I don't have one of these, but do have the predecessor; the B1ON. You can power this pedal with one of those inexpensive smartphone batteries via the USB connection. Am sure this is documented in the owners manual somewhere but I found out quite by accident. I had mine connected to my PC because I was messing with some of the patches in:
ToneLib – Complete guitar studio in your computer
I unplugged the power supply to my battery-less B1On with the USB cable still attached. The pedal remained on. My fist thought was that maybe I did put batteries in it and forgot I did it.
Nope. Battery compartment was as empty as I truly remembered it.
All that to say, while there may be no way to check your battery level, the next best thing might be an external phone battery and plug as a backup you can plug into the pedal's USB port. I've since experimented and found that 100% this does work. Also it works with my little iPhone charging cube with a USB x mini USB cable plugged in. This gives you some good options. Now-a-days you see those little USB sockets on power strips, so you got that going for you as well. Am assuming that the B1 four shares this functionality with the B1ON pedal I have. Can't imagine this wouldn't have carried over...