In our What the Tech? series, we break down complex terminology, take a look at emerging power trends, and shed light on the technology that goes into making our portable power, solar, and lighting solutions. Read on for a deep dive into the differences between MPPT and PWM charge controllers and the role they play in your solar charging experience.
Recharging your power station from the sun is a great way to keep your battery juiced both around the house and in off-grid situations when a wall outlet is not readily available. However, your solar charging experience can fluctuate depending on a few variables such as light, temperature, and the battery’s state of charge (percentage of capacity remaining).
The key to getting the most power possible from your solar panels at any given time is to utilize the right solar charge controller. The job of a solar charge controller is to regulate the power going from the solar panel to the batteries. However, not all charge controllers are created equal. Let's take a look at the options.
Pulse Width Modulation
All Goal Zero Yeti Lithium power stations come with an integrated Pulse Width Modulation (PWM) charge controller with reverse voltage, over voltage, and over current protection. What this means is that electronic switches in the controller are programmed to only allow a predetermined average current into the battery by rapidly switching the power from the solar panel to the battery.
Think of it like a person playing with a light switch that controls a single 100W(Watt) light bulb. Let’s say that person repeatedly turns the light on for exactly 1 second and then turns it off for exactly 2 seconds. This goes on for a full hour. What was the average power usage over that hour? Because the 100W light bulb was only on every 1 second in 3, the average power over this time period would be 33W (1 divided by 3). This is the same power usage as if you were to leave a 33W light bulb on for a full hour.
In much the same way, the PWM charge controller switches the solar power to the battery in the Goal Zero Yeti, but at a much higher frequency (~300 times per second). If the maximum input current to the PWM charge controller does not exceed the current rating of the electronic switches, the switches will simply stay on (i.e. removing the person controlling the light switch and leaving the light on) until the battery is at full capacity. At this point the switches turn off, and the battery is protected from any overcharging.
While the PWM charge controller is an extremely cost effective and compact solution, it has many inefficiencies. In order to understand these inefficiencies, let’s take a look at its use with the Goal Zero Boulder 100 solar panel. All Boulder solar panels operate at ~14V - 22V. In perfect conditions (low ambient temperatures and optimal sunlight), it is possible for the Boulder 100 to put out 5.88A(Amps). Given that Amps x Volts = Watts, if the panel operates at 17V, it will achieve its rated power output of 100 Watts.
However, when the Boulder 100 is connected to a battery through a PWM controller, the panel’s voltage is forced to drop to the lower battery voltage, which is ~9V - 12.6V. So, even though our panel is rated at 100W, charging through a PWM controller in perfect conditions, it will only be able to charge the battery at anywhere from 50W to 75W.
Maximum Power Point Tracking
A more efficient alternative is the MPPT charge controller. The Goal Zero Yeti 1250 and Yeti Lithium 3000 power stations come with an integrated MPPT and the MPPT Solar Charging Optimization Module is available as an external add-on for the Goal Zero Yeti Lithium 1000 and 1400. MPPT stands for Maximum Power Point Tracking and is a highly efficient DC to DC converter which uses closed loop feedback to maximize the power output of a solar panel. The MPPT controller looks at the output of the panel, compares it to the battery voltage, and determines what is the best power that the panel can produce to charge the battery. It then converts it to the best voltage to get maximum current into the battery.
Think of it like a transmission in a car. By the changing of gear ratios, a car can vary its speed and torque capabilities. The sacrifice of one, means the increased potential for the other. For instance, if you are in a low gear, your maximum speed is decreased, but maximum torque is increased.
In an MPPT, we are playing a similar game, but with voltage and current. If you draw a maximum amount of current, your voltage at the panel will sink, and the total power will decrease significantly. However, if you sacrifice your maximum current draw, there will be a point in which the MPP (maximum power point) is reached. The MPPT controller will automatically track this maximum power point and seek to maintain the current from the panel to constantly achieve it.
Let’s place the technique in a real-world scenario. If a cloud temporarily covers the sun while you’re charging from a solar panel, the MPP will change, and the MPPT controller will decrease the amount of current drawn in order to maintain a desirable voltage at the output of the panel. Then, once the cloud goes away, the MPP will revert to its original place, and the MPPT controller will allow more current from the solar panel once again.
Similarly, an automatic transmission in a car will automatically shift to a lower gear when going uphill to allow for more torque to overcome the forces of gravity.
By utilizing this MPPT technique it is possible to experience anywhere from a 20% to 40% improvement in solar output.