Not all power management boards accept batteries of a varying voltage. Check your board’s documentation to be sure.

When you cut off the old battery connector, cut each wire one at a time. Otherwise, you’ll short the battery together briefly which would be bad.

Make sure your housing is large enough or has some level of ventilation such that your Pi won’t overheat.

Here you can see me testing the PiJuice out with my Raspberry Pi 3B+.