You've probably heard that the Trik-L-Start DC-DC battery trickle charger has been discontinued, the only commercially available alternative is the Echo 15-Amp Charger by Xantrex. The Echo Charger is quite expensive at around $150 and only compatible with lead acid batteries.
I wanted to add a chassis battery trickle charger from my Solar Charge controller's auxiliary output but couldn't find an inexpensive low current DC-DC trickle charger for my application. The Trik-L-Start trickle charger would have been ideal, but they are no longer sold, necessitating the DIY approach. With the addition of a battery protection module to the circuit, it will mimic the Trik-L-Start trickle charger and can be adjusted to start/stop charging depending on the source voltage.