Have an 01 Screw with 80k, when should I replace the battery? Or should I wait til failure? Took delivery on Nov 2000.

 

Get a specific gravity tester - sears has em for $3.00 Looks like a turkey baster. It'll tell you the condition of the battery - if it isn't sealed.

 

There are ways to judge this.
Is the hot starting a bit slower turning over?
Lower capacity to run lights without engine running.
Voltmeter monitor. The one on the dash is no good for telling conditions.
Capacity test of the battery.
An old battery can be fine all summer but as soon as it gets cold, the truth will show.
If you need reliability, the store is waiting to sell you one, now.

 

Most car places will test your battery for free (Autozone, Pepboys, Advance auto). You might need to pull the battery out and bring it inside, it's a little hassle, but you can get peace of mind for 1/2 hour of inconvenience.

If you do replace your battery, I recommend Interstate batteries. My wife's car had a two year old Interstate in it, it sat outside for a couple of weeks outside at -40. It started on one hit of the key. It grunted and groaned, but she still fired up strong.

 

Average life of Ford OEM battery is approx 7 yrs, however if you want to get it tested for peace of mind then I agree with tsc to use Autozone, Pepboys, etc.

When you start experiencing long cranks in the winter, then it time to change.