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Old 18th Jan 2021, 12:37 pm   #1
Alan_G3XAQ
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Default AAA alkaline primary cell capacity at low current?

Manufacturers of run of the mill disposable alkaline batteries quote capacity only down to about 1mA load, typically citing 1.0 to 1.25 AH for a AAA (LR03) size.

I have a project that draws a tad under 100uA. Will the cell be flat after 1.25/1E-4 hours, so a bit under two years maximum? Or does the chemistry of these things mean it will last longer or shorter?

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Old 18th Jan 2021, 1:28 pm   #2
Radio Wrangler
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Default Re: AAA alkaline primary cell capacity at low current

Probably the damned things will have leaked before then.

David
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Old 18th Jan 2021, 1:29 pm   #3
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Default Re: AAA alkaline primary cell capacity at low current

Some years ago, I did test this aspect on Duracells (when they were the best) for items deployed underwater for long periods. Crudely, the answer was yes, the life does scale until you get to very low currents comparable to self discharge. In fact there was some data that suggested a better life at a few uA than at zero. I suspect you will be also at the mercy of quality and freshness in practice.
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Old 18th Jan 2021, 1:35 pm   #4
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Default Re: AAA alkaline primary cell capacity at low current

I find it hard to imagine a AAA battery pushing out a constant 1 amp for for a whole hour, or doesn't it mean that?

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Old 18th Jan 2021, 2:33 pm   #5
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Default Re: AAA alkaline primary cell capacity at low current?

For a cell with a nominal capacity of 1AH and discharged at 1ma I would by simple arithmetic expect about 1000 hours service.
At 100 microamps I would expect about 10,000 hours service, the run time at different low currents does scale linearly or very nearly so.

At 10 microamps one might expect 100,000 hours service, and this might be achieved under favourable conditions, but note that 100,000 hours is over ten years. Failure due to expiry of shelf life is a significant risk.

No great accuracy may be claimed in such matters. The actual cell capacity will vary between brands, and vary a bit between different examples of the same brand.
Temperature also has an influence.
As does minimum load voltage, a load that works down to 0.9 volts will give a longer run time than one that stops working at 1.1 volts.
The age of the cell when first put into use is also relevant. A fresh from the factory cell may have 10% more available capacity than one stored for say 5 years, but still just in date.

Alkaline cells typically have a stated shelf life of 7 years from production, or about 6 years remaining when purchased.

For a practical application, also consider the consequences of failure.
If human life would be put at risk, then precautionary replacement long before the theortical life is prudent.
If great financial loss would result, then again replacement on a planned basis is prudent.
For a non critical application, replacent only when failed should be fine.
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Old 18th Jan 2021, 2:37 pm   #6
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Default Re: AAA alkaline primary cell capacity at low current

Quote:
Originally Posted by Aub View Post
I find it hard to imagine a AAA battery pushing out a constant 1 amp for for a whole hour, or doesn't it mean that?

Cheers
Aub
No it does not mean one amp for one hour, it means a total capacity of one ampere hour at a much lower current for many hours.
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Old 18th Jan 2021, 2:48 pm   #7
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Default Re: AAA alkaline primary cell capacity at low current?

Alan, do not under any circumstances use Duracell AAA cells for this application. I would go so far as to say that one or more of them will leak before their official expiry date with a current draw of 100uA. Many discussions about similar problems elsewhere on the forum.

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Old 18th Jan 2021, 3:14 pm   #8
Alan_G3XAQ
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Default Re: AAA alkaline primary cell capacity at low current?

Hmm. Conflicting advice here, as I expected. Yes, the 2 years or so figure was an extrapolation, a I said in my post (1.25AH/100uA). My pack of AAA Duracell Plus batteries are dated 2029 so the shelf life expires 8 years hence. The data (real data) from Ambientnoise was what I was looking for: so a straight line extrapolation is more or less valid and the batteries will be flat in under two years. Nope, nothing I do in electronics puts life at risk, I'm just a hobbyist. This project uses three AAAs to back up a two quid Arduino Nano Morse code gadget and the batteries are housed in a 50p plastic holder off eBay. The CPU clock will probably stop before the cells reach 1 volt so even 2 years look likely to be optimistic.

Thanks for the help.

Alan
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Old 18th Jan 2021, 8:29 pm   #9
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Default Re: AAA alkaline primary cell capacity at low current?

Four decades ago I was reconditioning old Grundig CTVs. One rather favoured model, the W8240, had four AA cells for memory backup. I was getting the sets at maybe 7 years old. Usually Varta, often the cells had split cases, though not leaking. i always replaced with 4 new Duracells, and never had a problem, often seeing them again after another 5 to 7 years. all were still OK. Back then, they were not chronic leakers. my advice would be to change from AAA to AA, both giving longer life, and maybe less likelihood of leakage. more importantly, do NOT use the Duracel brand. I suspect "higher power" cells are more likely to corrode than "normal" quality.
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Old 18th Jan 2021, 10:39 pm   #10
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Default Re: AAA alkaline primary cell capacity at low current?

For your application you would seem to need "heavy duty" batteries which are designed for continuous drain applications rather than "heavy current" types. I discovered the difference some years ago after Screwfix changed from supplying Ray-O-Vac batteries to yellow Varta "heavy duty" types. I used to use rechargeable NiCd AAs in my first digital camera, but always carried some disposables for backup when the rechargeables ran out. Whereas the Ray-0-Vac AA cells used to give good service, the nominally identical capacity yellow Vartas only lasted around half as long. Varta currently do (at least) two ranges. The more expensive ones are designed for high drain items such as cameras and motors. The less expensive ones should be ok for low current drain applications.

Last edited by emeritus; 18th Jan 2021 at 10:45 pm. Reason: Typos
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Old 19th Jan 2021, 1:01 am   #11
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Default Re: AAA alkaline primary cell capacity at low current?

I use AA or AAA alkaline or zinc-chloride in several devices that stand unused for long periods, switched by soft switches, which results in a continuous drain typically of 10µA and invariably they will be completely flat after a few weeks. If you remove the cell(s) or insert a strip of thin plastic they remain serviceable until you need them. Particularly noticeable on bicycle lights
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Old 19th Jan 2021, 8:06 am   #12
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Default Re: AAA alkaline primary cell capacity at low current?

Alan, years ago I built a CMOS morse keyer and powered it from a PP3 battery permanently connected. I deliberately chose the oldest battery I could find which had already been used to power something else (being a skinflint in those days). It kept it working for many years, I can't remember ever having to change it. Of course batteries were less liable to leak in those days...

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Old 20th Jan 2021, 2:35 am   #13
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Default Re: AAA alkaline primary cell capacity at low current

Quote:
Originally Posted by Aub View Post
I find it hard to imagine a AAA battery pushing out a constant 1 amp for for a whole hour, or doesn't it mean that?
I think the standardised capacity is measured during a 10 hour discharge. The capacity will be lower when discharged faster and remain roughly the same when discharged slower.
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Old 20th Jan 2021, 11:21 am   #14
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Default Re: AAA alkaline primary cell capacity at low current?

Thankyou Julian, i think i need to check my bicycle lamps. (NOT the ideal place to have a quiescent drain, far from it!)
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