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Home > Archive > Alternative Power sources > January 2006 > A wall-wart alternative
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A wall-wart alternative
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| nicksanspam@ece.villanova.edu 2006-01-18, 5:21 am |
| Duracell says their alkaline batteries have a 7-year shelf life, defined
at 80% remaining capacity, with no discharge. I spoke with a Duracell ap
engineer who had a hard time understanding that's the expected life in
a circuit, if the battery powers the circuit and we supply the battery
with exactly the current the circuit requires.
He had a harder time imagining that the battery might last longer than
7 years if we supply an additional "shelf current" to make up for the
self-discharge over 7 years, saying that might avoid turning some zinc
into zinc oxide, but it wouldn't change the electrolyte dissolution or
materials breakdown that also degrade capacity.
He was also unconvinced that float-charging a battery with an intermittent
AC supply or load (eg charging a 4.5 V clock radio battery that supplies
120 uA to the clock alone but 50 uA with the radio running to 4.5 V using 2
5.1 V zeners in the circuit below) to its nominal voltage would extend it
beyond 7 years, saying one smoke detector manufacturer drastically reduced
battery lifetime by using a circuit that accidentally charged the batteries
with "a few milliamps," vs microamps.
Supplying a shelf current along with a load current is promising, IMO,
but supplying shelf current alone in a new battery-backed-up AC-powered
product with a battery test function seems too costly and inefficient,
given the tiny possible reliability improvement.
hot 10K || Cload
-------www--------||-------------->|------------>
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120 VAC /-/ --- to appliance
^ _
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neutral | |
------------------------------------------------>
20 DATA "AAA",0.27,"AA",1.1,"C",2.8,"D",8,"9V",0.5
30 FOR B=1 TO 5
40 READ TYPE$,AH
50 ILOAD=AH/8765'1-year discharge current (A)
60 ISHELF=.2*AH/(7*8765)'self-discharge current (A)
70 Cload=1000000!*(ILOAD+ISHELF)/(60*370)'1-year charge pump cap (uF)
80 Cshelf=1000000!*ISHELF/(60*370)'7-year charge pump cap (uF)
90 PRINT TYPE$;TAB(6);AH;TAB(12);1000*ILOAD;TAB(28);C1;TAB(44);C10
100 NEXT
Type Ah Iload (mA) Cload (uF) Cshelf (uF) Digikey # Price
AAA 0.3 0.031 0.0015 0.000039 P10799-ND $2.96/10
AA 1.1 0.125 0.0068 0.00015 P10806-ND $3.08/10
C 2.8 0.319 0.015 0.00039 P10811-ND $3.92/10
D 8.0 0.913 0.047 0.0012
9V 0.5 0.057 0.0033 0.000068 P10802-ND $2.96/10.
The curves on the technical Duracell web site don't define capacity. After
the above Ah discharge, the output voltage reaches 80% of nominal voltage,
eg 1.2 V for a 1.5 V cell at 0.25 watts. AAAs store a lot less than AAs...
Nick
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<nicksanspam@ece.villanova.edu> wrote in message
news:dqkucs$fp5@acadia.ece.villanova.edu...
> Duracell says their alkaline batteries have a 7-year shelf life, defined
> at 80% remaining capacity, with no discharge. I spoke with a Duracell ap
> engineer who had a hard time understanding that's the expected life in
> a circuit, if the battery powers the circuit and we supply the battery
> with exactly the current the circuit requires.
> Nick
The chemical reactions are not as simple as the textbooks imply. There are
contaminants, side reactions and surface effects that even the designers
don't fully understand.
Generally any of the simple primary cells CAN be re-charged, because the
reactions are reversible. The problems are practical engineering. There is
one battery engineer that sometimes posts on
sci.chem.electrochem.battery that might be able to help.
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