8.4 volts (when freshly charged), and around 7.5 volts over most of the run time, just won’t cut it for a lot of devices that require 9 volt batteries. I have smoke detectors that start sounding the low battery warning at around 7.6v. I use NiMH 9v batteries in them, but they use 7 internal cells for a total of 9.6v.
This battery would start sounding the warning way too soon. Though, the greater capacity might still give it a decent run-time.
BTW, how do you charge it? It must come with a special charger?