Starter or battery problem

2004 camry 4 cyl. turns over slow but starts. I checked battery voltage (car not running) it is 12.6 volts. when cranking in dropped to 6.??. The post terminals and cables looked ok but I did not disconnect or remove them to check. How can I verify if it is a bad battery or a bad starter.

6V is too low. Did you measure this at the posts, or the clamps? Either way, I’d remove clean, and reattach the cables (a battery brush is cheap). If you can, I’d also charge up the battery and have it tested. If you do all that and there’s still a problem, then the starter would be suspect. But only if the battery and alternator test out good (car parts places will test them for free in the car).

How old is the battery? If 5 years or more, it’s probably near the end of its life and I’d start by replacing it. If it’s cranking slowly now in warm weather, by winter it’ll probably be dead.

Battery or connections. Usually starters will just not work or make clicking noises.