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What Is A Solar Charge Controller And Why You 100% NEED One

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Perhaps you’re wondering if you can simply hook up your solar panels straight to your batteries. And the quick answer to that is NO! Under most circumstances, a solar charge controller will need to be installed in between your batteries and solar array.

In this post, we will go over what a solar charge controller does and why it’s 100% necessary to have one.

So if you’re ready, let’s get to it!

Purpose Of A Solar Charge Controller

The simplest definition of a solar charge controller is that it regulates the voltage from your solar array to keep your batteries properly charged and healthy.

There are a couple of additional important features that we will also go over.

A solar charge controller serves four important functions to your solar array and batteries.

1. Regulates Voltage To Correctly Charge Batteries

Solar Charge Controller Reducing Volts Before Feeding Battery
Solar Charge Controller Reducing Volts Before Feeding Battery

When trying to understand WHY a solar charge controller is important for charging batteries, it’s critical to know two important facts.

  1. Batteries can only be charged by being fed power from a small voltage range (usually between 13.6v – 14.4v depending on the battery chemistry and state of charge).
  2. The voltage produced by a solar array is wide ranging and depends entirely on the size of the solar array and whether series and/or parallel connections are used.

To simply feed a battery a wide range of voltages is dangerous and can permanently shorten the lifespan of the battery.

Therefore, a solar charge controller is used to regulate the wide ranging & inconsistent incoming voltage from the solar array into a more consistent and usable voltage for the battery to accept.

2. Prevents batteries from being overcharged

Imagine sitting at a buffet with a table full of your favorite foods. Though you’d be tempted to try and eat everything at the table, your stomach and brain work together to prevent you from overeating and harming your body.

A solar charge controller acts similarly to your stomach and brain.

When a battery has been significantly discharged, the charge controller will allow the safe maximum amount of volts to flow into the battery to quickly recharge it.

When a battery nears it’s fully charged state, the charge controller will gradually lower the voltage to maintain the health of the battery.

And finally, when a battery is fully charged, the charge controller will stop the flow of power into the batteries to prevent overcharging.

3. Prevents Reverse Current Back To Your Panels

Volts usually flow from high voltage to areas of lower voltage.

This is great during the day time when solar panels are producing power and pushing volts towards the charge controller and battery.

But at night, or in shaded areas, the voltage coming from the panels drops significantly and well below the standing voltage of the battery. When that’s the case, power will want to flow from the batteries back towards the panels.

This is called ‘reverse current’.

But a charge controller blocks the voltage from flowing backwards.

Whew!

4. Provides Valuable Solar Harvesting Data

Although not critical to the function of a solar panel system, certain charge controllers can provide valuable real-time and historical solar energy harvesting data.

We use a Victron Bluetooth-enabled solar charge controller, which we can pair with our smartphones to see insightful solar data as power comes in.

From our smartphones, we can see the instantaneous watts being sent directly to our batteries, as well as the volts and amps breakdown of those watts.

Solar Charge Controller Energy Harvesting Data On Smartphone
Data from Victron Solar Charge Controller

Amp Ratings For Solar Charge Controllers

Solar Charge Controller Amp Ratings

If you scan through all the different solar charge controllers on the market, you might notice that each charge controller comes with an current (amp) rating.

Some controllers, like this Renogy Wanderer, are only rated for 10A.

Others, like this Victron charger, are rated for 30A.

What does this mean?

These ratings are the amount of current that a charge controller can feed to your batteries.

So if your charge controller is only rated for 10A, then only a max of 10A will flow into your batteries. And a 30A charge controller will be able to output a max of 30A into your batteries.

How To Select The Right Size Solar Charge Controller?

The right size charge controller depends on the size of your solar array.

Our rule of thumb: Divide your total solar array wattage by 13.6 (the minimum volts to charge a battery) and round up to get your ideal solar charge controller size.

Or, simply follow this guide.

  • 10A charger: For up to 130w on a 12v system (260w for 24v system)
  • 20A charger: For up to 270w on a 12v system (540w for 24v system)
  • 30A charger: For up to 400w on a 12v system (800w for 24v system)
  • 40A charger: For up to 540w on a 12v system (1080w for 24v system)
  • 50A charger: For up to 680w on a 12v system (1360w for 24v system)
  • 60A charger: For up to 800w on a 12v system (1600w for 24v system)

For example: We have 350-watts of solar on our van roof and so we selected a 30A solar charge controller.

PWM vs MPPT Solar Charge Controllers

There are two different charge controller technologies on the market today:

  • PWM (Pulse Width Modulation) Controllers
  • MPPT (Maximum Power Point Tracking) Controllers

The technical difference between the two is how they manipulate the volts coming from the solar array in order to charge the batteries. It can get quite technical.

But for the purposes of this post, you only need to know that:

  1. PWM controllers are considerably cheaper than MPPTs, as much as 5x cheaper.
  2. MPPTs are roughly 20% more efficient at charging the batteries from the same amount of sunlight.
PWM vs MPPT: Solar Harvesting Data
PWM vs MPPT: Solar Harvesting Data (source: MarineHowTo.com)

In the PWM vs MPPT post from MarineHowTo, the author experimented between the two types of solar charge controllers and found that MPPT controllers were 20% more effective than PWM controllers.

The graph above from MarineHowTo shows how many amps an MPPT and PWM controller harvested in different lighting situations across a 7-day span during the spring season in Maine.

  • MPPT 7 Day Total = 220.44 Ah
  • PWM 7 Day Total = 182.48 Ah
  • MPPT = 20.8% Boost In Efficiency

MPPT or PWM: Which Is Better?

When it comes to deciding which type of charge controller is best for your needs, only you can decide that for yourself.

The MPPT controllers are more efficient, but more expensive. And if staying within a tight budget is critical for you, a PWM controller might be the way to go.

But if you can afford the extra expense, especially if you have already invested in a larger solar array system, an MPPT charge controller should be your choice. The ~20% gain in energy harvesting efficiency versus a PWM is worth the extra cost, especially if off-grid living is important to you.

We use a Victron 100V/30A MPPT charge controller and have been happy with the solar harvesting results we’ve gotten.

Recommended Solar Charge Controllers

Here are our top three solar charge controller recommendations, based on price.

Cheapest

Renogy Wanderer PWM Solar Charge Controller

Renogy Wanderer

Best Value

Renogy Rover

Best Overall

Victron Energy MPPT Solar Charge Controller

Victron Energy

Final Thoughts: A Solar Charge Controller Is An Essential Component For Any Solar Panel System

By now, you should have a good idea about what a solar charge controller does and why it’s essential for any solar power system.

But getting the correct solar charge controller is only one component of a proper functioning solar system. Check out these other posts to learn more about how to put together the best solar array for your camper van.

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