I have an ATK SIM (~same board as yours, but EU version) powered from 2x D-type alkaline long life batteries in series. I hooked them to the battery terminals. Solar, UV and wind sensors are attached. I think you're not exactly after this setup, but definitely a working "external" powering solution. It has been working for more than a year without a hitch. I think it can last 2-3 years but I replaced the first set of batteries after about year after some reception problems, but it turned out it wasn't caused by the batteries. They barely lost charge. So, it's a way to keep in mind if one wants a non-tethered, external, long term powering solution.