I have made several over complicated circuits over the years to trickle charge my start battery
off my solar
system. Some controllers do this but most don't.
So if you want a quick and simple aux charge circuit all you need is a high wattage resistor and a diode.
On my system with agm's the charge voltage goes to about 14.4v so a 1.2ohm resistor and a IN54001 diode is for my system perfect. Trickle charges between 200-500mA . I would make the resistor about a 10w one .
Hope this helps someone !