rechargeable batteries many years ago, perhaps 10 years ago and their capacity was then about 1800 and 650 respectively. Theses are reasonably clear cut numbers: if I use a single AA battery which draws 100 mA of current, it should last 2850/100 = 28.5 hours. Of course if you double the current it may not last 14 hours, and the exact performance of the battery as a function of the charge drawn may not be known by the consumer, but the information given is a tremendous improvement on non-rechargeable batteries.

The performance of the Duracell AA battery for fixed 100 mA current at 0 C and 21 C, as a function of time, as reported by Duracell.

In all fairness to Duracell, you can obtain technical information on their batteries, but not all manufacturers make this effort. This shows that when drawing a current of 100 mA, their AA batteries last a little less then 25 hours, at 21 C [it seems that they prefer medieval units of 70 F for measuring temperature], but only 15 hours at 0 C. This would suggest a capacity of 2500 mAh at 21 C, but we can't compare with the rechargeables, as it is not clear which temperature their rating is valid at, nor the current implied. Certainly, I can't believe that rechargeables are better than regular batteries.

The question I'm always faced with when I buy batteries, then, is: Should I pay more for the well-advertised batteries like Duracell, or go for a much cheaper alternative which may be almost as good. I should be able to calculate the cost of the batteries in pounds (or dollars if you come from across the pond) per Amp hour, just like apples are priced per kilogramme. It frustrates me that I can't do this and have to resort to consumer reports or engineering reports. I can't do this if I'm comparing prices in the shop.

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Website revised by John Austin, 29/1/2015. © Enigma Scientific Publishing, 2015.