Solar panels and batteries are frequently used together to power devices like telematics systems, starting batteries, refrigerated trailers and power stations, but they operate quite differently.
This blog post will explain the critical distinctions between how solar panels and batteries produce voltage and current. Understanding these differences is essential for designing effective solar power systems and avoiding potential safety issues.
A grid-connected power source can supply nearly unlimited power since it comes from the local electrical utility. This is quite different from solar. We discussed these differences in a previous blog post called Power Supplies vs. Solar Panels.
A battery is similar to a solar panel in that it has a limited amount of energy available. The battery's output is not a constant voltage but will change with load conditions and depth of discharge. However, it differs from solar in several critical ways.
Let’s talk about how they differ.
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Battery Characteristics
We often hear, “Can I run my device directly off the solar panel without a battery?”
The answer to this question is … maybe.
A battery's voltage decreases slowly as it discharges from full charge to empty. It also decreases more quickly at higher current draws. These characteristics are independent of the load connected to the battery. The chart below shows the capacity of a lithium-ion battery in relation to the battery voltage. Notice that even when the battery has fully discharged its 3.07Ahr capacity, it still has a voltage. If a battery discharges to 0V, it will damage the battery.
Solar Panel Characteristics
In the case of a solar panel, the amount of light absorbed by the panel and the resistance of the load will determine how much power the solar panel produces. A solar panel’s operation is dictated by its characteristic IV (current vs voltage) curve. Below is a group of IV curves for a given panel in low-intensity indoor lighting conditions.
The amount of current a solar panel can produce is proportional to the amount of light illuminating the panel and the panel’s total area. The plot above shows that, for a given panel, the current produced at 0V (shorted circuit current) will increase proportionally with light intensity.
The amount of voltage produced by a solar panel is a characteristic of the material it is made from. Each solar technology will have a different voltage at which maximum power is produced. The voltage at 0A (open circuit voltage) will change significantly at low light intensities and is not proportional to intensity, but for intensities greater than 10,000 lux, the voltage changes only slightly as intensity increases.
A solar panel's power depends on the short circuit current, open circuit voltage, and fill factor. The fill factor describes how sharp or square the IV curve is. It depends on the material technology and the quality of the solar panel. Depending on the technology and panel design, typical fill factors are in the 50-80% range.
Impact of Load Resistance
Unlike a battery, a solar panel does not output a specific voltage regardless of the load connected. The resistance of the load will dictate the operating point on the IV curve. For example, on the IV curve shown below, if the load is 0.7mΩ, the panel will operate on the IV curve at the point where the voltage is 1.2V and the current is 1.8mA (See blue arrow in figure below).
This is because the voltage on the load must equal the amount produced by a panel, and the current must be the same through any circuit loop. So, if the load is 0.7mΩ, the point on the IV curve where the panel will operate will be when Voltage/Current = 0.78mΩ or 1.2V and 1.8mA. The power output into a 0.7mΩ load would be 2.2mW (1.8mA * 1.2V). If we look at another point on the IV curve denoted by the red arrows in the figure above, we see that for a resistance of 2.3mΩ, the voltage would be about 3V, and the current would be about 1.28mA. In this case, the panel would produce 3.8mW of power. Finally, a third point on the curve denoted by the green arrows would be for a 4.1mΩ load and generate 2.6mW of power at 3.3V. As you can see, the power will initially increase as you move along the IV curve until a maximum point is reached. After that, the power will decrease.
Depending on your load resistance, there are often better options than powering your device directly with a solar panel.
Power Management Techniques
A power management integrated circuit (PMIC) or a power point tracking charge controller is typically added to the system to collect the optimal power from a solar panel regardless of the load resistance. These controllers control the resistance at which power is collected to optimize power collection from a panel and then convert it to a usable voltage to power the actual load. They aim to always collect power from the maximum power-producing point on the IV curve regardless of load resistance or battery voltage.
Adding a battery or capacitor can provide a more stable voltage to your load and allow it to operate when light is reduced.
Safety Considerations
Short Circuit Currents
The second question we often hear is, “What are the safety considerations if there is accidental shorting?”
In a lead-acid battery, the current produced is limited by the resistance of the wires, leading to shorts and internal resistance. This leads to shorted currents as high as 4000A (1). This amount of current can burn up conductors, melt wire insulation, create large arcs, and start fires.
In contrast, when there is a short in a solar charging system either between the solar panel and the charge controller or between the charge controller and the battery, the current produced will be the short circuit current of the panel at the present lighting conditions. If you have a panel with a short circuit current at full sun intensity (1000W/m^2) of 6A, you will not need to worry about the panel producing much more than that in a shorted state. It is important to have a fuse next to the battery, so if a short occurs between the charge controller and battery, the fuse will blow and disconnect the battery from the system instead of allowing the battery to short.
Sometimes outdoors, depending on temperature, time of year, and location, you may see sun intensities above 1000W/m^2 by 10-20%, but not much more. If the wiring in your system can handle the short circuit current plus a safety factor, you will have no potential for the overheating or melting seen with a shorted battery. Secondly, the voltage will drop to 0V when the panel is shorted, typically within milliseconds. There may be a small arc, but it cannot be sustained.
Open Circuit Voltages
One final thing to consider when looking at solar panel safety is that when disconnected from a load, a solar panel's voltage will be at its open circuit voltage, which can be up to 50% higher than its operating voltage. That means that your system must be able to handle that high voltage and that you must ensure that the open circuit voltage is in a safe range for human contact or that there are appropriate protections in place to ensure no person can come in contact with the high voltage.
In conclusion, understanding the distinctions between how solar panels and batteries produce voltage and current is essential for designing effective solar power systems and avoiding safety hazards like overheating or arcing.
If you have any questions about this post or want to talk about a custom solar application, contact us and let’s start a conversation today!
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