The warning light doesn't tell you how healthy the battery is, it tells you if there are any problems with the charging system. A flickering light means charging is intermittant for some reason, so it would be worth checking the alternator or voltage regulator, which might be on the way out. It's worth also checking that the warning light circuit and bulb are ok - if it lights up brightly and continuously when you switch the ignition on, then goes off when you start the engine, it's ok. Hope that helps.
It is possibley your alternator or battery
On a 2004 Mercury Sable : The battery is in the drivers side of the engine compartment at the front
On a 2001 Mercury Sable : The battery is located in the front of the engine compartment , on the drivers side
check the recalls!!
Because you are about to throw a rod. You need to find a mechanic quick!
It's probably misfiring. You would need to diagnose why.
The most likely cause is a dirty fuel filter that starves the engine.
On a 1997 Mercury Mountaineer : The " CHECK ENGINE " warning light is just to the left of the battery voltage gauge
Clean the battery cables and make sure the wires to the alternator are tight and without corrosion.
I believe it is mounted below the battery tray in the engine compartment
If this is a general noise, it could be the transaxle. When you are accelerating, the engine is pushing the gears, and when you let go of the pedal, the vehicle starts to shift back down. If this is an engine noise, it could be predetonation or a valve problem.
If the loose cable is the main one from the negative side of the battery, then everything will flicker simultaneously with the key on and the engine stopped. When the engine is running, the alternator will keep things energized. If you have a loose groung cable on one particular accessory, only that accessory will flicker or just remain off.