How Solar Charge Controllers Work

How Solar Charge Controllers Work

Power in equals power out. Power out equals more fun!

It’s a beautiful summer evening. A warm breeze blows as you kick back in the Adirondack chair on your dock. The stereo plays as you reach into the fridge to grab another cold one. You turn the LED lights up so that you can finally catch up on your reading.

Is this heaven?

No. It’s just your boat dock since you installed a solar power system.

But if your solar charge controller didn’t charge your batteries all the way it’s lights out for this relaxing night by the water.

Solar Charge Controllers

Solar charge controllers are battery chargers powered by the sun. When they have been correctly matched to your solar panels and batteries your batteries charge faster, more completely, and last longer. You’ll have many heavenly nights on your dock.

Solar Charge Controllers

Solar charge controllers manage the power from the solar panel to the battery. They put the perfect amount of volts and amps into the batteries to bring them to a full state of charge. Charge controllers have three main functions.

  1. They stop batteries from overcharging.
  2. To stop current from flowing from the battery back into the solar panel at night.
  3. To disconnect the battery if it’s voltage gets too low.

The solar charge controller monitors voltage and current coming from the solar panels and adjusts it to give the batteries a full charge. Most charge controllers charge in three stages.

  • Bulk Charge- Bring batteries to a 70% charge with a constant current charge.
  • Saturation Charge- A lower current ads the remaining 30% charge.
  • Float Charge- Replaces current lost to self discharge. Keeps the battery fully charged.

There are two different types of solar charge controllers, PWM and MPPT. Each uses a different algorithm to charge batteries.

PWM Charge Controller

PWM stands for Pulse Width Modulation. This type of charge controller directly links the solar panel to the battery. It sends pulses of voltage and amperage to the battery, stops the pulse, then checks the state of the battery. With a discharged battery these pulses happen often and last for several minutes. With a charged battery the pulses happen several minutes apart and last only a few seconds.

PWM charge controllers are less expensive than MPPT charge controllers. They are also less efficient. Because PWM charge controllers are linked directly to the battery, the solar panel will only put out as much voltage as the batteries current state of charge. A fully charged 12-volt battery has 14 to 14.5 volts and around 11.9 volts when fully discharged. A 12-volt solar panel puts out between 16 and 20 volts. If the battery has a partial charge of say 13 volts, the solar panel will only put out 13 volts. This causes the solar panel can waste between 20% and 60% of its voltage.

With PWM charge controllers the solar panel must match the battery. You must use 12-volt solar panels with 12-volt batteries or two 12-volt solar panels, in series, with a 24-volt battery bank.

MPPT Charge Controlers

MPPT stands for Maximum Power Point Tracking. A solar panel’s power output changes with temperature and the amount of sunlight hitting the solar cells. The amount of power it is putting out at any given moment is its Maximum Power Point (MPP). It is the point where the volts times the amps equal the most watts.

An MPPT charge controller continuously compares the solar panels Maximum Power Point to the battery’s voltage. The charge controller changes the solar panel voltage to match the battery voltage. This raises the amperage to the battery so that it always gets a maximum charge from the solar panel.

MPPT charge controllers are more efficient than PWM controllers providing 10% to 30% more power to the battery. They are also more expensive.

MPPT charge controllers allow you to use higher voltage solar panels with 12-volt batteries. By matching the voltage from the solar panel to the voltage of the battery a 20-volt or 24-volt solar panel can be used with a 12-volt battery. Two 20-volt or 24-volt solar panels in series can be used with a 24-volt battery bank.

What’s the Best Charge Controller for Your Dock?

Solar charge controllers are one of the most important parts of any solar installation. They ensure your batteries will stay charged and healthy for many years.

The best charge controller for your dock depends on many factors including your location, how much solar power you need to generate, and how much room you have for solar panels. However, most docks will only need a limited amount of battery capacity. In this case, a PWM solar charge would work well.

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Further Reading
Battery Tips
Charge Controller Calculator