Reluctant Start HD

I know the mornings have been cold lately but I am really growing tired of my car struggling to start every morning. Every day it's the same thing, a battle to get the engine started before the battery dies. Either the car starts and runs badly, or I end up having to bump start it in reverse - which I hate! Today the battery grows weaker & weaker, I start to doubt it'll start this morning atall... To my surprise, it does! I rev the car to warm it up and it sounds like it is running suspiciously good for a change....