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!
Confused where to start with DIY solar? Check out our comprehensive camper van solar system guide to start from the very beginning.
What A Solar Charge Controller Does
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
When trying to understand WHY a solar charge controller is important for charging batteries, it’s critical to know two important facts.
- 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).
- 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.
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.
What Size Solar Charge Controller Do You Need?
If you scan through all the different solar charge controllers on the market, you might notice that each charge controller comes with an 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?
This rating is the max 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 to your batteries.
How To Select The Right Size Solar Charge Controller?
The optimum size solar charge controller depends on the total wattage of your solar array.
Follow the table below based on whether your batteries are 12v or 24v.
Table 1.1 - Solar Array Size To Charge Controller Size
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:
- PWM controllers are considerably cheaper than MPPTs, as much as 5x cheaper.
- MPPTs are roughly 20% more efficient at charging the batteries from the same amount of sunlight.
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 love our Victron MPPT charge controller. To learn more, check out our Victron solar charge controller review.
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Recommended Solar Charge Controllers
Here are our top three solar charge controller recommendations, based on price.
When you’re ready to install one, check out our post How To Wire A Solar Charge Controller In A Van.
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.