Universal Lead-Acid Battery Charger from Laptop Power Supply

[drdm]

Hello everybody,
I have to make a charger for a 6V 12Ah battery, witch I initially planned to do the old-fashion way(a transformer, a rectifier and an lm317 adjusted to 7,5V), but I've discovered that for the same price as a 1A 12V transformer I can buy an old laptop power supply, rated at 19V 4A. I've decided to use one of these and make a regulator, that can charge 6 and 12 volt batteries (I recon 7,5V and 14V output voltage respectively). From what I've read about lead acid batteries they must be initially charged with 1/10 of their amphour rating (1.2 amps for a 12Ah battery) and after they reach a certain voltage it should be maintained till full charge.I'm planing to make an adjustable voltage regulator based on lm317 with adjustable current limiting.
These two circuits are given in the datasheet, but i can't figure out how to combine them(it is also not entirely clear to me how the high current circuit works).
Does my plan make any sense or is there something fundamental that I am missing?

[Kilrah]

Twice as much energy lost as heat as what goes into the battery? We're in 2016

[drdm]

That is not of importance to me, but if you have a better solution I will be glad to see it. My main concern is if a laptop power supply is suitable for the task.

[Seekonk]

You have a laptop power supply, no need to invent a new one.    These all have an opto isolator and zener to set voltage.  Switch zener with LM431 and make it adjustable.

[drdm]

I still haven't bought the supply and for a coupe of bucks more i can get an universal one that has voltage adjustment build in. The bigger problem is that i need a current limiter since I don't want to feed 4-5A into a 12Ah battery. The limiter could be adjustable so different batteries could be charged.

[knks]

I use Laptop Power Supply (19V or so) to charge my car battery. Schematics very simple - just one resistor in series 
Of course I have to manually monitor the battery voltage, but since I only use it ~ once-a-year I think it's good enough

[jeroen79]

My main concern is if a laptop power supply is suitable for the task.

Not by itself.
It's output voltage is too high and it may not have a suitable constant current limiter.
But it can be used to bring the mains AC down to a more managable DC level to power the charger.

How about geting a cheap CC/CV buck regulator.
Then all you'd have to do is modify the CV pot so to switch between 6 and 12 volt batteries.

[tszaboo]

It is better to get a nice IC doing everything, like an L200, instead of coming up with a discrete solution.

[danadak]

http://www.ti.com/lit/an/slua115/slua115.pdf

http://www.ti.com/lit/an/slua055/slua055.pdf

http://ww1.microchip.com/downloads/en/AppNotes/01015a.pdf


Regards, Dana.

[agha.akhlas7]

You need to charge 12 vdc batterry with 14.5 vdc by regulating the laptop power supply

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