LM338 as battery charger?

Discussion in 'General Electronics Chat' started by fireofenergy, Dec 26, 2011.

  1. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    Hi,
    Just wanted to know what happens when an input voltage dips below the drop out voltage for a linear regulator. In this case, I want to charge up 4 lithium iron phosphate (not li-ion) at about 3.5v ea in series = 14v...FROM a solar panel... which, being rated at 18 volts (and higher in cold temps) can, in higher temps, drop considerably. Now, the voltage difference in the lm338 specs say like 2.5 volts.

    If, theoretically, the panel only put out 15v, and the LM338 was set to 14v, I know I would lose voltage, but I would lose current too?

    Thanks
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    You can reduce the voltage headroom the reg requires. You could use a low dropout reg like LM2941 that only needs less than a Volt across it to regulate.

    You can also get "semi LDO" regs like the LT1085 that only need about 1.3V across them to regulate.

    How much current do you have to have?
     
  3. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
    20
    0
    I'll look them up... I figure less than 3A for current which will decrease to almost nothing as the batteries charge up to the set voltage (or so I think).
    I'm just in the "figuring what to do" stage with solar and lifepo4 batteries...
    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Don't forget that you will need to keep the solar cell from discharging the batteries. Once the light fades past a certain threshold, your solar panels will start draining your batteries instead of charging them.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Lithium battery cells are never charged in series because each one has a slightly different capacity. They are charged separately with a "balanced" charger circuit (these circuits can be in series) and the battery has a separate charger wire from each cell.

    If charged in series, the weakest cell is fully charged first then is over-charged when the other cells are still charging.
    Don't gamble with Lithium. Sometimes it burns like a magnesium flare.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    True, the LiFe packs being used in the auto apps have "monitor/balancers" across each cell monitored by a central controller.
     
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