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Wiring solar panels, charge controller and battery together

03 Mar 2021
8 min
Intermediate

Andrei GorichenskiiAuthor

One of the most common questions we get is "Can I connect my solar panels directly to the battery?" While technically you can, we strongly advise against it. A battery is a fragile thing and high voltage of solar panels can easily destroy it. A charge controller acts as a safety barrier between panels and a battery and should be a part of every home solar panel installation. In this article, we'll explain how to wire together solar panels, a regulator and a battery.

Charge controller keeps your battery safe

But what does a battery fear? From what does a controller actually protect it? Well, a charge controller

Whenever you add energy storage to a solar system, add a charge controller in between the panels and the battery

• Lowers the voltage of panels down to the level of the battery. When the battery is directly connected to panels whose voltage is higher, the battery heats up. Not only does it decrease the lifespan of a battery, it can potentially lead to its explosion.
• Prevents overcharging. If you charge the battery after it's full, its temperature rises because it tries to accept more current than it is capable of. As a result, you have faster aging, decreased capacity and explosion in the worst-case scenario.
• Stops the usage of a battery in the state of deep discharge. Regular deep discharge of a battery lowers the number of charge cycles a battery can survive.
• Blocks reverse currents. At night electricity can flow back to panels from the battery, which can lead to temperature rises and fires in the PV modules.
• Allows monitoring the state of the panels and the battery.

Solar controllers handle the voltage of panels differently. PWM (pulse-width modulation) controller simply brings it down to the level of the battery. MPPT (maximum power point tracking) controller, on the other hand, uses extra voltage of the panels and turns it into electricity.

PWM and MPPT are interchangeable in cases when the voltage of the solar panels is slightly higher than the voltage of the battery. For example, you can install either of them with 30-cell panels and a 12V battery or 60-cell panels and a 24V battery. However, even in these cases, an MPPT-controller allows you to harvest 20-25% more energy. This type of solar regulator is especially good in winter, when panels reach their voltage peaks.

PWM-controller	MPPT-controller
Pros • lower price: $20-$100 • 15+ year lifespan • 2-4 times smaller than MPPT	Pros • 90-95% efficiency • converts extra voltage of panels into current • manages charging process • built-in Bluetooth to connect to to a laptop or a smartphone
Cons • 75-85% low efficiency • works well only when the voltage of panels is slightly higher than the voltage of a battery	Cons • 10-15 year lifespan • more expensive than a PWM-controller: $100-$1,000

Look for maximum volts and amps when sizing a controller

Before purchasing a charge controller, make sure it fits the solar panel system. The main parameter you're looking for is maximum amps. Amps of a controller must be bigger than the combined power of all solar panels divided by the voltage of the battery.

Let's say we have two 300W panels and a 12V battery. Now we calculate the amps:

300W * 2 panels / 12 V = 50 A

Let's add 25% for safety. On cold days panels produce more current than usual and it's better to be ready for it:

50 A * 1.25 = 62.5 A

The size of a controller must be bigger than 62.5 A.

Connecting battery, controller and panels

Whether you have a PWM-controller or an MPPT-regulator, the procedure of hooking it up with the battery and panels remains the same. Normally there are three wiring sections on a charge controller: one for panels, one for a battery and one for DC loads.

Battery

1. Take a simple stranded copper core wire.
2. Use the black wire to match the charge controller "minus" with the battery "minus".
3. Use the red wire to match the charge controller "plus" with the battery "plus"
4. Screw the wires tightly into the charge controller

Turn the charge controller on: it should be able to measure the charge of the battery.
In the user manual of a charge controller, there should be a wiring diagram, which you can consult if in doubt.

It’s advised to wire the controller to the battery first before connecting it to a solar array. Controllers often have to perform an initialization when they get connected to a battery during which the regulator evaluates the battery's state. If you connect the solar panel to a charge controller first, it may not initialize correctly.

PV modules

After you've connected the charge controller to the battery, it is now safe to connect it to the panels. Out of the junction box of a panel come two cables, a positive and a negative. In some situations, it's just two wires that go straight to the controller. More often, there are so-called MC4 connectors on each end, a "male" connector for the positive cable and a "female" for the negative one.

1. Connect MC4 connectors with a complementary pair of connectors with wires on other ends. If you have two wires coming out straight of the junction box, skip this step.
2. Plug these wires into the regulator: positive into "plus", negative into "minus".
3. Once you've connected the panels to the controller, it should be able to recognize them. Check the status of your array on the charge controller screen.
4. PV modules start to generate electricity as soon as they face the sun.

Here's the diagram, which gives an idea on how to connect these parts of a solar panel system together. We have one 12V KiloVault solar battery, one 96A Midnite MPPT-controller and two 330W Panasonic solar panels.