Ok, I will try to make a short story short. This one has my husband and me pretty puzzled. Here are the details… I have a 1999 Mercury Moutaineer and changed the battery almost four years ago. Within the last week, maybe week and a half, my vehicle has not been wanting to start. It will do a slow crank and after a half hour of just letting it sit it starts fine. But the thing I noticed when it left me stranded an hour away from home by myself, is that the radio and lights would come on but yet it refused to start. And the thing that has my husband confused is that the battery runs at the right range (not low) when the vehicle is actually started but yet it runs really low when just left in the run position. Any help on this would be greatly appreciated. Thanks to all 
I forgot to mention we had the battery tested and they say its a good battery and my husband checked all the grounds that he knew to check. Thanks again.
How are you determining that the battery runs at the right range? By a gauge on the dash? That gauge is reading the performance of the alternator once the engine is running. If the engine is running, and the gauge is low, then the alternator, not the battery is bad. If the gauge is reading right when the engine is running, then the battery is bad, not the alternator.
Go to an auto parts store, like Autozone or Pep Boys. They can test the battery for free.
BTW, any battery 4 years old or more is suspect of going bad at any time.
The gauge only reads low when I just have it in the run position and the vehicle isn’t started, it reads fine while the engine is actually running.
Aha! During the starting process. Now it makes sense.
The starter is the most power drawing component on a vehicle. This is why it is imperative the battery and battery cables/connections must always be in good condition. (and of course, the complete charging system)
Offhand, it sounds like a failing battery to me. Considering the battery is 4 years old it’s quite possible that the battery is on the way out.
There are several methods of battery testing; a really good way and several others. Much depends on the person doing the testing.
Since no details are provided about the battery testing, I will only say that the battery should be put on the battery charger for a MINIMUM of 1/2 hour (preferably one hour) and a carbon pile should be used to apply a load that is equivalent to roughly 3 X the normal starter amperage draw on a warm engine. This load (normally around 300-375 amps) should be applied for 15 seconds and at the end of that time the battery voltage should not be lower than 10.2 volts.
Under 10 makes it a very questionable battery and around 9 makes it a junk one.
If this range you’re referring to is the dashboard voltmeter then that could verify if the battery is going bad. If the voltmeter is showing less than 10 volts when you originally turn the key to crank the engine over then that could point to a weak battery.
Another way to check battery capacity, is to start with a fully charged battery (trickle charged, etc.), Check that its “resting” voltage is at least 12 1/5 (preferably, 12 3/4) volts. While the truck is being cranked, keep the voltmeter connected to the battery. If the battery voltage falls to 10 volts, or lower, during cranking, change the battery. Though this won’t be the load (it’s much lighter) that Mr.ok4460 advises, it’s do-able in the car/truck. This method won’t rule out the possibility of a short somewhere in the battery cables, or starter; nor, maximum load the battery. So, if there is still a question about the battery, take it out and have it tested as Mr.ok4460 advises.
Another little note. Alternators are NOT designed to charge dead or very low batteries. Get that battery changed out so that your alternator doesn’t go PLAPHOOIE! Batteries are a lot easier and cheaper to replace than alternators. By your description of your vehicle, it sounds like your present battery is at least the second battery for this vehicle. Did you keep the receipt? You are most likely eligible for a prorated credit depending on your battery’s warranty period. It might only be a couple of bucks, but a buck is a buck, eh?
And the thing that has my husband confused is that the battery runs at the right range (not low) when the vehicle is actually started but yet it runs really low when just left in the run position. Any help on this would be greatly appreciated. Thanks to all
The reason your dash gage is doing this is because the gage is reading the voltage of the alternator, aprox 14.7 volts, when the vehicle is started. When the vehicle is turned off and the key is in the run postion it is reading the voltage of the battery, could be up to or around 13 volts.
I agree with ok4450 that it is your battery. But to be on the safe side I would have the battery tested again and also the alternator as will as the
starter. If all these checkout all right then I would suspect a bad connection in the starting system