Choosing a charge controller

February 21, 2010
Solar panels and wind generators produce electricity, but their output varies widely based on the available sun or wind speed. Therefore there is a need to convert that output to something a little more consistent using a charge controller.

Do I need a charge controller?

Generally, you will not need a charge controller if you simply plan to sell power back to the electric utility company. You will need a charge controller if you plan to:
  • use batteries
  • use an OutBack Power System (which requires batteries)
  • use solar and wind power in the same system
  • go off-grid, independent from utility power (usually a mobile or remote home)
  • have a backup power source during outages (which requires batteries)
  • power a portable inverter or otherwise large device (requiring a significant battery source)

What kind do I need?

There are almost as many different types of charge controllers out there as there are solar panels. However they all essentially do the same thing, which is charge batteries. The most common battery type used is sealed lead acid (SLA) or gel lead acid, which is very similar to SLA. Click here to see more.

If you plan to use wind generators, the charge controller is usually built into the unit or comes with it separately. Therefore, you don’t need to worry about choosing a charge controller unless you plan to make your own wind generator.

What you need is largely based on what you plan to power and how often you plan to use it. In my article about choosing solar panels I discussed calculating power:
  • V (Volts) * A (current in Amps) = Power (Watts)
Keep in mind that the sun is not always shining and you’re probably not always using power. I will discuss how you can calculate your specific needs in a later article. There is a little math involved.

How do I connect it?

Connecting a charge controller is fairly simple. There are usually two things to connect, sometimes three. You will always have to connect the solar panel(s) and the batteries. The labeling is fairly explicit. Consider these and look at the pictures: Notice the Prostar 15 had third thing called “load.” A load is what you plan to power. Instead of connecting something directly to the battery, the charge controller should turn off the load when the battery runs low. Most inverters do this automatically.

Don’t be intimidated by the FLEXmax. It also has two simple connections, PV +/- and Bat +/-, it just requires larger wire. The FLEXmax is generally used in larger solar systems.

Wiring details should be outlined in the instruction manual for your specific charge controller. I’ll try to cover wiring in more depth at a later time.