cheepo car battery chargers

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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
increments.

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.


Re: cheepo car battery chargers



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right??
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and


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.

Re: cheepo car battery chargers



"lentildude" <

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** 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





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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.

Ross

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Clip it onto a suitably rated electrolytic and measure the voltage again.



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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.

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I've been told they do.



--

Bye.
   Jasen

Re: cheepo car battery chargers


Thanks for feedback,


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


Re: cheepo car battery chargers



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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
adapter.

Re: cheepo car battery chargers




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The Optimate II I bought a couple of years ago seems to work well enough, I
keep a spare motorcycle that's left behind the shed all year round which so
far only gets an outing once a year for its MOT test, the battery gets
brought indoors and de-sulphated as soon as winter ends and another charge
at the end of summer just before the test, this has stopped the battery
becoming scrap.

Its worth remembering that most cheap & cheerful chargers are "impedance
wound" so the off load peak voltage will be somewhat higher than the correct
charging voltage and drop considerably on normal load. This of course means
that a normal power transformer will not be suitable for making a LA battery
charger.

The Optimate charger claims a 20V desulphating phase current limited to
200mA, for reference the Yuasa "Little red battery book" recommends
desulphating at 29V current limited to 1/3 of the Ah capacity rating, this
book suggests that as the battery starts to draw charge current the terminal
voltage should fall to the deep discharge value of 10.8V - at which point a
fully automated charger should switch to constant voltage charging at the
recommended charging voltage for the battery type (open & SLA differ). Its
quite possible that a cheap charger with an off load peak voltage anything
over 20V probably won't be far behind expensive chargers like the Optimate
in terms of recovering mildly sulphated batteries.



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