AA battery shoot-out. What's in a brand?

Dr. Slack, Tue Jan 03 2017, 03:04PM

After I stopped dismantling single use cameras a few years ago, my stock of AAs has gradually dwindled to nothing, and I need to *shudder* buy some batteries. I do use rechargables for a few specific high drain applications, but there are plenty of clocks, effects pedal and other miscellaneous spare torches that make having a few ordinary batteries sensible.

On the shelf in my supermarket, I have one the one hand Duracell Ultra (<deep voice>our most powerful AA ever </deep voice>) at £4.59 for 4, and on the other Wilko's 'long life' alkaline at 50p for 4. That's practically an order of magnitude difference in the RRP. I wonder if there's an order of magnitude capacity difference?

I've just been hooking my arduino up to my PC, to see if I can get the serial going, so a battery tester is a perfect 'Hello World' application. While the arduino's ADC is something of a toy, it's more than adequate to draw a curve of a discharging battery. I ran the Wilko and another cheap battery (25p to 30p each depending on quantity from Asda) yesterday, and I have the Ultra and (just for comparison) a NimH on there today. I'm testing at quite a high current, so it's not hanging around forever, a constant resistance load of 7.4ohms which is easier to do than a current sink, so load varying from 200mA down.

Before I post the results tomorrow, does anybody have any guesses as to what the ratio of performance between the brands will turn out to be?

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guesses so far
Hydron Wilko ~= 0.67 Duracell
klugesmith Wilko >= 0.67 Duracell
Sulaiman Wilko = 0.50 Duracell
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Yay! Everybody has won! And not surprisingly, the more expensive the battery, the longer it lasts, at least for these three samples.

The industry seems to use constant current loading, which is perhaps more reproducible across different tests than constant resistance. What I've done is to normalise the x axis to the charge delivered.

By picking different endpoints, I can accommodate all of the guesses. If the endpoint is 1.2v, then the Ultra delivers twice the charge of the Wilko (and some appliances I've met *do* have a very inefficient high endpoint like that!). If we take the more standard 0.9v as the endpoint, then the ratio is less than 1.5:1. @Klugesmith that's why I wanted a full plot, not just a time to a single (probably poorly chosen) endpoint.

I suspect the (relatively) poor performance of the cheap battery at 1.2v compared to 0.9v is down to internal resistance depressing the voltage, rather than early charge exhaustion. This is also consistent with its lower voltage right from the get-go.

It's not clear from the plot, but I did hammer all three alkalines down to below 0.4v on load. Interestingly, all had recovered after 12 hours off load to 1.1v or more. Rate of depolarisation is obviously then a mechanism for the excess drop in voltage at higher current.





I also compared the Duracell against its specification (graph not shown), which was done for 100mA and 200mA. It slightly under-performed the 200mA line for the first Ah, and then as the current dropped, ran between the 200mA and 100mA lines for the second Ah, before beating the 0.9v/100mA spec by about 10%. One thing I haven't controlled for well is temperature, though my room is a 'cool lab' somewhere around 20C. I was surprised to see in the Duracell spec that the batteries lose 33% of their capacity in going from 21C (the spec temperature) to 0C, and more than 50% in going to -10C, so the tempco is fairly vicious.

I may burn another £1worth of batteries (I did not pay full price for the Duracells, they were on a half price offer) for a lower current run, when I've improved the test setup to add switchable loads so that I can track the internal resistance separately.

The 'jaggy' plots are probably due to the fact I was using VCC for the ADC reference, which comes straight from the USB, and I was using the PC from time to time while the test was running. If I do a second run, I'll use a more stable reference. I notice there's also a 'quiet ADC' mode that sleeps the processor when making a conversion, I might try that as well.

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