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cheepo car battery chargers

Discussion in 'Hobby Electronics' started by lentildude, Apr 5, 2007.

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  1. lentildude

    lentildude Guest

    I have a cheepish 12v 4amp car battery charger with led bargraph
    for bat voltage. (brand projecta )

    Question is it is a 12v charger, voltmeter reads 11.5v with no load.
    Car batteries are 13.8v, how is a lower voltage going to top up a car
    battery? I have some older batteries in the garage but they are
    sitting on 12v. Battery chargers 11.5v aint gonna do a thing?? right??

    Opening it up it has no adjustment, just 2 diodes, a thermal fuse and
    leds with different zener diodes to display voltage in 1/2 v

    Maybe I should build a proper charger from scratch, anyone have any
    good, simple reliable circuit links? BTW I assume those desulphators
    dont work, silicon chip had one a while back.
  2. Ross Herbert

    Ross Herbert Guest

    Measure the o/c ac voltage from the secondary (each half of
    secondary). If your meter reads around 18V rms (or higher) then the dc
    rectified pulses will be sufficient to charge a 12V lead acid battery.
    If it doesn't measure at least 18V o/c per half secondary then it
    probably won't do the job.

    Does the charger have an indicator showing when the thermal switch
    opens and closes? This will probably be a led which goes on and off
    during charging. If it does have such a led and it does go on and off
    then the charger is most likely working as intended.

    These el-cheapo chargers are basically just junk and I wouldn't even
    look at them let alone buy one.
  3. Phil Allison

    Phil Allison Guest

    "lentildude" <

    ** The LED volt meter does not read accurately with no battery connected.

    It needs it there to smooth the rectified AC.

    Same for any "average responding " volt meter - it will read only 64% of
    the peak value when there is no smoothing.

    Learn some basic AC / DC theory anytime, dude.

    ......... Phil
  4. RMD

    RMD Guest

    Just stop getting all theoretical and charge up your batteries.

    It will work and work well.

    Put it on a timer so you don't overcharge the battery.

  5. jasen

    jasen Guest

    it's not providing 11.5V continuously,

    the peak voltage is more like 16V, the diodes inside it will ensure that
    when its output is lower than the battery it doesn't discharge the battery.
    I've been told they do.
  6. lentildude

    lentildude Guest

    Thanks for feedback,

    I just charged an electrolytic and measured 17.5v so those cheepo
    chargers do work!
  7. Ross Herbert

    Ross Herbert Guest

    Right, now charge your battery but don't leave the charger (still
    powered ON) connected to the battery when it is fully charged. Either
    disconnect the charger or do as RMD suggested - use a mains timer
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