People with minimal power needs (a few amps a night) may be able to get by using just a bit of charge off the vehicle's starter battery. It can be done for about $25, or about $100 with 100w of solar if one shops around.
Note: this is a streamlining of the shallow cycling article.
The 20A example pictured here was ~$15 at the time of writing. They are simple, generic devices sold under many names. The 10A versions are closer to $10.
This kind of controller is used because:
Here is how they are wired:
The hardest part of the installation will be running a wire from the battery to the controller. You may be able to leverage holes in the firewall2) for other wiring, or find other holes that are presently closed by rubber plugs.
One might use a $10 adapter like this one, cut off the single male plug and insert the wires into the LOAD terminals.3)
Users with more complex power needs might wire a fuse panel to the LOAD terminals and from there to the various loads.
There are three setpoints that need to be configured. Terminology will vary but basically:
Here is an overview of the controller on youtube. The configuration begins at 11:35. He refers to the charging voltage as “float voltage” although the distinction is not important; there is only one charge stage. For reference, in the video the controller's setpoints are CHARGING VOLTAGE 13.7v, the LOAD CONNECT 12.6v, and the LOAD DISCONNECT 10.7v. Refer to the list above for settings more appropriate for your starter battery.
Insert the panel wires into the SOLAR terminals and enjoy the life upgrade.
For portable panels where you will be plugging/unplugging a lot a quick-disconnect of some kind would be useful. A common solution is to use an MC4-to-SAE adapter on the panel side and a short SAE connector on the controller side. Those willing to drill can install a port to allow plugging in the panel into the body of the vehicle instead of running wires through a window.
One can also buy a set of MC4 extension wires4) which are wired it into the controller. This provides the disconnect and will also allow you to deploy portable panels in the sun while the vehicle remains in the shade.
This kind of controller will harvest more power when used with panels that have a lower Vmp / higher Imp rating. Luckily these are often found on lower-priced polycrystalline panels. Big box stores like Home Depot often have 100w panels for $80 or less. Watch for sales. Otherwise $100 for 100w shipped is common on sites like Amazon.
If this is not an option you may want to deploy portable panels which are stored when not in use. Because of their light weight and storability (?) flex panels are often used as portables. They tend to cost about 2x as much as framed panels, do not last as long, and can difficult to prop up in breezes.
You may want to install an isolator to pull more 12v power into the cabin while driving. This does not require an aux battery setup.