For starting batteries, I'd use a very small 12v panel and no controller. If your panel is small enough, you don't need a controller, just make sure it's small enough.
It's not like you are using your starting battery for any loads between engine starting, so you only need enough to maintain that battery. Years ago I had a panel that was about 4" x 12". I used it to maintain a starting battery.
There's an article by Don Casey on boatus.com that talks about this:
Installing a Solar Panel to Maintain Batteries by Don Casey
Wet-cell batteries self-discharge at about 1% per day-more in hot weather, less when it's cold. So to float charge the battery, i.e., maintain a full charge, we need an equivalent charge rate. For a 100-amp-hour battery, that means a solar panel with a daily output of about one amp-hour.
Solar panels, however, are rated in watts. To determine the output current in amps, divide the watt rating by 15-the approximate true output voltage of a typical solar panel. Thus a five-watt panel has a maximum output of about 0.33 amps. But rated output only occurs at high noon; the rest of the time, the panel puts out less. Expect average output to be the equivalent of about five hours of rated output per day. That makes the expected daily output of a five-watt panel about 1.65 amp-hours (5 x 0.33).
Allowing for recharging inefficiencies, this is still about 50% more than necessary to float a 100-amp-hour battery. A good rule of thumb is 3.5 watts per 100 amp-hours of battery capacity. But throw in an occasional sunless day, put blocking diodes in the circuit, and let the bilge pump run once in a while, and five watts will be about right. A small amount of extra capacity won't damage the batteries as long as you maintain the water level in them. ...
I would err on the side of fewer watts for a starting battery, and use a maximum of 2 watts of solar power for each 100AH of battery. It's a simple solution requiring no charge regulator.
Depening on the size of your batteries, your 5w solar panel could possibly be used if you split-up the charging between the house and starting batteries, using diodes. Casey talks about that too.