Voltage control for solar panels on a sun tracker with arduino

When I was young some 30 years ago .. I studied automatism and I discovered just recently the wonderful world of arduino ... watch a big step !!!
I’m currently building a XY sun tracker which uses DC motors (boats power trim items).
They run with 10 amps and I’m planning to use a dedicated solar panel (around 100Wh at pic) connected with a charger regulator with a 12v battery.
My issue is that during bulk charges, the voltage can be regulated over 18V while it goes down to 13V during floating. If I connect a step down alimentation, like I’ve seen on numerous item, it will follow the delivered tension as a ratio ?
How can I manage this for my arduino?
I’ve already received and currently testing (or better said trying to discover) a mega 2560 to drive the tracker.
Thanks for your inputs
PS: the mechanical part are almost finished I’ll post some pictures
It moves 6x300 Wc panels

Step down regulators usually have fixed voltage outputs, regardless of input voltage (within limits).

Pololu has an excellent selection.