If you have a means of monitoring the voltage to your battery, you can just tie the panels to the battery, and watch the voltage. If it gets over 15 V, disconnect panel (or part of the panels if you can), or put a load on to lower the voltage. An inverter tied to the system can be used to load it down, adding load to the output can vary the drop in voltage.
This is not a great way, but if you need to charge it will work.
I have some fixed panels, and a portable for when the roof panels don't get enough sun or are not enough. I just tie the portable to the battery.
Doug