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Laptop power supply for battery charger

Discussion in 'Electronic Basics' started by Harry, Sep 22, 2008.

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  1. Harry

    Harry Guest

    I have a 19 volt 3.5 amp laptop power supply that I want to use for
    equalizing lead acid batteries (car batteries and 75ah rv batteries).
    I know I can't leave it unattended because the voltage will fry the

    If I cut the end off the cord, attach alligator clips, and attach to
    battery until it boils will it work?
  2. Sjouke Burry

    Sjouke Burry Guest

    It is likely to fry your laptop supply, and if it doesnt,
    and your lead battery is small , you will miss the important
    moment and fry your battery.
  3. ehsjr

    ehsjr Guest

    There's more to equalizing than just charging the battery
    until the cells boil. Connecting as you mention might work,
    but you stand the chance of burning out the supply and killing
    the batteries.

    To protect the supply and provide a proper equalizing
    voltage and current, I would recommend this:

    | |
    D1 | ----- |
    | ----- |
    | + Adj [.5R]
    [Supply] | |
    | - +----------+---> To Batt +
    +---------------------------------> To Batt -

    D1,D2 are 1N5408's (3 amp diodes) and the .5R resistor
    is 3 1.5 ohm 5 watt resistors in parallel. The LM350
    must be installed on a heatsink. Allelectronics sells
    the resistors at 3 for $1.00, CAT# 1.5-5. They also have
    the diodes, the LM350 and heatsink.

    Equilization requires a limited current (about 3 to 7 percent
    of battery capacity) and a voltage between 14.5 and 16 volts.
    The circuit above limits the current to 2.5 amps. At 2.5 amps,
    the 1N5408 drops about .9 volts, the .5 ohm resistance drops
    another 1.25 volts, and the LM350 drops about 2.5 volts, so you
    have about 3.9 volts dropped in the circuit. Thus, with your
    19 volt supply, the battery receives about 15.1 volts at 2.5
    amps, which is about right for equalizing your 75AH batteries.

    The circuit does NOT protect the battery from being charged too
    _long_. It provides a proper equilization charge current and
    voltage, and protects the supply, but does not prevent the
    battery from overheating.

  4. neon


    Oct 21, 2006
    2.5 amps is not floating charge is a very good charging current for a car .5 amps is floating charge. And if you don't put a diode across the reg. you blow the reg.
    Last edited: Sep 24, 2008
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