My back up battery. Velcros to the back of the middle strap. Kinda at the crown of my head toward the back.

[Battery] https://www.amazon.com/dp/B01CU1EC6Y/ref=cm_sw_r_other_apa_i_xOO7DbXYN8NJD)

Not sure what qualifies as a "smart" battery. Maybe it's about the fact some batteries shut down when the current draw is too low? Anyway, I've used the earlier version of this one https://www.amazon.com/Anker-PowerCore-Ultra-Compact-High-Speed-Technology/dp/B01CU1EC6Y/ with no issues.

As everyone has said, get an Anker. I use the PowerCore 5000, and it has yet to let me down. It’s small and lasts me through a full CD of playing with plenty of battery to spare