Our initial tests were with the Lithium Heavy Duty Solar USB Charger 2.0 from Brown Dog Gadgets.

Here are our initial results from field tester Matthew McDonald:

Afternoon Folks,

Wanted to let you know what I found with the solar charger.

Regardless of available solar power the charging circuit will only output about 530mA to the battery. This was confirmed with an external DC supply and a fluke current meter connected at the battery terminals.

I made the following assumptions to estimate power consumption:
- spark core consumption continuous @ 300mA.
- Battery capacity is 6600mA-hr
- hive activity will be monitored 365 / 24 hrs
- shortest day in a year has about 9.5 hours of sunlight
- solar panel see's sunlight throughout the day
- max charge rate is 530mA

In a 24 hour period on the shortest day there is a potential for a 5035mA-hr charge. A 300mA consumption for 24 hours would require a 7200mA-hr charge. In order to break even the spark core will need to consume at most an average of 209mA. I'd recommend a little less to give the system more headroom, so maybe shoot for 180mA for a daily average on the spark core consumption and go from there.

One thing to note is that the solar panel suffers severely with any shade or overcast. I measured a max of only 55mA from the panel this afternoon with an overcast sky. I was only recently able to acquire the data logging software and will have that running tomorrow evening which will give a more accurate estimation. My guess is that 180mA estimate will need to come down lower.

Also, NREL has amassed significant solar data which may be worth looking at . [ http://www.nrel.gov/solar_radiation/data.html ]

Let me know if you guys have any questions, comments or suggestions.
~Matt