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How-to Choose a Solar Charge Controller for a DIY Camper Van Electrical System

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How-to Choose a Solar Charge Controller for a DIY Camper Van Electrical System

What is a Solar Charge Controller?

The Charge Controller takes the power made by the solar panels and transform the ‘solar panel power’ into a form of power that the batteries can use.

Quick note before we get started.  This is just one part of a overarching “How to Install a DIY Camper Van Electrical System” series.  If you’ve just stumbled on this article directly without seeing that, there are likely some things we’ve already covered.  If you want to check out that step by step guide, you can do that here: https://www.explorist.life/diy-campervan-solar

Also, we have interactive solar wiring diagrams that are a complete, A to Z solution for teaching you exactly what parts go where, what size wires to use, fuse size recommendations, wire lug sizes, and all kind of other stuff to help save you time and frustration.  You can check that out here: https://www.explorist.life/solarwiringdiagrams/

Finally, for this blog post you’re reading right now, we have a calculator that will help you choose a charge controller. I HIGHLY recommend reading this post to truly learn how a charge controller works, but if all you need is the calculator, here that is:

How does the Charge Controller Work?

Solar panels typically put out a voltage that is too high for batteries to use. If you have your solar panels wired in series like I recommend, you could possibly have over 100 volts coming out of the solar panels. If you connected your 100 volts from the solar panels directly to the battery, it’s not going to work. The Charge Controller regulates the voltage from the solar panels back down to the 12.6 – 14.6 volts that the batteries can store/use.

Charge Controller regulates the voltage from the solar panels.

MPPT vs PWM CHARGE CONTROLLERS

There are two main types of charge controllers. They are MPPT and PWM. This blog post is a crash course in solar design and getting into the specifics of the differences is out of the scope of this blog post. Here’s what you need to know regarding MPPT vs PWM charge controllers MPPT is the newer, more efficient technology. From here on out, any time I talk about charge controllers, I will only be talking about MPPT charge controllers as I want to guide you to build a high-end, expandable solar setup.

HOW TO MATCH SOLAR PANELS TO A CHARGE CONTROLLER

One of my favorite series of charge controllers is the Victron BlueSolar MPPT Charge Controller. If you’ll notice, there are MANY different sizes of charge controllers:

WHAT DO THOSE NUMBERS MEAN?!?

Lets use the Victron SmartSolar MPPT 100 | 30 for example. The 1st number, 100 means the maximum input voltage the controller can handle. In other words, the Victron SmartSolar MPPT 100 | 30 can handle a max of 100 volts coming from the solar panels into the charge controller. The 2nd number, 30, represents the max amount of amps the controller can output going INTO THE BATTERIES.

*MATH ALERT*

Let’s say, for example, you have 4 x 100 watt solar panels with the following stats.

EACH 100w solar panel has an Open-Circuit Voltage (Voc) of 21.6 volts. and an Optimum Operating Current of 6.72 Amps. Those are the only two numbers we are concerned about for now. I generally recommend just wiring all of your solar panels in series for simplicity and efficiency sake. Which means: Those 4 x 100 watt solar panels get wired together like this:

Since they are wired in series, the voltages get ADDED together for a total of 86.4 volts. (Open-Circuit Voltage (Voc) of 21.6 x 4 panels) The amps on the “upstream” side of the 100w solar panels remains 6.72 since in series, the voltages get added and the amps stay the same.

So, the 86.4 volts is under the safe threshold of the 100 max volts of the Victron SmartSolar MPPT 100 | 30 solar controller.

100 is the first number. What about the 2nd number, 30?

The 30 in the Victron SmartSolar MPPT 100 | 30 is the MAX resulting amps AFTER the solar controller has worked it’s magic. We need to do some math to determine the amperage. Here are the things we know:

  • We have 4×100 watts of solar panels totaling 400 watts of solar.
  • Assume batteries are 12.6v
  • Amps = Watts / Volts

This means, that at 400 watts and 12.6v we can expect up to 31.74 amps coming out of the solar controller.


400 watts / 12.6 volts (Battery) = 31.74 amps coming out of the charge controller.

Now, we are talking about that Victron SmartSolar MPPT 100 | 30, we have to compare that 2nd number, 30.

31.74 amps is a bit over the 30 amp threshold. BUT…

Solar panels rarely put out their full wattage. AND…

In the Victron SmartSolar MPPT 100 | 30 manual, they say their controller is good for solar arrays up to 440 watts:

It’s always good practice to trust manufacturer specs and recommendations.

AND… If you happen to go ‘over’ on your Amperage, it’s not that big of a deal in terms of damage. It’ll just be lost power that the controller won’t convert.

So, basically, the Victron SmartSolar MPPT 100 | 30 is pretty perfect for those 4 x 100 solar panels.

But what if you like playing it safe? What if you want some wiggle room? Great! Size up to the Victron SmartSolar MPPT 100 | 50. Sure, it’s a little more money, but if it’s worth your piece of mind to have an extra 20 amps available to you, go for it.

Now, Why would you want wiggle room or safety margin? Let’s talk about temperature

Solar Controller vs Temperature

DID YOU KNOW… As temperatures drop, solar panels actually put out MORE power.

Totally honest though, the math gets messy, SO I made a calculator that you can input all of the values for your setup so YOU can see how temperature affects your solar panel setup AS WELL AS will give you a recommendation on what solar controller you need taking solar panel temperature into account.

There’s a video below the calculator you can check out if you need additional instructions on how to use it:

Now that you know what kind of charge controller is compatible with your solar panels, it’s time to learn how to choose an inverter for your DIY Camper setup.  Check that out here:

https://www.explorist.life/how-to-choose-an-inverter-for-your-diy-camper/

Everything that you are learning here is put to use in our FREE Interactive Solar Wiring Diagrams.  If you haven’t yet, check them out as they are a complete solution for a camper van electrical system.  Check them out here: https://www.explorist.life/solarwiringdiagrams/

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here: https://www.explorist.life/diy-campervan-solar

Finally, If you found this guide helpful, It’d truly mean the world to us if you’d share it with somebody who can use it, pin it to pinterest for later reference, or share it to a facebook group when somebody has a question about this subject.  Click the bubble in the lower right corner to subscribe to be notified of future updates and as always, leave any questions you’ve got in the comments below.

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Scott

Monday 28th of December 2020

Hi, Nate--just watched your YT video on wiring different size or mismatched panels. . .at the 11 minute mark you talked about adding that extra panel that would REDUCE the voltage of the other panels. . . As stated previously I have 5 panels in parallel that are EACH putting out 85 voc--is it possible to add another panel with a 17-22 volt voc and bring the others down to that level so I can use them in a 12 volt battery bank? Thank you!

Scott

Tuesday 29th of December 2020

@Nate Yarbrough, I won't ask anything more after this: I cannot afford $550 for the controller and if I cannot get the voltage down for use with my established 12 volt system the 5 panels will be of no use at all and essentially dead. Last question: how much wattage will be left by adding a 17-22 voc panel? Thank you, Nate, you have greatly added to my knowledge. Scott

Nate Yarbrough

Tuesday 29th of December 2020

The voltage would come down, but so would the total array wattage as there would be pretty significant power loss. Just wire your panels in parallel and use a 150V charge controller (https://amzn.to/38E0V2w).

Scott

Monday 21st of December 2020

To further clarify my 12/20 6:39 posting: Each 95 watt panel has a voc of 100 and each one put out about 85 volts yesterday in full sun at about 65 degrees. I am using a 12 volt deep cycle battery bank. Not knowing ANYthing much at the time -- I purchased from "a guy" these 5 panels not knowing they were not 12 volt panels. . . WIll the 100/50 controller help me here? Other advice?

Scott

Thursday 24th of December 2020

@Nate Yarbrough, These are in parallel and not series. The tag on the back of each individual panel says 95 watts and 100 voc. I guess this is extremely rare since others have said these specs are not possible. The tag says this per panel and the voltmeter verifies it--per panel. Thank you for your reply, Nate. /Scott

Nate Yarbrough

Tuesday 22nd of December 2020

If your panels VOC is 100V... that will exceed the max voltage of your 100V charge controller when the temperatures get cold (solar panel voltages increase as temperature decrease.) You'll likely need a 150V MPPT charge controller and wire all of your panels in parallel.

Scott

Sunday 20th of December 2020

Help! Just discovered my private party purchase of (5) 95 watt 100 voc panels put out 85 volts and I just literally fried a 500 watt 12 volt charge controller. Is there any hope for me to use the high voltage output on a 12 volt battery bank?

Nate Yarbrough

Tuesday 22nd of December 2020

To keep the array voltage low, those panels must be wired in parallel. The will also need to be paired with a charge controller that can handle that kind of voltage. I suspect you would need a 150V charge controller, but here's the specifics on that: https://www.youtube.com/watch?v=MxziHKvTRh8

Mallory

Tuesday 29th of September 2020

Also, what makes Victron so much more preferable to Renogy? It is twice the cost, and I'm not sure what the additional benefits are.

Nate Yarbrough

Thursday 1st of October 2020

The Victron SmartSolar charge controllers allow usage of the VictronConnect app which will show all of the data from the solar array on your phone as well as historical data figures. This is nice because you can see a day-by-day graph of solar production so you can see if your solar array is ACTUALLY performing as it should be and can help troubleshoot problems if there are any. Since it is connected to an app which has internet access, it can receive firmware updates if Victron pushes out new features. For example... soon, Victron will release a firmware update that will allow for COMBINED solar and alternator charging at the same time when a SmartSolar charge controller is used in conjunction with the Orion Smart DC-DC Charger.

Mallory

Tuesday 29th of September 2020

Hi Nate! You're blog has been incredibly useful in learning about solar and general building recommendations. I am purchasing 2 residential panels, which have much higher voltage than the usual RV panels. I'll have (2) 325W panels in parallel, and in calculating the size MPPT controller I need I am getting conflicting information. Some resources say to take your wattage (650w) divided by the battery system voltage (24V Nissan leaf), which would equal 27a, necessitating a 30a controller. Other sources say to take the short circuit amperage (6.03a) and multiply it by the number of panels in parallel (2), which would equal 12a, necessitating only a 15a controller. What are your thoughts on this? How does that short circuit amperage come into play?

Nate Yarbrough

Thursday 1st of October 2020

This is best explained in a video. Here is a link: https://www.youtube.com/watch?v=MxziHKvTRh8