@jmosk, the Sparkfun shield does have an I2C battery gauge. The difference is that the Particle shield is shipped WITH a 400ma LiPo which is not included with the Sparkfun unit. Also, the Particle unit has a screw terminal for external power (wall wart, solar cell) whereas the Sparkfun unit requires an extra barrel jack (not included). Finally, the Particle board is smaller and sits tightly under a Core/Photon. I believe you get value whichever board you chose though Particle provides a ready-to-go solution. 