Alternator sounds shagged. Under load it's not providing enough current to charge your battery when all systems are being used. Have a double check of your belts and make sure they're tight enough (not too tight though!).
It should be reading 12-12.5 with ignition on, over 14 when the car is started, and it shouldn't drop when you boot it.
Quote Clarkson, Top Gear, 17th July: "I'm not having a stroke... just driving a Nissan!"
Quote Ry, Mallory Park, 3rd Dec: "Ahhhh crap... Just a bit too over eager really!"
"I used to be an adventurer like you, but then I took an arrow in the knee"