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Understanding the functionality of solar charge controller
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Understanding the functionality of solar charge controller

15 mins 26 Jan 2021
Charge controller is an essential part of any solar panel system — it keeps your batteries safe and helps to store the accumulated energy. But how exactly does it function? What helps the controller to understand when the battery needs to be charged and what is the core difference between PWM and MPPT controllers? In this article we'll focus on the main functions of the solar charge controllers and the details behind them.

Controller manages the battery charging process

First, let's go over what solar regulators actually are. With solar power generators it's always a stand-alone device, whose main function is to manage the charging process of the battery: keep it from overcharging and not to let it be used when empty. This device is pretty much obligatory for a solar panel system. The only exception when a solar system with batteries can function safely without a charge controller is when battery capacity greatly surpasses the wattage of panels, like in solar vehicles, and there is basically no risk of overcharging it.

There are two main types of solar controllers: PWM-controller (pulse width modulation) and MPPT-controllers (maximum power point tracking). The first is much cheaper but should only be used when the voltage of panels is equal or slightly higher than the voltage of batteries. MPPT regulators, on the other hand, are more expensive, more efficient and sometimes have more features. Now let's look at their functions more closely.

Controllers prevent overcharge and deep discharge

The main reason why we don't want our batteries to be overcharged is simply because it drastically shortens their lifespan. The voltage of a full battery under continuous charge gets too high and the water inside starts to separate. In some situations it can lead to the explosion of oxygen and hydrogen inside of the battery. In other cases high voltage might cause the malfunction of home appliances or provoke the inverter to shut the system down.

At the same time, batteries don't like to be used when they are half-empty. Yes, deep-cycle batteries that are used in PV modules are designed to be deeply discharged, but it doesn't mean that it's healthy for them. This is why the charge controller disconnects the battery when it's half-empty so it can charge up back again and then continue working. The way that controllers track the load of the battery is by measuring its voltage. A fully charged battery has higher voltage than a half-empty one. So when the voltage of the battery drops below a certain level, the controller disconnects it. This function is called Low Voltage Disconnect (LVD) and is relatively modern — nowadays almost all models have it.
A fully charged battery has higher voltage than an empty one

Regulators block reverse currents

During the daytime solar panels generate electricity and it flows to the battery and home appliances. But in the nighttime when panels stop working, some amount of current can start flowing back from battery to panels. This is called "reverse current". Charge controller prevents it. In the old days there were special relays in regulators that would switch off at night to block the reverse current. Now there are semiconductors in regulators, which make a "one-way-road" for the current. Another way to block reverse currents is to install the blocking diode, which also contains some kind of semiconductor — generally silicon.

Charging process is different with PWM and MPPT controllers

Both PWM and MPPT controllers are pretty much the same when it comes to prevention of overcharge/deep discharge or blocking reverse currents. However, the ways that PWM and MPPT controllers approach the task of charging the battery and keeping it full are different.
PWM-controller first makes sure the battery is fully charged (or keeps the current flowing until it is). Then it opens and shuts the flow rapidly so that the battery is being charged by the trickle of pulses of energy. The voltage of panels in this case gets equalised to the voltage of the battery. This is why PWM-controllers aren't the best choice when the voltage of panels is substantially higher than the voltage of the battery — extra voltage of panels just goes to waste. What is more, the voltage of a half-empty battery is lower than when it's full, and that makes the PWM-controller even more inefficient during the charging process. MPPT-controller tackles this issue.

The voltage and produced current of solar panels changes during the day. For example, the hotter it gets, the lower the voltage of the panels is. Usually, Power = Voltage * Current, but when it comes to solar panels, an increasing voltage past a certain point leads to the loss of power. MPPT-controllers are always trying to find that certain point where voltage and current can be maximized. It lowers the voltage to the level of the battery just like a PWM controller does, but the excessive voltage also turns into current and goes into the battery. MPPT controllers perform especially well on cold sunny days when panels reach maximum voltage (Vpp) and barely lose any energy during the charging process. As a result, an MPPT-controller is 20-30% more efficient than a PWM-controller even in the right scenario for PWM when the battery and panels voltages are matching.
Let's set an example. If a 60-cell 300W panel with operating voltage of 32 V is connected to a 12 V battery, then the PWM-regulator cuts the voltage from the panel to 12 V. The current from this panel that goes to the battery is approximately 9 A (300W/32V=9.3A). The loss of energy is evident: since the voltage is halved by the regulator, the panel functions as if its power is slightly higher than 100 W (12V*9A=108W). An MPPT-controller, on the other hand, lets the panel retain its power, but at the same time lowers the voltage to the safe level of the battery, so you get around 25A of current (300W/12V=25A). On a cold day this panel reaches its open circuit voltage (Voc) of 39V, which means that the current which goes into batteries is closer to 29 A — three times better than with a PWM-regulator!

Charge regulators help monitoring solar panel system

Owners of solar panels might have a hard time understanding the state which their system is in just by looking at it. Panels generally don't have any way of telling you how they're actually doing. It can be equally hard to understand whether your battery is full or it needs charging, or it has been broken for several days. Solar charge controller, being a stand-alone device, actually gives you an option to monitor the state of your system. Most controller models have a display that gives the most basic information about the flow of solar energy. Basic controllers just show you the voltage of panels, the load of the battery and whether it's charging right now or not. More advanced ones measure the temperature, the current that flows to the batteries, let you customise the preferable time for charging and so on. Some charge controllers can be managed through the Bluetooth or via Wi-fi connection.
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Illustrations – Marina Fionova