20W solar...need controller?

I would be afraid to connect 17 volts to a battery connected to a vehicle - who knows how much voltage the computer can handle? A Toyota mechanic told me our Highlander can handle 16 volts. It’s easy to drop the voltage by 1 volt, just run the current through two diodes in series - there is a drop of about half a volt in each diode. The Highlander parasytic current draw is less than half an amp so a 1 amp battery maintainer is adequate for its current addiction. I have heard that disconnecting the battery on a late model vehicle can cause problems, such as having to replace sensors because the computer learns to live with sensor aging and when it loses memory it has to learn again from a new sensor.