I think possibly that my Alternator on 2001 Navigator is going out. Just about every year since I have owned it when it starts to turn cold outside and I let my car sit for three days or more the battery is dead, not completely dead but enough to not be able to start it, so I usually put it on a trickle charger once a week.

A year or so after I bought it I had to get a new battery, the battery is a Motorcraft Tested tough 850 and it seemed like just after I installed it that I left the dome light on all night and it drained the battery so I charged it the next day and it was fine but since then I have this problem where if I let the car sit for more than three day with out driving it then I have to jump it or charge it, the battery is about 4 years old.

Yesterday it was dead after sitting three days and it got down in the high 30's, so I charged it engough to be able to start it and drove it across town for about 15 minutes (in town driving), I went in a store for about 45 minute to an hour and I when I came out it just would click, so I got a jump and drove home on the freeway at 70 mph for about 10-15 minutes.

I had to go to another store so I borrowed my parents car and the Navigator sat for about another hour, when I came back it just clicked, so I put the trickle charger on and have not checked it yet. I am going to go today and get the Alternator and the battery tested.

If I drive the Navigator consistently every day or every other day then it is fine, I am thinking maybe I am lucky and it will just be the battery and not the alternator.

I am thinking it could possibly be my driving style and my stereo, I have a 400 watt amp running my speakers and a 300 watt amp running a subwoofer, when I drive the Navigator it is usually short in town trips and I am thinking that maybe with my stereo system and everything else in the car running lights ect and the short in town trips that maybe it is just not long enough high rpm driving for the alternator to fully charge the battery?

This could have possibly been a strain on the battery all these years and combined with the cold weather finally killed the battery?

If it is the Alternator should I use a Motorcraft Alternator? I looked them up and my Alternator is originally a 130 amp but now they only sell 115 amp reman units, I am worried that this will not be powerful enough with my sound system and I would prefer to by a new alternator over a reman unit, I do not think I would need a High output Alternator with only a 700 watt system.

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If it is the Alternator should I replace the belt tensioner and the serpentine belt while I have it all apart for preventative maintenance or would that be a waste of money? My Navigator has 62,500 miles and the alternator and the belt are the originals.

If it is the battery should I stick with the Motorcraft? I know tone of people who have had good luck with Motorcract batteries, my brother had one in his Grand Marquis for 10 years.

According the the Motorcraft Prorate chart I should only have to pay around $48 as my battery is around 40 months old. Am I right about the $48 for the battery? I am not certain how they do the prorated discount but to me it looks like I should only have to pay $48 as my battery is around 40 months old. Look at the chart and scroll down to where it says tested tough max $119.95 retail value batteries.

http://spautoparts.com/documents/Mot...te%20Chart.pdf

Sorry about the long post, I just wanted to make sure you guy know everything as I am kinda stumped right now on what to do until I get the alternator and battery checked.

 

An easy test to check your alternator is to disconnect the battery while running. An alternator should generate enough amps to keep the engine running.
If it immediately quits, the alternator is not charging.

 

Many people think that disconnecting your battery when running is a bad idea. The electronics in modern cars are not as robust as in days of old. The battery is a clean steady DC source which the alternator is not.
The best way to test the alternator is with a volt meter. 12 Volts when not running, around 14 Volts when running.

 

If you want a bigger alternator, there are a lot of aftermarket high power options which could be a better choice for you since you have an increased power demand with your sound system

 

Truly don't disagree, just sometimes keeping it very simple.
I myself, always use a VOM at the alternator rear terminal to get a good measurement of volts.

 

No matter what you replace you need to start with finding your power draw with the engine off. Its obvious to me if you have a verified good battery you are losing the charge due to residual power draw. I would look towards any and all aftermarket electronics. Is the deck constant hot? The amps constant hot? A alarm system? All of these will do it.