For free open source software, hardware recommendations,
instructions, plans and user manuals see HiveTool.org
1. Operate through a week of rain (battery life of 10 days).
2. One day of sun provides 2 days of operation (operate unit 1 day and charge batteries 1 days worth) so need an average of 50% sunny days.
3. Low profile (solar array fits on the top of a 10 frame Langstrough hive.)
Average power consumption:
Pi A: 200 ma @ 5V = 1 W
Scale: 50 ma @ 12V = .6 W
(The actual measured Pi power consumption is 120 Ma.)
Assume PS efficiency is 85% 2 watts/.85 = 2.4 watts
2.4 watts / 6 VDC = .4 amps
.4 amp x 24 hours = 9.6 AHr
derate 200 AHr battery to 100 AHr
100 AHr / 9.6 AHr/day = 10.4 days
Goal: 4.8 watts (2x consumption)
5 watts x 24 hours = 120 watt hours
120 watt hours / 20 watt = 6 hours at maximum output (12 hours at 50% output
Derate panel to 15 watts = 7.6 hours
(note: 20 watts is maximum output and that probably won't ever happen - especially if the panel is flat on the top of the hive. I would feel more comfortable using a 40 watt solar panel 2.3 amps max output.)
1. Battery Amp Hour ratings are theoretical. A 200 AHr battery in theory would supply 200 amps for 1 hour or 1 A for 200 hours. In practice this is far from true - there is a better chance of getting closer to 1 A for 200 hours than 200 amps for 1 hour. The point is, the AHr rating of the battery must be derated (depends on the rate of discharge and temperature).
2. 6 V golf cart batteries are better deep cycle batteries (thicker plates) than 12 V marine deep cycle which are a compromise. Battery life depends mostly on Depth of Discharge (DOD) - the bigger DOD, the shorter the battery life.
3. A solar "Charge Controller" probably won't work - DO NOT USE THIS:
40 Watt solar array:
$54 on ebay
$95 on ebay, probably find locally (Sam's Club)
(This is about $170 not counting shipping/taxes.)
May need gain antenna: