What causes the battery to go dead every 2 days?

Answer 1

Dead cell in the battery or something is on pulling power from the battery. Examples are, trunk, under hood, glove box, brake, or dome light.

Most likely the alternator is not charging the battery and would cause battery failure within 2 days of driving. A vehicle that sits with a constant drain (not including lights) will take over a week to drain the battery. I am sure the owner of the vehicle would know if the lights were on or not.

I beg to differ. If the under-hood, trunk, or glovebox light is on the owner would never know this. If the alternator were not charging the battery the red charging system light would be on, which I am sure the owner could easily see. Not to say the alternator cannot be defective if the light is burnt out. A battery with a dead cell will run down overnight every time. Also a trunk or or other under-hood light can easily run a battery down if it is not a very strong fairly new battery. But a defective alternator is not out of the question as it can be strong enough to not turn on the charging system warning light, but weak enough to not keep the battery fully charged. Easy to check the output of the alternator. If you read 13.5 to 16 volts at the battery posts with the engine running the alternator is more than likely good. Less than that or more than that and the alternator is defective.