Trickle Charging with a Solar Panel

CHAPTER 5: Batteries and Power 103

a 100Ω load resistor inside the light box that I use for my photography. That equates to just 1.8 mA.

I find a spreadsheet a useful way of recording how the solar panel performs. Figure 5-15 shows an excerpt from the spreadsheet, complete with graph. You can then file this away until the next time you wish to use a solar panel in a project.

The spreadsheet can be downloaded from www .hackingelectronics.com, but there is really nothing complex about the math.

As you can see, the solar panel produces only 1 or 2mA indoors even under bright artificial lighting. The results outdoors with a clear view of the sky are better, but it really only produces quite high power in direct sunlight.

Trickle Charging with a Solar Panel

Since the solar panels produce a reasonable voltage, even in relatively low light conditions, they can easily be used to trickle charge a battery. However, you should always use a diode to protect the solar panel from the situation where the battery is at a higher voltage than the panel (say at night), since such a reverse flow will damage the solar panel.

A typical simple trickle charge schematic is shown in Figure 5-16.

Figure 5-15 Solar panel data