How to Connect a Solar Panel to a 12V Battery?

Connecting a solar panel to a 12 Volt battery is not a hard thing to do.

In this article, I will explain how to connect a solar panel to a battery, step by step.

I will also share a few tips you need to know along the way.

Let’s start!

Step 1: Connect battery to charge controller

In the first step, you are going to wire the battery to a charge controller. It is important to wire this component first before you wire the solar panels. If you wire the solar panels to your charge controller first, the fuse of the charge controller might blow.

If your charge controller has no fuse, you can’t use it anymore.

Always connect your battery to the charge controller first!

What is a charge controller and why do I need one?

A charge controller is a device that regulates the current and voltage that goes to the battery. To understand this situation, picture a scenario where your solar panel is getting sufficient sunlight. Your 12V battery only needs around 13.6 volts from a 12V solar panel to get charged.

However, the output from your solar panel can range from 17 to 48 volts depending on the type. Can you see the problem here? If there is no charge controller to provide a regulated voltage to your battery, it will get overcharged and damage will occur.  The charge controller also senses if the battery is full. When the battery is full, the charge controller will stop delivering energy to the battery.

This is why you can’t connect your solar panel directly to your battery.

Wiring the battery to the charge controller

The thickness of the wire will depend on the type of charge controller you get. If you get a 20 amp charge controller, you need to use a wire thickness that is able to carry 20 amps without overheating (AWG14 / 2.5mm² or bigger). Flexible wires with a nylon coating are preferred (THWN-2 cable). Learn how to select wires and thickness in my book.

You need to keep these wires as short as possible because they will be expensive.

The biggest mistake is choosing wires that are too small which increases the risk of fire.

When connecting the battery to the charge controller you need to wire the black (negative) wire first, then the red (positive) to the charge controller. There might be a small spark but that is normal (charging of the capacitors in the charge controller).

Step 2: Wiring the Loads

The following step is to wire the loads. These can be an inverter, 12 volts dc box or both. You have two options here:

  1. Attach the loads to the charge controllers output terminal
  2. Attach the loads to the battery terminal

Option 1:

Only use this option if you have a small DC load. These terminals cannot deliver high current but will protect your battery from over-discharging.

For example, the renogy rover 20/40Amp series, can only supply 20 Amps at its load terminals both for the 20 amp and 40 amp version. This means that you can only attach 240 Watts (20 amps x 12 Volts) to the load terminals. You see that this is very limited.

DC fuse box attached to the load terminals of a charge controller
DC fuse box attached to the load terminals of a charge controller
Never attach an inverter, battery charger or a device that draws a lot of amps to the load terminal of a charge controller.

Option 2:

If you have a device that can draw a lot of amps like an inverter or battery charger, you need to attach the wires directly to the battery terminals. From here, you can wire it directly to the inverter or to a busbar. See the following image for reference:

charge controller connected to battery and loads
Charge controller to battery and loads

Using this setup, you have to monitor the battery levels in order for them not to be depleted.

Before you go to the next step, make sure you have programmed your battery type in the charge controller.

Step 3: Connect solar panels to charge controller

The third and final step is to connect the solar panels to the charge controller.

If you have more than one panel and are unsure if you need to connect it in series or parallel, check out my article here.

Confused about specifications of the solar panel you got? Check out my article where I explain all the parameters of a solar panel here.

Use the cables that come with the solar panel, these are UV-resistant cables. Other cables will crack and break over time when they are exposed to UV radiation. This is the part when you need to oversize the diameter of the wire to account for voltage drop. Try to keep the voltage drop under 3%. Click here to calculate the voltage drop.

What is voltage drop?

Voltage drop is a phenomenon that appears when you have a long cable that delivers energy from point A to point B. The length of the cable increases the resistance in the cable which will reduce the voltage at point B.

For example, your solar panel delivers 18 Volts DC and 5.8 amps to the charge controller and the wire length is 40 feet.

how to calculate voltage drop
Voltage drop in a single panel system

As you can see in the calculation, the voltage drop is 4.09% which will reduce the overall power delivered to the charge controller.

Let’s calculate the amount you will lose: 0.74 volts x 5.8 amps = 4.3 Watts.

This is not much, but if you have multiple panels, this can add up quickly. It could be that in your country the voltage drop may not be above 3% by law. To reduce the voltage drop, you use thicker wire.

Try to limit voltage drop to 3% because it will increase power loss in the system.

The final version will look like this:

Complete schematic of how to wire a solar panel to a battery with loads attached
Complete schematic of how to wire a solar panel to a battery with loads attached

Fuses

You need to have fuses in between your devices. The main objective of having fuses is to protect the wires from overheating or catching fire, not to protect the device. This is because you are going to size the wires to the amount of current that is able to flow from one device to the other.

You need fuses in the following places:

  • From solar panel to charge controller
  • From battery to charge controller
  • From battery to inverter
  • From battery to DC fuse box

I will write an article about fuses soon because it will be too long to include in this article. This is also something I write about in my book.

Conclusion

You have come to the end of this article. Hopefully, you now know how to connect a solar panel to a battery.

If you have questions, please write them in the comment section below.

2 thoughts on “How to Connect a Solar Panel to a 12V Battery?”

  1. Hi, do you need to match the voltage of the solar panel to the battery?
    So a 12v panel for a 12v battery? Or can the panel be 18v or 36v etc.

    Reply
    • Hello Rob, you do not need to match the solar panel to the battery. The charge controller will take care of the voltage transformation. For example, you have an 18 volts panel connected to a 12 volts battery. The charge controller will transform the 18 volts down to the ideal voltage to charge the battery. Do not forget to adjust your charge controller to match your battery pack voltage.

      Reply

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