I have a 2005 Mustang convertible that I purchased new. If I let the vehicle sit for a week the battery will be dead. If I drive the car, on a regular basis, there is no problem. In addition, a high quality battery will only last one to two years before I have to replace it.
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Your batteries are only lasting 2 years because you let the car sit for long periods and let the battery run down.
Now as for why the battery runs down, cars draw energy from the battery even if turned off. The battery loses charge even if disconnected! That is normal. If you let the car sit for a month or better between drives, the battery is pretty run down and will require a long drive to recharge it. A short trip means the battery didn’t get fully charged. That hurts the battery and it is more likely the car won’t start the next trip. If this is normal for you, buy a Battery Tender or similar product designed to keep the battery charged up between drives. Your battery life will double or triple and the car will start when you want it. Just unplug the charger and go.
If it won’t start after 5 days or so… does the car have a aftermarket alarm, stereo or remote starter? If it does assume it has gone bad and is draining your battery. They are common culprits. Your mechanic needs to run a parasitic drain test to find out if the add-ons or something Ford put in the car has gone bad and is draining the battery.
Just adding to what Mustangman says: Check out the battery considerations paragraph in this story. That section was written by a CarTalk contributor and BestRide editor (Craig Fitzgerald). You may find it helpful.
A good article . . .
But I don’t know where you came up with that 85 milliamps of parasitic draw
Sure, there might be a higher draw right after you lock a vehicle and arm it with the key fob, but many of the modules will go to sleep after awhile