What does it mean when your instruments peg out when you start your car?

are you sure it wasn't doing it before you had the battery changed?some fords do a gauge sweep test when key is turned on.not sure when they started it,but somewhere around 2002 or 2003.my 2000 don't do it unless the battery is bad/weak.

Brand new battery. Light still on.

if you mean the battery light is STILL on,then it's not the battery.it's the alternator that is bad!it has drained your new battery down to nothing.when battery light comes on,it usually means the alternator ain't working!who said you needed a new battery?go have charging system tested.battery MUST have a full charge on it before alternator can be tested while still mounted on the vehicle!!