The Sunny Buddy's efficiency has repeatedly measured to be about 80% in tests. The Sunny Buddy is a switching supply the output power is given by the equation Pout = Pin * Efficiency.
This is another place where the Sunny Buddy is better than comparable linear solutions. EfficiencyĮfficiency in any power supply system can be said to be the ratio of power out to power in. That's an extra 33% more charge current available to your battery over a comparable 5V charger. The circled region of the graph shows this: the highest current the small solar cell can deliver is around 180mA, but the Sunny Buddy pushes out 240mA before entering current limit. The Sunny Buddy locks in on that point in the curve, pulling the maximum current the cell will provide, but no more, and turning it into charge current. The solar cell curves will be compressed along the X-axis in lower light conditions, and, while the unloaded voltage may remain quite high even in low light, the amount of current which can be drawn from the cell decreases rapidly with the amount of light available. This point, called the maximum power point, is crucial to squeezing the most efficiency out of a solar cell.įinding that point is the key here. There's a point on that curve, in the "knee" region, where the power transferred to the load is at its peak. They slowly droop until they reach a certain point, then decline increasingly rapidly until even a small increase in current draw causes the output voltage to plummet. The solar cells, however, behave quite differently. Since I was actively increasing the load to stress the supplies, it folded back to a lower voltage to gracefully handle the excessive load without bursting into flames. In a charging application, that's the point at which it would have settled in and charged the battery.
You can clearly see the point at about 240mA where the Sunny Buddy could no longer safely draw more current from the solar cell. For a sort of baseline comparison, note that the output of the bench supply, the battery, and the Sunny Buddy are pretty flat. The chart compares output voltage versus load current for the five sources listed above: in short, how much current each is capable of providing.
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