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A wall-wart alternative

Discussion in 'Home Power and Microgeneration' started by [email protected], Jan 18, 2006.

  1. Guest

    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--------||-------------->|------------>
    || | |
    | |
    120 VAC /-/ --- to appliance
    ^ _
    | |
    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
     
  2. bw

    bw Guest

    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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