For small solar systems (10 amps, 170 watts and under) the two I like are the Sunsei CC-10000 and Morningstar SS10-12. Both employ pulse wave modulation technology that helps you get the absolute maximum amount of power possible from your solar panels.
The Sunsei controller, the one I use, is the cheaper option ($40) and very simple to hook up. You connect your solar panels to one pair (positive and negative) of terminals and a second terminal pair to your battery
charge and neutral lines. Once installed the controller has a single LED indicator light
that glows green when the unit is busy taking solar-generated power and feeding that voltage to the battery.
The Morningstar controller is slightly more complicated and slightly more expensive ($60). It has three sets of connectors: one pair for the solar panels, one pair for the battery, and a third pair for your trailer's 12 volt electrical
gadgets, plus a jumper you should set if you're using gel-cell batteries (most of us don't). This unit has two LEDs: a charge indicator that glows green when the battery is being charged and a LED that glows red when the battery has fallen below
The Morningstar unit has some advantages and disadvantages compared to the Sunsei unit. Its major nifty feature is a battery management system that disconnects the battery from the trailer's 12v lights
and appliances when the battery voltage falls below 11.6 volts and prevents excessive damage by a process called sulphation that occurs when batteries are excessively discharged. A very useful feature if you plan to use an inexpensive "flooded" battery, but not quite so important if you plan on using a more expensive "AGM" (Absorbed Glass Mat) battery.
The down side of this feature is there are times when your trailer will suddenly go dark and the total "load" of all your trailer's lights
and appliances need to be limited to 10 to 12.5 amps, a figure that can easily be exceeded when a trailer's water pump, furnace
, and a few incandescent lights
are turned on at the same time.
One item of note with the Morningstar unit is it comes set with the "sealed battery" (gel cell) jumper installed. When this jumper is installed the controller limits the system's charging voltage to a maximum of 14.1 volts to protect a gel-cell's "gel" electrolyte from boiling and creating battery killing bubbles in the gel. If you use standard "flooded" or "AGM" style batteries you should remove this jumper to increase the maximum charging voltage to 14.1 volts.
Either of these two controllers are excellent products. Since our trailer has an AGM battery and I don't need to worry about excessively discharge quite as much I chose the simpler and less expensive Sunsei controller for my 105 watt setup. If, on the other hand, I planned to use less expensive and less durable flooded cell batteries I would have gone with the Morningstar unit.