Hi C3PO,
I just got a Watson BP-U90, and it works with my FS7, and with the charger which came with the camcorder. It seems to be communicating charge level with the FS7 OK, but I won't know how accurate that is until the charge wears down some. I assume the Watson BP-U30 would behave the same way.
I have the Hawk Woods BP75UX batteries and they are awesome. The two dtap ports are very useful, I use one to power my SmallHD 501 and I get about 3 hours powering both camera and monitor.
I use the Swit S-8U93, gives me almost 4.5 hrs. when shooting in HD 23.98p. With that, I've also purchased the Dolgin dual turbo charger TC200. Only $180 from B&H, works like a treat. The battery and charger have been equally good, reliable and cheaper alternatives to the Sony products.
Have heard good things about the Dolgin charger, but went with SWIT's dual charger because the form factor is a bit smaller. Have two of them in my ThinkTank roller, & shelved Sony's.
I use the Vivitar BP-U90 clones and get five hours on each battery, full functionality with firmware v 3.0.
But making the upgrade to the extension with V-mount adapter is really nice, as long as you don't mind the extra size.