How To Connect Two Solar Panels To One Battery

Note: This article contains an affiliate link to My Generator. If you click through and make a purchase, we earn a small commission at no extra cost to you.


We have a Wedgetail Camper with a REDARC Manager30 and a 100Ah Revolution Power Lithium battery. We figured out a way to connect two solar panels to the camper at the same time.


It’s All In The Wiring

The Wedgetail Camper has two solar inputs via red Anderson plugs. The first is on the roof. This is used when the solar panel is stored for travelling.

When the Wedgie is open, the roof hinges over 180 degrees and becomes the base of our bed. It doubles as an awning over the outside kitchen. The solar panel can be removed and used as a portable panel with a 10 metre extension lead.

Our Wedgetail Camper has 2 solar inputs, allowing us to connect two solar panels at once.
This solar input is used when the solar panel is stored for transport. We use this to plug in our solar blanket.

The panel is mono crystalline, 200W maximum output.

When used as a portable panel, there’s a second red Anderson plug on the front right corner of the Wedgie. This is where you plug in the extension lead when the panel’s being used as a portable unit.

The 2nd solar input on our Wedgetail Camper. By using both inputs, we can connect two solar panels to our REDARC BMS.
The second solar input on the front right hand side. We plug the portable solar panel into this.

In this configuration the red Anderson plug on the roof (now on the underside of the outside kitchen awning) is not used. I’ve always wondered if it’s possible to use both red Anderson plugs at the same time… have one panel connected to each.

We also have a REDARC 115W SunPower mono crystalline solar blanket. It was supplied with a 10 metre extension lead. REDARC use standard grey Anderson plugs, so I had to make a short adapter lead with a red Anderson plug on one end and a grey one on the other.


Note: For a 12 month review of our REDARC solar blanket, go here.

Why did I need the adaptor plug?

Our adaptor cable allows us to use red and grey Anderson plugs interchangeably.
Our red to grey Anderson plug adaptor.

Well we often leave the solar panel on the roof and use the solar blanket instead. It’s just easier to get out and set up compared to wrestling with the large panel. And interestingly the 115W solar blanket’s output is consistently lineball with the 200W solar panel’s output. You simply can’t beat quality…

But sometimes it would be handy to use the blanket and the panel at the same time… especially on cloudy days.







Ask The Experts!

I spoke to the people at Wedgetail Campers about how the two solar inputs were wired. They assured me they’re individually wired to the REDARC Manager30 solar input, with no tricky devices in between.

The terminal strip on top of our REDARC BMS. The solar inputs are individually wired, so we can connect two solar panels to one charge controller.
Double-checking the wiring on the Wedgetail Camper’s REDARC BMS.

All good so far.

The next step was to contact REDARC. I gave them two important pieces of information:

  1. how the solar inputs to the BMS were wired, and
  1. what type and size both the solar panel and solar blanket were.

They confirmed there would be no issues running these simultaneously. Thanks to Jessica and the tech team at REDARC for your help.


A Good Result

So now we can use the portable panel and blanket together. With both panels connected we push about 12A into the system. This is about double compared to when just one panel is connected.

How to connect two solar panels into one battery system.
Our panel and blanket working together.

We did 2 tests to compare to outputs of the panel vs the blanket. Here’s a few figures for you:

Conditions:

  • Solar blanket flat on the ground.
  • Solar panel angled perpendicular to the sun.
  • Sunny day, no clouds, January (perfect conditions!)
  • The individual contributions of each panel/blanket were measured by disconnecting the blanket and checking the revised current.

Test 1:

  • Input voltage = 14.2V
  • BMS output to 2nd battery system = 11.5A total
  • Power (14.2 x 11.5) = 163W
  • BMS output, Panel connected = 6.4A (or 56% of total)
  • BMS output, Blanket connected (11.5 – 6.4) = 5.1A (or 44% of total)

Test 2:

  • Input voltage = 14.4V
  • BMS output to 2nd battery system = 12.0A total
  • Power (14.4 x 12.0) = 173W
  • BMS output, Panel connected = 6.6A (or 55% of total)
  • BMS output, Blanket connected (12.0 – 6.6) = 5.4A (or 45% of total)
The REDARC battery monitor showing the input voltage with two solar panels connected.
The REDARC battery monitor showing the input voltage with two solar panels connected.
The REDARC battery monitor showing current output to battery with 2 solar panels connected.
The REDARC battery monitor showing current output to battery and loads with two solar panels connected.

A couple of comments on these results:

  • The solar blanket’s maximum output is only 58% of our solar panel (115W vs 200W).
  • The solar panel was angled at the optimum angle to the sun, whereas the blanket was simply laid flat on the ground.
  • Yet the REDARC solar blanket consistently contributed 45% of the total input power. You could say it’s punching above its weight!






Some Considerations

You too can connect two solar panels into one system, just like we do. REDARC sell a ready-made cordset with grey Anderson plugs. It allows you to connect two panels, then connect them into one solar input.

Just check a few things first:


Are the solar panels compatible?

The best way to find out is to contact your solar panel supplier. I contacted REDARC because the solar blanket was their product.


What is the maximum rated input power of your BMS?

The REDARC Manager30 has a maximum solar input rating of 520W. Our panels are rated at 115W and 200W for a total of 315W. So we’re well within allowable range.


Is the solar input cable heavy enough to handle the higher current?

If your BMS has been wired correctly, the solar input cable should be large enough to handle the maximum input current that the BMS is rated to. You can ask the manufacturer of your camper or caravan to confirm this. The last thing you want is to melt the cable and start a fire.

Alternatively, you can check it yourself. Use this handy calculator to check the required cable size.


Do either of the panels have a built-in regulator?

If so, you might not be able to connect them. Alternatively you might have to remove the regulator. Contact the supplier of your battery management system.







In Summary

We now have the ability to re-charge our second battery system quickly and efficiently. On clear sunny days, one panel is more than enough. But on those cloudy days, you need every bit of solar power you can find.

With some simple investigation, you might find you can do the same.


If you’re looking to buy, My Generator have some good deals on REDARC Solar Blankets and Battery Management Systems.



FAQs – Common Questions About Connecting 2 Solar Panels


Can you connect two solar panels to one battery?

You might think we’ve already answered this. However, there’s one consideration…

To power a dual battery system, you’ll need to connect the panels in parallel.

How to connect two solar panels in parallel.
Connecting two solar panels in parallel. Connect the negatives to the negatives and positives to the positives.

If both panels are rated at the same maximum voltage, then you’ll get maximum output. For example, our 115W SunPower solar blanket has the following ratings:

Maximum rated voltage = 19.8V

Maximum rated current = 5.8A

Maximum rated power = Volts x Current = 19.8 x 5.8 = 114.8W

If you connect two of these blankets in parallel, you’ll get:

Maximum rated power = 2 x 19.8 x 5.8 = 229.6W

However if you have two panels with different output ratings, the maximum voltage for the system is determined by the panel with the lowest maximum rated voltage.

Confused? Let’s look at what happens when we connect our solar panel and solar blanket.

Panel:

Maximum rated voltage = 18.0V

Maximum rated current = 11.1A

Blanket:

Maximum rated voltage = 19.8V

Maximum rated current = 5.8A

When you connect them in parallel, this is what you’ll get:

Maximum rated power = (18.0 x 11.1) + (18.0 x 5.8) = 304.2W

So the output from the solar blanket will be de-rated to (18.0 x 5.8 =) 104.4W or by 10%.

In our case, it’s not a big deal.

It’s more when you have a bank of panels connected in parallel, for example 6 on the roof of a motorhome. Then you start to lose significant capacity.


Can I connect two different solar panels together?

See “Can you connect two solar panels to one battery?” above.


What about connecting multiple solar panels to one battery?

You can connect them in parallel, providing your battery management system can handle the maximum input power.

How do you calculate maximum input power? See “Can you connect two solar panels to one battery?” above.

In our situation, maximum input power is 304W. Our REDARC Manager30 is rated to 520W max. solar input, so it’s okay.

In contrast, the REDARC Manager15 is rated to 260W max. solar input, so would be too small for our application. It would still operate okay, but you’d be wasting the extra capacity of the solar panel/blanket combination.


How do you match a solar panel to a battery?

Sorry, there’s no easy way to answer this! I hope you like maths…

  1. Figure out what devices you want to run. For example:
  • A fridge draws 7A and runs for 20% of the day (24 hours). So average current draw in one hour is 7 x 0.2 = 1.4Ah
  • An inverter draws 45A for 5% of the day. So average current draw in one hour is 45 x 0.05 = 2.25Ah
  • USB ports and LED lights draw 5A for 20% of the day. So average current draw is 5 x 0.2 = 1.0Ah
  • Water pump draws 7.5A for 5% of the day. So average current draw is 7.5 x 0.05 = 0.375Ah
  1. Add all these together:
  • (1.4 + 2.25 + 1.0 + 0.375) = 5.0Ah.
  • Let’s round it to 6Ah.
  • What does this mean? Well, in 24 hours you’ll use 24 x 6 = 144Ah. Say 150Ah.
  1. Your solar panels must be capable of supplying this. Let’s size your solar panels:
  • Assume a maximum of 6 sunlight hours.
  • So in 6 hours they must provide 150Ah to the battery or 150/6 = 25A.
  • At 25A and assuming the solar panels supply 12V, you’ll need a 25 x 12 = 300W of solar panel capacity.
  1. Allow at least 10% extra capacity:
  • So you need 300 x 1.1 = 330W of solar panels.
  • Your most likely match will be 350W.
  1. What size battery do you need?
  • Deep cycle batteries don’t like to drop below 50% capacity (Lithium batteries can go much lower without damage).
  • You use 150Ah in 24 hours. But you can’t discharge below 50%. So you need 300Ah.
  • The solar panels supply 150Ah in 24 hours. They will recharge your battery to 100% every day (assuming  hours of usable sunlight). 

In Summary:

You don’t just match the solar panel to the battery. You have to know the loads on the battery, size the battery to suit the load and then size the solar panels to suit both the load and the battery.

I told you the answer wasn’t simple!


How many solar panels does it take to charge a 12 volt battery?

See “How do you match a solar panel to a battery?” above.


How many batteries do I need for a 400 watt solar system?

See “How do you match a solar panel to a battery?” above.


Can a solar panel overcharge a battery?

Yes! Never connect a solar panel directly to a battery. It needs a regulator between the panel and the battery.

The regulator will either be mounted on the back of the solar panel or within a battery management system (BMS). The regulator regulates (funny that!) the voltage from the solar panel and protects the battery from over-voltage.


What types of solar panels are available on the market?

There are three main types. They are:

  • Polycrystalline
  • Monocrystalline, and
  • Thin film.

SolarMagazine.com have a really useful and informative article on the differences between these 3 types of solar panels, plus a whole lot of other useful information.


Access Travellers Guides and a whole lot more at our Free Resources pages.

Get your Traveller’s Guides

… and a whole lot more at our FREE RESOURCES Page!


Any questions or comments? Go to the Comments below or join us on Facebook or Twitter.

Any errors or omissions are mine alone.


NOTE: The REDARC solar blanket was supplied to us free, in return for a review. (That said, our reviews are always honest and authentic – we tell it how it is).


For more Product Reviews and useful Articles, go here


Leave a comment