We tried to run the Thread Sensor Tag with a AEM10941 Solar Harvesting IC (Module can be found here: https://www.tindie.com/products/jaspersikken/solar-harvesting-into-li-ion-battery/ ) from E-peas (see image below) and a small 45x45 4.5V Solar Panel. This was supposed to harvest energy inside a building but apparently it did not work out as expected. Somehow the battery voltage droped lower and lower even with brighter light. Not sure whats the cause of it and we have to investigate more but the idea was that this device will never need a battery change again!