Warning lights appear, quick fix, a few days later warning lights reappear

There was a new guy at Auto Zone who didn't really know how to read the codes, said nothing was showing up. Came back and said his manager told him it meant the battery was dead?! Um, I may not be mechanically inclined, but even I know a car won't start at all if the battery is dead! A female employee then said I had to pull all of the fuses and test each one to figure out which one was blown to replace. A friend of mine is an ME, he pulled the fuses and found none were blown, cracked, etc. He blew out the air filter, told me to replace it, and add quart of oil. The weird thing is, after that the warning lights stopped for a few days, until yesterday afternoon a mile from home. I will check all the fluids, get the oil, pull the fuses and see if that resolves. I'm just praying it's not the main battery. I had to replace the other battery, which was about $125 about two years ago? I have to look at my records, I swear the dealership replaced the water pump about the same time. The car's 12 & 1/2 years old, with just over 130k miles. I'm trying to get it to last as long as I possibly can and back to walking, riding ten speed until I can get this resolved. Lost job, that's why it's sitting in the garage until I can afford repair.