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

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Dr. Slack

Tue Jan 03 2017, 03:04PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

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 Â£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?

--------------------------------------------- ---------------------------

edit
guesses so far
Hydron Wilko ~= 0.67 Duracell
klugesmith Wilko >= 0.67 Duracell
Sulaiman Wilko = 0.50 Duracell
/edit

---------------------------------- -------------------------------------------------- -------

edit
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 Â£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.

/edit

...

Tue Jan 03 2017, 03:58PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The cheap ones are probably dry cells, so should have a capacity of about 200-500mA*hr depending on construction. The Duracell should be alkaline which will be just over 2000mA*hr. If it is the newer 'lithium' chemistry it might have upwards of 3000-4000mA*hr

Hydron

Tue Jan 03 2017, 04:01PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I'm going to guess at the el cheapo cells being about 2/3 the capacity of the ludicrously expensive ones.

On another note, we have some Varta "Industrial" cells knocking about the stationary cupboard at work - I wonder what the "Industrial" thing actually means?

Hydron

Tue Jan 03 2017, 04:05PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

... wrote ...

The cheap ones are probably dry cells, so should have a capacity of about 200-500mA*hr depending on construction. The Duracell should be alkaline which will be just over 2000mA*hr. If it is the newer 'lithium' chemistry it might have upwards of 3000-4000mA*hr

The cheap cells were specified to be alkaline. I would never touch a zinc based dry cell - they literally eat through the casing when left sitting, and I've seen enough electronics ruined by leaky cells thank you.

klugesmith

Tue Jan 03 2017, 04:05PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

As you said, the load is a bit heavy for alkaline AA's. I bet they will all be exhausted (what the OEM's call 0.9 volts per cell) at times within a factor of 1.5. Including the NiMh.

If you just want to measure hours, instead of mAh or mWh, it gets even simpler. Put your 7.4 ohm load in parallel with a cheap quartz wall clock, whose big and little hands have been set to 12. If the clock is in customary vertical-plane orientation, I bet you will also confirm that eventual stoppage comes with the seconds hand at about 9.

@hydron: Alkaline and "zinc-carbon" cells both consume anodes made of zinc.

Sulaiman

Wed Jan 04 2017, 06:09AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I am very interested in the results
I guess

Wilko = 0.5 x Duracell

(I buy Poundland batteries, mainly due to route to workplace)

Dr. Slack

Wed Jan 04 2017, 02:30PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

results edited into the OP

Hydron

Wed Jan 04 2017, 08:04PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

That graph tells me that when I need a high performance battery, why not pay a little bit more than a Duracell for a decent low self discharge NiMH of similar capacity. Will keep using the cheap alkaline cells for remote controls etc

Uspring

Thu Jan 05 2017, 11:05AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Dr. Slack: That's one of these little measurements I've always wanted to do myself but somehow never got around to. Since you are using these batteries mostly for low drain purposes (as I), you've probably thought of adding some unloaded recovery periods in order to simulate these kind of conditions(?)

Dr. Slack

Thu Jan 05 2017, 11:26AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'll get around to a low drain measurement in the fullness of time. As I said, this measurement was little more than a Hello World for hooking up my Arduino to python on the PC. I'll put python on my server so I can leave it running for a week. First I'll look at reducing, or at least understanding, the noise on the ADC, if I can be bothered to do that before I add the standard 16 bit ADC that everyone seems to use.

I like to have evidence based answers for 'should I buy generic, or brand?' I wonder, should I write to Duracell, not with a complaint, but just the observation, to see whether they offer me any swag to keep quiet, well, no more noisy anyway?

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