Battery warning light

Here are a couple of sheets that we at RepairPal put together to help people with the Diagnosis of a Charging System. It is meant for a shop and may be somewhat over kill for you, but follow it and you will get to the root of your problem. https://repairpal.com/estimator/charging-system-diagnosis-cost https://repairpal.com/low-charging-system-output-diagnosis

It is an absolute NO-NO to remove battery cables on a OBD 2 generation car. You probably have a bad alt. Take it to a auto parts house that will check the complete charging system free. As to unhooking the battery while it was running, Pray that you haven't screwed up the other electronics in the car.