I'm strongly leaning toward your battery having reached the end of its life. Batteries, of course, rely on chemical reactions to generate electricity, and over time the chemicals used degrade and can no longer hold a charge. Think of your battery as a jar. You fill the jar with water up to 100%, just as you charge the battery to 100%. However, each time you fill the jar, you add a marble. One marble doesn't make much difference, but over time, the marbles eventually fill up the jar. Now you fill the jar with water, but the amount of water you can put in (the amount of charge the battery can hold) is only a fraction of what it was when it was new. So even though it's fully charged, when you start using it, it drains immediately.
If would be worth your time to do some troubleshooting first, but I think you'll have to replace the battery. The fact that you keep having to reenter the CMOS settings tells me the battery is so low that it can't even maintain those - and they take a microscopic amount of power (your laptop doesn't have a separate CMOS battery; it relies on the main battery always having at least a minimal amount of power).
That all being said, go through the iFixit troubleshooting page for your laptop and see what you find. In particular you'll want to run the battery diagnostics using Dell's SupportAssistant software as described.
Dell Laptop Battery Draining Fast - iFixit Troubleshooting
Good luck and be sure to come back and let us know what you find; your experience will help the next person who comes along with the same problem. And if you end up needing to replace the battery, we can probably help find you a guide to that repair or help figure out your next steps, depending what your results are.
