Wiring solar panels, charge controller and battery together

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

• 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, it's temperature rises because it tries to accept more current that 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 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.

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. During 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.

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.

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.

After you've connected the charge controller with the battery, it is now safe to connect it to 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 "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 recognise 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.

If you have several solar panels, like on the diagram, the positive cable of one panel usually goes to the negative terminal of the adjacent one. Then, the negative cable of the first panel and the positive cable of the last panel go into the charge controller. These panels are connected in series, which means that their voltage is combined, but an amperage stays the same.

In this small panel system, each of the panels has a voltage of approximately 38V. Since panels are connected in series, their combined voltage is 38V * 2 = 76V. Their amperage is

330W ÷ 38V = 8.7A

The voltage of the battery is 12V. A PWM-controller would just cut down the voltage of panels down to the level of battery. Therefore, you would make use only of 8.7A * 12V = 104W of power for the whole array of 660 W. However, since we have an MPPT charge controller, it lets panels retain their power and speeds up the charging process of the battery. This battery in theory can receive 660W ÷ 12V = 55A of current.