I have a 2008 MB CLS550 and recently my car didn't start when I tried to crank it. I took the battery out and took it to O'Reillys to test and charge. They said that it was good and they re-charged it. I installed the battery and the next morning at 7am it turned on fine and I went off to work. Around 2pm I turned the car back on but it struggled as if there was not enough voltage in the battery. I then stopped the car for about 1 hour and when I went back it no longer turned on. I needed to get jump started. This is the original battery of the car and is 3 years old.

Can anyone tell me where I can get a substitute battery but at a cheaper price?

thanks.