Hi Lea,
Just had a look at my 2010-ish version, it has thin rubber strips on front and back faces of the battery compartment. The battery caddy is an interference fit between them, but it does go with a push. It’s quite fiddly to get in, you have to feed the end with the plastic insulation flap in first and make sure the connectors don’t get trapped.
There’s also a couple of very narrow foam strips stuck to adjacent sides of the battery compartment lid.
Some years ago I did some experiments to determine the effect of varying supply voltages on '817 power output, sadly I can’t put my hand to the results at the moment but ISTR that on HF the power output decreased fairly linearly from 5W at 12v down to 4W at about 9v and then diminished rapidly, the radio switching itself off at about 8.5v. On VHF/UHF it behaved differently, with 13.8v being required for 5W out and decreasing to about 2W by the time the supply voltage was 10. Tests done on CW mode.
HTH
73 Paul G4MD