To store the power from your solar panel in your auxiliary battery, you need a charge controller to regulate the voltage from the panel. This 10A PWM charger from Renogy is the best entry level controller you can buy, and pairs perfectly with a 100W panel to keep your battery topped off and your fridge, Propex, and interior lights powered.
Charging is done over 4 stages, with selectible charge profiles for Gel, SLA, flooded, or LiFePo4 batteries. The controller is capable of unlatching the BMS on a LiFePo4 battery that has been fully discharged, frozen or overheated. Basic status feedback is provided through the LCD screen.
This PWM controller isn't as efficient as the MPPT controller in the DC-DC Charger and Solar Charge Controller - you should expect as much as 30% of your panel's power to be lost through the PWM regulation. However, if you don't need to squeeze every bit of power from your panel into your battery, this PWM controller comes at a price that can't be beat.
Note: wiring and hardware are not included, as they will vary depending on where you locate this controller and your battery.