Assuming you don’t have any peripherals plugged in that are eating your battery, your issue is probably a run away application or process. It is usually pretty simple to diagnose.
Step 1: Charge the computer fully and unplug all peripherals.
Step 2: Open Activity Monitor and select the CPU tab, then click on the first column header (% CPU) so that the applications are sorted from highest use to lowest. When you look at the applications and the first column you’ll get an idea of how CPU intensive the applications are and the most CPU hungry programs will be at the top of the list.
Step 3: keep the AM open but put it in back. Every once in a while take a look at the AM and see what the CPU usage looks like. Typically you’ll see programs jumping up and down as their usage peaks and falls. Generally a run away app will have a constant or at least a very high near constant percentage.
Step 4: Once you think you’ve figured out the offending app(s) try quitting them and restarting them. Sometimes that’s all it takes. You may find some programs have constantly high power usage. This is why I don’t use Chrome.