When a car is new, there is no problem to start the engine if the car was not run for a week. After the car runs about 40,000 miles, the engine could not be started if the car was parked for a week until it was jumped up. I was unable to recharge the batter before jump up. The dealer tested the battery and said it was good. I told the dealer the battery was good because it had already been jumped up at home and it was recharged by driving to your location. So, you saw the battery was good. Then the dealer replaced the battery with a new one under the warranty.

After a half year, I got the same problem. I am unable to start the engine if the car was not run for a week unless it was jumped up. Of course, the engine can be started if I keep driving the car such as every 3 days instead of one week. So, I have questions.

Why can a new car engine be run even if the vehicle was not run for a week? Why the battery was easy to be dead completely when the car becomes old even the battery is new? The battery should have no problems because it has been replaced for only a half year. The problem is if the car was not run for a week, then I am unable to start the engine unless jump it up. Please note there was no such a problem when the vehicle was new. I usually dive my car once a week. Is it normal for such a problem when the car becomes old or there is something wrong?

The age of a vehicle usually doesn't affect the battery, it is the battery age. However, it would appear you have a slow current drain in one of the power systems. You will need to find what circuit, a shorted wire for example, is causing the drain. You may have a light on under the hood or in the trunk. You will need to do a little detective work with a 12 Volt tester. With everything off, including the key, see if any of the fuses or breakers are allowing a current flow. Sometimes, the starter lead wears thin and allows a small current to flow to the frame. Look for anything with a frayed or burned wire.

If the problem is due to a slow current drain in one of the power systems, how to explain why there is no problem to start the engine if a battery is new and even if the vehicle is not run for a week. Does the slow current drain not happen to the new but only old battery?

It happens to both batteries but the new battery holds the charge better. That's why you can drive the car every week and it keeps the battery charged.

Besides a low drain, you may have an Alternator just not wanting to put out full charging voltage?