Sounds like you might be wanting to repurpose a larger panel you acquired into a trickle charger? If it is going to be perpetually hooked to the battery to “maintain” it should have a charge controller so it goes on “float” when full. A larger panel usually puts out 17-22 volts.
The batteries will take the voltage, but without being controlled to go on float and stop charging when the battery is full, 18-22 volts will evaporate all the electrolyte out of it in a short time. Even continuing to put 14-15 volts to a battery after it is at full charge will also evaporate the electrolyte prematurely. Car alternators have cutoff charge controllers for the same reason.
I have used 20 volt panels as car chargers many many times. We go camping out in the sticks sometimes 40 miles out. We always take a 100w panel with us just in case we need to charge the battery for some reason. But when using it to charge a car battery back up I don’t use a charge controller because I will be physically monitoring it and unhooking it right away after it is charged. I am physically the charge controller.
But if you are looking to leave something hooked to it perpetually to maintain a car battery, that $20 charger is a DEAL! :)