for dc to dc charging you need both a boost and buck converter. The boost is to take the input voltage up to 18 volts, and the buck to take it back down to 14.6 volts without losing any amps during the process.
You can find the boost/buck converters on ebay, they have different wattages for how many amps you want to charge at. I charge at 6 amps with my boost/buck charger, I'm never in no hurry. The converters you can find for around 10 dollars each for the low wattage ones. The converters that can handle more amps are in the 30/40 dollar range and are much larger. Since the output voltage amps are adjustable, you can use it to charge any battery li-ion,lead acid, anything that requires cc/cv charging. These chargers work great, I charge all my batteries with these, with the LED meter there is never any guess if your battery is almost charge, the amps drop to about 100ma when it reaches full charge.
Also get a volt/amp led meter (about 6 dollars) so you can see how the charge is progressing. Put a fan on the converters they get hot while running. If the boost converter doesnt have a fuse, add one at the input. I never had a buck converter fail but lost 2 boost converters if they get too hot, thats why I use large fans and keep charging at 6 amps.
6 amp boost buck converter (400 watt boost/ 280 watt buck)
This is the LED meter, note the large fan.
