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The purpose of the charge controller is to take the power generated from the solar panels and charge the batteries. If you were to connect the solar panels directly to the batteries without a charge controller, it would end up overcharging the batteries permanently damaging them.
There are two main types of charge controllers:
This is the most common type of charge controller in small systems like portable solar generators, because it is very simple, light, and inexpensive. The problem is that it’s not very smart. It’s basically an on and off switch. It’s on when the battery is low, and once the battery starts getting full, it turns off for a short period, then back on. The duration when it’s off gets longer and longer until it is totally off when the battery is completely full.
This type of charge controller is larger heavier, and more expensive, but it has a several important advantages over a PWM charge controller. First of all, the MPPT stands for maximum power point tracker. If you remember from the solar panel section, your solar panels have a Vmp (the voltage where they produce the most power). This is never the same voltage as the battery. A 12V solar panel usually has a Vmp of arount 18V. This is because if it were at the voltage of a 12v battery (about 13V full), it would not be able to charge at all when unless there was perfect sun pointed directly at the solar panel. It would also charge slowly because the absorb voltage (the voltage needed to charge the battery) is around 14v. The point is when you connect the battery to the solar panel with a regular PWM charge controller, it will pull the solar panel voltage down to the battery voltage, which is not the Vmp of the solar panel, and thus your solar panel will not be able to produce its rated power. The MPPT charge controler is able to find the Vmp of the solar panel (even on cloudy days when it is not the same as the rated Vmp), and keep the solar panel at that voltage to produce its maximum power. It then steps that voltage down to the battery charging voltage while at the same time increasing the charging amps. By doing this, it can actaully charge your battery at the full power of the solar panels. This can increase charging by up to 40% on cloudy days when you need it the most. So a solar generator with an MPPT charge controller will actually out perform one with a PWM charge controller even if it has less solar panels. This is really good for a portable system because you can get away with less solar.
Another really nice thing about an MPPT charge controller is that it can have a much wider range of solar input. Good charge controllers can allow for over 100V of solar, where with a PWM, you need to stay very close to the battery voltage. This means you can add more solar panels easier with an MPPT charge controller. Also, since it can regulate the current (rather than just using a switch), you can add extra solar panels and go over the output watts of the controller without any worry (on good MPPT charge controllers). So if you have a 1000 watt charge controller, you could connect 1500 watts of solar, and even though you would never produce more than 1000 watts, it will not damage anything.
Doing this can be extremely beneficial for days of cloudy weather. So for example, lets say it is cloudy or overcast, and your solar panels are only putting out 30% of there rated output with an mppt charge controller, and 20% with a PWM charge controller. Now if you have both charge controllers rated at 500 watts of power with 500 watts of solar connected, the pwm will be putting out 100 watts, and the mppt charge controller will be charging with 150 watts of solar (both are much lower than the 500 watts).
Now lets say you have an extra 1000 watts of solar. If you connect these extra solar panels up, the mppt charge controller will charge at a rate of 450 watts, and PWM charge controller will charge at 300 watts (on a cloudy day). Not bad right. Well we have one little problem. What happens if a cloud moves away, and we have full sun? 1500 watts of solar is going to burn up your 500 watt PWM charge controller (or its connecting wires, etc). However the MPPT charge controller will regulate the power, and keep it at 500 watts. This way you can connect extra solar panels, and don’t have to remember to unplug them when the sun comes out.
Here is what you need to know:
PWM charge controller:
A simple on/off switch to keep from over charging the batteries
No current or voltage regulating
MPPT charge controller:
Much more advanced
Voltage and current regulating
More efficient (up to 40% more efficient)
More expensive (about 5X the price of a PWM)