Solar Charge Controllers

Solar System Charge Controllers

If you have batteries in your system, you'll almost surely need a charge controller to prolong battery life.  The only exceptions are very small systems using PV panels of 5 watts or less. Charge controllers sense the battery state of charge by monitoring its voltage.  Lead-acid batteries gradually rise in voltage as they charge.  But batteries are like buckets, you can only fit just so much into them.  Overcharging will use excess water, and if severe, will cause physical battery damage.  So a charge controller's primary duty is to limit, and eventually shut off input as the battery approaches full.

Controllers are rated by what voltage or voltages they can operate at, and by how many amps of current they can pass at maximum.  Be sure your controller can handle the maximum rated amps of your array.  Most controllers will sustain permanent damage (read crispy critter) if significantly over-amped for more than a few minutes.

Optional Charge Controller Duties

Beyond the basic duty of preventing overcharging, you'll find many controllers offering other bells and whistles, many of them very useful. The most common are:

• Voltage Indicators using simple LED lights, often red/yellow/green, that give a rough idea of the system state of charge.
• Volt or Amp readings using more sophisticated LCD displays that can show how many amps are flowing in or out through the controller.
• Load Terminals with automatic Low Voltage Disconnect to shut off all, or parts of the household power, if battery voltage gets too low.  This isn't very nice to the users, but it's very nice for the batteries and their life expectancy.  The more polite LVDs will blink the lights 5 minutes before shutoff to allow the user a chance to reduce power.  Once triggered, LVD won't turn back on till the batteries get at least some modest charging.
• Lighting Controllers to automatically turn on lights at night.  Often used for sign and security lighting.  Can be set to run dusk to dawn, or for a certain number of hours.  Triggered by sunset, not time, so seasonal adjustment is automatic.

Controller Operation, PWM Vs MPPT

There's a major change underway in how controllers operate.  Standard controllers simply open and close the circuit between the PV array and the batteries, usually with a power transistor using PWM (pulse width modulation) control that switches hundreds of times per second.  This pulls the PV voltage down to whatever voltage the battery is operating at.  PV modules are designed to always make more voltage than the battery wants, because electrons only flow from higher to lower voltage.  But if we could run the PV module at its Maximum Power Point, we could get more wattage out.  This is what the new generation of MPPT controls do.  They run your array at whatever voltage delivers the most wattage, with the excess voltage that would toast the batteries converted to amps the batteries can happily absorb.  MPPT controllers typically extract about 15% more energy year round.  They do their best work in winter, up to 30% boost, when PV temperatures are low, and battery voltages are generally lower.  At this time, the OutBack MX60, Apollo Solar, and the BZ Products controllers are MPPT types, others are standard PWM.