Is my battery dying?

I usually drive about 16 miles per day, but when I get home today, the gauge shows "charge battery". So I took the car for a long drive for about another 30 miles half highway, half local. However, when I got home after it, it still shows "charge battery, car will turn off all electric usage" and the car shut down completely after I turned off the engine.

So for the first time, I put the battery maintainer to use (it's the diehard battery maintainer platinum Chris recommended last time). The maintainer shows the battery still has 48%. So far after I plugged in for one hour, it charged to 55%.

But how come after 30 miles driving the battery is not even charged to normal level? Is it time for me to replace it? I hope not since I heard it's pretty expensive.