Dim head Lights

I have a !994 cutlass supreme oldsmobile special
edition (se) mileage 1416330. When I turn my
night lights on they go dim and I can not hardly see
the road at night. The car starts to slow down hardly driving. The Battery gauge goes very low below 13
almost 9 and 18 is the max. At day time it stays at
13 all the time, but I feel the car running slow and
spending to much gas. The engine is a 3.1ltr. 6 cyln
but it was not spending so much gasoline when I bought it 4 months ago everything else I guess is fine
i changed wire and sparkplugs. But still have the same problem I was told it was electric problem and afterwards I was told it was a mechanical problem i
got a new alternator and still having the same problem the car is charging find, is not the battery
is in good conditions not unless I should buy a new one the one I have is not that old. Please help me
if you can. Thanks.