What is a Charge Controller in a Solar System

What is a Charge Controller?

 As we know that  we get more voltage from the solar panels as the brightness of sun increases.  The excessive voltage could damage the batteries if the batteries are directly connected to the solar panels.  A charge controller is a device which is used to maintain the proper charging voltage on the batteries.  When the input voltage from the solar panels increases, the charge controller will regulate the charging to the batteries so that over charging is prevented.  A Charge controller regulates the voltage and current coming from the solar panels to the battery.  Most of the 12 Volts solar panels produce 16 to 20 volts and if connected directly to the batteries will damage them due to over charging.  Most of the batteries need around 14 to 14.5 volts to get fully charged.

Modern Charge Controllers 

Most of the modern charge controllers undergo what is known as 3-stage charge cycle which is shown below.

• Bulk Phase : During this phase of charge cycle, the voltage rises gradually to the bulk level ( normally 14 volts) while the batteries draw the maximum current.  Once the Bulk level of voltage is reached, the absorption stage starts.
• Absorption Phase :  During the absorption phase, the voltage is maintained at the Bulk level for some specified time which is typically 1 hour while the current gradually tapers off as the batteries get charged up. 
• Float Phase :  During this phase the voltage is lowered to a float level ( normally 13.3 volts to 13.7 volts) and the batteries draw a small current until the next cycle.

    You can see the relation ship between the current and voltage during the above 3 phases of charge cycles in a graph which is shown below.

Is the charge controller always needed?

No,  it is not always needed.  Generally a charge controller is not required for a small systems like 1 to 5 watt solar panels.  A general rule is that if a solar panel puts out about 2 watts or less for each 50 battery amp-hours, then a charge controller is not required. 

Why 12 volts solar panels actually produce 17 volts 

You may get a doubt "Why solar panels are not designed just to produce 12 volts".  The reason is that if you design a solar panel to produce just 12 volts,  the panels will provide power only under perfect conditions and full sun.  This is not the situation in most of the places.  The panels have to produce extra voltage so that when the sky is cloudy and sun is low, you should still get some output from the panels.  A fully charged 12 Volts battery is around 12.7 volts at rest so the solar panel has to at least produce more than 12.7 volts in worst conditions.  

One thing to remember is that the solar panels work best at cooler temperatures.  A solar panel rated at 100 watts at room temperature produces 83 watts at 110 degrees temperature.

A solar charge controller regulates the 16 to 20 volts output of the solar panels down to what the battery needs at the time.  It may vary from 11 volts to 14.6 volts depending on the state of the charge on the battery, the type of battery and in what mode the controller is in and the temperature. 

The charge controller is installed between the solar panel array and the batteries.  The charge controller maintains the charge on the batteries using the 3 stage charge cycle described above.  The power inverter can also charge the batteries if it is connected to the AC utility power.

Summary :  If you are  using four 80 to 100 watts solar panels,  your charge controller should be rated up to 40 Amps.  Even though the solar panels normally do not produce that much current, there is an 'edge of cloud effect'.  Due to this phenomenon I found that my four 7.5 amps panels ( 4 x 7.5 = 30) pump out more than 35 Amps which is well over the rated 30 Amps maximum.