Why use a solar charge controller?
Solar charge controllers are needed in most PV systems to protect the batteries from overcharge and excessive discharge. The minimum function of the controller is to automatically assure the battery is fully charged and to keep the battery fully charged without damage to the battery.

The basic criteria for selecting a solar charge controller includes nominal operating solar PV voltage and the solar PV panels rated current. A solar charge controller should be able to handle the rated current of all the panels combined.
A 85W solar panel is rated at 5.02 amps. The controller selected should handle 5.02 amps.
Two 85W panels in parallel 5.02 amps + 5.02 amps = 10.04 amps.

The next criteria is selection of the desired features. Is an indicator required to indicate the solar panel and charge controller is working? Is an LED charging indicator sufficient or would you rather have a meter that displays battery voltage and charging current? Is conformal coating an important feature? Is Gel, AGM, or lead-acid battery selection important?

Solar boost controllers maximize the power transfer from the solar panels to the battery through Maximum Power Point Tracking (MPPT). The charge controller with this feature constantly seeks the maximum power point on your solar panel to transfer its energy. Typically this converts, for example, the 17 V from the solar PV panel down to the 13 V needed to charge your battery. During the conversion, the current increases from the solar PV panels up to a possible 25%.

Diversion Load Controllers are controllers that monitor your battery voltage and limits its maximum value by diverting some of the energy into loads. When used in combination with solar PV panels, a diode is inserted between the solar panel and the battery bank eliminating discharge power in the evening. Most Diversion Load Controllers use a resistor to dissipate the excess energy in the form of heat or in some cases the energy is placed into the onboard hot-water tank.