Question: how do you know when you need a new battery?
Answer: The engine cranks, but won't start. This is most likely to be the result of a flat battery – it could also be an engine ignition or fuel system problem. ... 
The engine won't crank (and the accessories and lights are off) ... 
You've had to jump start your car a lot. ... 
Your car battery is cracked, swollen or leaking.