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.
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.
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.
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.