I’m using a 2007 MacBook. Changed the HD to 500gb. I’m on the third battery and it only lasts about 1.5 hours.
I use Firefox and some other workarounds, but it’s still chugging along.
About $900 over 10 years...not too expensive.
I have a 2006 MacBook. On the second battery, and it lasts all day. What you should regularly do, is take apart the case, and use an aerosol spray can to clean out the dust bunnies. I bet your fan is clogged and allows your system to overheat, running down your battery. It's amazing how dust can build up inside over a period of months, let alone years. I just upgraded to a 2016 MacBook Pro. Not because the old one is no good, it is still going strong. I wanted to newer OS and faster speed of the new ones.