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Best Strategy for 13.8V 2A charger Using Transformer that Puts out 12.4V / 25V Answered

I salvaged a 240V --> 12.4V or 25V transformer and would like to create a 13.4V 2A charget for my lead acid car battery.

Which Voltage should I use and - any recommended strategy?  

I know I need to add a rectifier and capacitor in there is about it so far after days of research.



4 years ago

I would use a switch mode constant current constant voltage (both user settable) to deliver power to the battery. When the battery is drained, and the battery acts like a 'short circuit' or low impedance, the regulator will limit current. When the voltage is reached, and the battery begins to draw less current, it will go into constant voltage mode, as the current that is drawn is no longer is attempting to exceed the threshold. That should work well enough.

A linear solution may work, but it will be very power wasteful, and you will need to worry about getting an appropriate heatsink. You can figure out what heatsink you need with the info here: http://www.ti.com/lit/an/slva118/slva118.pdf

A better solution would be to use a proper lead acid battery charging chip:


Lots of choices here, may seem a bit scary. I narrowed down the parametric search for your needs. I am looking at this part, the LT1513 as probably the most suitable:


It meets all the criteria, and should work a treat if you can build it up on a PCB. Don't bother trying on a breadboard, since high frequency switch mode high power stuff does not work well.


4 years ago

Set R to about 2.4K = 2400 ohms see pp21.

The LM317 TO-220 Package is limited to 2 Amperes and can be heat sinked if you like.

Remember a 12.4volt AC subtracting 0.7v for the diode gives 11.7volts AC.

Then multiplied by 1.414 ( square root of two ) to get the peak of 16.5volts DC.

Easily enough to regulate 13.4 volts DC for your battery.

As Steve would say " good enough napkin math " :-)


Answer 4 years ago


I geuss he didn't like my answer from yesterday :(


Answer 4 years ago

Thanks! The more I read, the more I understand that I don't understand haha.

Is ground on the bottom a label for the wire or I need to plug it somewhere?

What resistor should I put on the right of the LM317? Last but not least - I bought a bridge rectifier with four legs, could I use that instead of the diodes? How would I plug it in?

I'm ready to pop out and buy what you put up there, I finally have a shopping list, thanks :) (hopefully will know more with an extra answer :D)

I have a relay switch here if it can be used to regulate.. I might be completely off here and feel free to ignore this one. :) -http://www.mouser.com/catalog/specsheets/833H.pdf
Last question - before building the circuit I am just going to see if the charger I took it from fries if I try to use it (its suppposed to deliver 1.5A, delivering 5A now). The transformer is warm (but not hot), is this fine? Theres also a larger resistor getting hot, but guessing this is normal? Adding image of the circuit