The information I am about to give you is 100% accurate and will save you hundreds of dollars.
I had carpenter ants once as well and did a lot of research. Got rid of them in 2 weeks. Seriously.
This is what you need to kno:.
First, don't bother killing the ants you see as they are a tiny percentage of what you are actually trying to kill. Furthermore killing too many ants will result in the queen laying more eggs to ensure the future of the nest. If she looses enough workers she will command that the nest be split which results in twice as many ants.
DO NOT SPRAY ANYTHING. DO NOT BUY ANYTHING FROM LOWES OR HOME DEPOT. Unless you are able to directly spray the nest itself, these sprays are worthless and force the ants to find new routes, making it harder for you to track them. If you get a good kill with the sprays, remember the queen will just make new ants.
Instead of killing the ants you see, OBSERVE them. Seriously. They will lead to their trail and to the nest itself. Try it, you will be surprised.
The random ants you see look line loners, like they are operating as individuals but tehy are part of a trail. Watch them, and you will find the trail.
This is important because you need to bait the trail, rather than spread ant killer all over the place. Basements are easy; they are probably moving in a line up and down the wall. So tonight, right around 6pm or so, start looking for ants and find out where they are going.
Tomorrow, I will tell you exactly what to buy to kill the entire nest.
Why am I waiting until tomorrow? Because I have the stuff at home and can't remember the name.
It's only available online and your local pest control company doesn't want you to know about it.
Any advice you get other than what I just told you is incorrect.