battery charging

Discussion in 'General Electronics Chat' started by djbrooksy, Apr 25, 2010.

  1. djbrooksy

    Thread Starter New Member

    Sep 19, 2009
    HI guys and girls

    this is going to be a stupid question :D:D:D i know ya all love them

    ok i have a pit bike or a minni bike whatever you wish to
    call them.[​IMG]
    i have a EFI kit to go on

    now my problem is i dont wish to run a full size battery that you would
    find on these bike as it will only be needed to power up the ecu before
    i start the bike.
    the battery i wish to use is a 12v 1.2amp VRLA as its small and light
    but dont wish to cook the thing with charging circuit from the bike.
    The bikes output is 12 to 14 volts at 4 to 6 amps
    do i need to panic about cooking the battery
    any help guys would be great

    cheers mick

    PS sorry about spelling and gramma its not one of my strong points
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    I'd think something like a small power diode and a low value resistor in parallel.

    Connect that in series with the battery.

    The resistor will limit the maximum chanrge current to the battery, and the diode will bypass that if a heavy current is drawn to prevent excess voltage drop.

    The resistor value should be high enough to limit the charge current to whatever the battery can handle, assuming the charge system is regulated to about 15V.

    eg. if the battery can take half an amp and it's nominal voltage is 12v, dropping 3V at 0.5A needs 6 ohms. Try a 5.6 or 6.8 Ohm resistor.
  3. djbrooksy

    Thread Starter New Member

    Sep 19, 2009
    thanks for the quick reply just so im geting what your laying down
    [​IMG]something like this
    so the resistor keeps current down but the diode will let the ecu have full power when needed

    the max charge current is 0.24A at 13.8 volts
    thats a float charge anyway but it will only ever need to be topped back up
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    From the charger to the battery.

    Your resistor/diode layout is correct.

    That way, the power coming from the charger to the battery passes through the resistor, limiting the charging current. But If the ECU (or anything else) needs more current FROM the battery, the diode will allow it to bypass the resistor.
  5. djbrooksy

    Thread Starter New Member

    Sep 19, 2009
    ok so now i have that under control
    working with the calculator here

    the DC output after the reg/rec on the bike
    is about 13v at 6 amps = about 78 watts

    78 watts with a 1.5k Resistance gives me 0.22amps

    am i working it out the right way or am i way of track :confused::confused: