Battery light

Hello,

If the alternator is providing the correct out put, then the light is on likely due to a wiring issue.
It could be as simple as a loose wiring in the circuit or a connector could be loose as well.

I have attached the charging system wiring schematics (look below). Check all the wirings make sure they are secured. Also be sure to check the ground for the charge indicator harness.

Here is a helpful article on how to test wirings:

https://www.2carpros.com/articles/how-to-check-wiring

Please let me know of any questions.
Thank you.

Hello, I'm Danny.

If the alternator checks out fine its possible you may have a weak, failing, or bad cell in the battery. You might want to perform a battery load test. Most local auto parts stores will test your battery for free. Here are a few tutorials for you to view for batteries:

https://www.2carpros.com/articles/how-a-car-battery-works

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

https://www.2carpros.com/articles/symptoms-of-a-bad-car-battery

https://www.2carpros.com/articles/car-battery-load-test

If your car has an unsealed battery it might be low on water. I've attached picture steps below on how to remove and replace the battery on your Mercury. Hope this helps and thanks for using 2CarPros.