From the Phidget documentation, we know that the Single Board Computer (SBC) will run at 1.2 watts with no power consumed by devices in its USB ports, and 2.5 watts maximum if all USB port devices are drawing power to maximum specification. Because the power to all sensors and USB devices is included in this estimate, this is what we use to pick a battery and a solar panel.
Although we chose our Phidgets first, and are now designing a power system to support them, the Phidget selection also included some power concerns. With a little forethought, we can guess that the wireless internet adaptor is probably the most power hungry thing that we can plug into a Phidget SBC. A wireless adaptor has two important benefits:
- You can download data over the network
- You can change code, settings, and scheduling of data gathering as the station is operating
You don't necessarily need an internet connection to use the wireless, as you can connect to it via its phidgetsbc.local local link address. On the other hand, without a wireless adaptor, the SBC is essentially running autonomously. You can save a lot of power this way, but if the SBC gets into an undesirable state (extreme weather causes it to reboot, a USB Phidget wiggles loose and doesn't properly attach in software, etc) your only options are to either reboot, or add a network connection to log in and change things. As we do not use wireless here, but do use a webcam, we use an estimate of 2.0 watts to run the SBC.
We would like to have a power setup that will operate continuously, rather than having to replace the battery. This involves solar power, and it also involves knowing something about the expected weather (namely, the sunshine) in the installation location. Your solar watt capacity should be big enough that in periods of sun it can recharge the battery much faster than the SBC will drain it.
To take an example as to why this matters, imagine installing a 2.0 watt solar panel into your system. If there were sunshine 100% of the time, this would be a closed, self-refreshing system because the SBC would draw 2.0 watts from the battery, and the solar panel would put 2.0 watts back in. But with only a short period of dark, the battery will be drained slightly and never refreshed. So we need to consider all of the factors that could cause darkness (or relative darkness) and determine from them how big a solar panel we need.
We start with choosing amorphous solar panels because of their low cost, and (more importantly) their ability to charge a battery in low or indirect light conditions.
Then, we account for nightfall. This at least doubles our solar needs, especially in winter when nights are long. We assume 3/5 dark time, as twilight conditions are poor for power generation, and the station will be installed in a valley with high ridges blocking the sun for morning and evening. So even assuming every day is sunny, we will only receive 2/5 charge time, and will need 5/2 (5 watts) of power via the solar cell simply due to location and season.
Then, we account for weather. An average long storm for the interior Rocky mountains is about two weeks. Although we chose amorphous solar panels for their low-light performance, the conservative assumption is that the battery will get little recharge during such a storm. Therefore, we want to recharge quickly between storms. In late winter, the mountain sky is cloudy about 2/3 of the time, leaving us one week to top off the battery after two weeks of drain. From the more general power needs section, we can learn that our 2.0 watt SBC will draw 0.17 amps if we use a 12 V battery. So our drain - at worst - will be:
- Two weeks = 24 hours per day x 14 days = 336 hours
- 336 hours at 0.17 amps = 57 amp-hours
To recharge 57 amps at 12 V, with a 10 watt panel this would take (the concepts are from the more general power needs section):
- 57 amps * N hours = 10 watts / 12 V
- N = 68
..68 hours. At 2/5 charge time from the nightfall calculation (giving ten hours a day of charge), a 10 watt panel would recharge in the expected week (6.8 days). This gives us an idea of what class of solar panel we are looking for, and from here we can examine 10 watt and larger panels with respect to cost and size.
After examining cost, a 10-watt panel was nearly the same size and cost as an 18-watt panel ($80), and so an 18-watt panel was used here. The 18-watt panel also would help add a buffer when - even on non-storm days - high mountain clouds form and further reduce the available sun.
Whatever you choose for your solar panel, you should include a charge limiter, and a way to reliably attach the wires to a battery (clips are for testing only, the post clamps are a more permanent install). The cable on the right is outdoor grade power hookup wire: