A solar module converts sunlight into direct current. So the current from the solar module could be directly fed into the battery which also stores the electricity as direct current which in turn could be used to serve DC consumers.
The solar modules’ voltage, however, is higher than the voltage of a fully charged battery and can therefore damage a battery when it is directly charged. Moreover, a deep discharge of the battery may considerably impair its durability when the loads are used excessively. The charge controller protects the battery from deep discharge, overload and short circuits. It extends the durability of the batteries considerably and should be on principle applied with self-sustaining solar systems.
When the end of charge voltage is reached, a series controller interrupts the module power using relay or power conductor and switches it back on after a defined voltage drop. It disables further current flow into batteries when they are full.
A shunt controller continuously reduces the module power when the end of charge voltage is reached. But since the module continues producing power, the unneeded part of the module's power is used as a short-circuit current without any problems. This method is ideal for the battery as charging is safe and swift.
MPPT solar controllers make the solar panels operate at their optimum power output voltage. The current is regulated by the battery voltage, therefore they make the system accept high input voltages from the solar array. The generated power is able to be used more efficiently.
What PWM means?
Pulse Wide Modulation is a special charging algorithm that is used in most charge controllers and also in other electric devices. It is the most effective means to achieve constant voltage battery charging by switching the solar system controller´s power devices. When a battery voltage reaches the regulation set point, the PMW algorithm reduces the charging current to avoid heating and gassing of the battery. The result is a higher charging efficiency, rapid recharging, and a healthy battery at full capacity.
The choice of the appropriate solar charge controller depends on three factors:
These factors are predetermined by the components to be applied.
Module current and load current
The charge controller must be able to absorb the maximum current that flows in the system. Therefore you have to check first which current flows with the module (module current) and which current flows with the consumer (load current).The chosen charge controller should at any rate cover the currents to be expected while the possible currents of the charge controller should not be exceeded.
12 VDC systems are often used, both on the consumer side (load current) and on the battery side. It is, however, also possible to use battery systems with 24 VDC and 48 VDC. The data of module, load current and voltage are specified by the manufacturer.
Basically, solar charge controllers are distinguished between shunt controllers, series charge controllers and charge controllers with MPPT function.
The load voltage with shunt and series charge controllers is determined by the battery. Chose a shunt or series controller when battery current is higher than the module current. Charge controllers with MPPT (Maximum Power Point Tracking) function adjust themselves at any time to the maximum voltage of the module bringing higher voltages down to the voltage required on the battery. Thus they make sure that the current yield is higher. Choose a charge control with MPPT when battery current is lower than the module current.
Besides, using a much higher DC voltage on the input side allows using thinner wire, decreasing the wire cost and making the installation easier. Especially when used in cooler regions or when the distance from the solar module to the battery is long (long cables), the MPPT controller ensures higher current yields. Small appliances are supplied often with 5 VDC by a USB-plug. Pico systems as well as some charge controllers or inverters provide USB outputs, not for data transmission but for 5 VDC power supply. These USB outputs can charge mobile phones and battery packs or supply tiny loads like lamps, radio, MP3-players.
PWM solar charge controllers
A PWM controller connects a solar module to a battery, the current then flows through the controller to the battery. The module voltage almost breaks down to the battery voltage. Basically nothing else happens than in the example above when we connect a solarmodule directly to a 12 V battery. But when the battery becomes full (the absorption voltage is reached), the solar regulator starts working. It separates the module and battery from each other and when the battery voltage has dropped a few millivolts, the solar moduleis switched on again. This process takes place several times per second. This control mode is called pulse width modulation (PWM). Solar modules deliver a certain current depending on the sunlight. This current is independent of the module voltage. Consequently, the same current flows at 18 V or 13 V. However, the module delivers a power (measured in watts). The power is the product of voltage and current. Anyone who paid attention in physics at that time knows that it is possible to calculate electrical power simply by multiplying voltage and current. As a result, the power at 18 V is higher than at 13 V if the same current is used as a basis.
MPPT solar charge controllers
MPPT = Multi Power Point Tracking - in English about as much multi point tracking. An MPPT controller scans the power curve of the solar module and finds the highest power point. Usually a module delivers the maximum power at a voltage of 16-18V. The power of the module is then converted to the battery voltage, like a voltage converter that converts 12V into 230V or 24V into 12V. This method is so effective that despite the losses in the regulator, much more power is transferred to the battery through the voltage conversion than with a PWM regulator. Here is a short example:100 Wp bring (exemplary!) in the sun 18 V and 5 A (corresponds to 90 W). With a PWM regulator you could charge a 13.5 V battery with 5 A charge current. Which corresponds to a power of 67.5 W (at exactly 13.5 V). With a MPPT regulator the 5 A and 18 V are converted to battery voltage and at 13.5 V 6.66 A (90W) flows.