I have a couple of the typical gray "Walmart Batteries". One puts out 9.2V and the other 9.3V, charged a couple of days ago. That's not under load...
Rayovac RV-DVD-03 5400mah LIon
generic EDV004SU 4500mah niMH
I ran my UA-5 off a tired old 12V SLA... after a long festival day the optical output died... I checked the battery with a meter right then and it was down around 6volts (I don't remember exactly). But the point is that 8.4 should run the UA-5 OK.