Had a dead battery. Installed a new battery and the Legacy started, no problem. Did not drive the care for a week. Battery completely dead. Not even enough power to work the electric lock. Had the battery checked and it was OK. Jumped the battery, ran the car for an hour or so, and again it was OK. Checked to make sure no dome light or other items were left on. They were not. Battery was fine for 3 days, then did not drive the car for a couple days. Again, the battery was completely drained. Any ideas?

Seems you have a dead battery. Could be anything related to the charging system (alternator, control, wiring) or the battery itself. It takes a methodic process of testing to determine the cause(s).

Start by having battery tested and charged. If the battery checks ok and holds a charge then "likely" the problem is with the charging system, but it's a multistep process of elimination and very few parts store do a complete full load battery test.

If the battery test fails then you'll need to replace the battery and then promptly have the charging system tested to verify its working or you'll have another dead battery.

Once the battery and charging system has been verified functional you need to have a parasitic current test done. Excessive parasitic draw can ruin a new battery in a few months depending on your driving habits. Is this a daily driver or every other week or longer?

I'd suspect the battery right off, disconnect the battery overnight and if it's dead in the morning or even just low replace it, Do this even if the battery tested good or is new. If you satisfied that the battery is good feel the starter solenoid after it sat long enough for the engine to cool off if it's warm the hold in coil in the solenoid is shorted, not uncommon. I hope this helps. Take care.

I ran into a similar problem with a Cavalier, turns out that the on-board computer needs to have constant power. Even if the battery is very low or even dead, it still has to be connected. If the battery is removed it breaks the circuit and stops the command that lets the engine start. After the new battery was installed it would crank but not start, the dealership told me that I had to have it towed to them to have the comp reset, which would have cost around $150 total. Thankfully a mechanic friend of mine had a diagnostic machine and was able to reset it for me - cost me a case of beer. He then showed me a (not so simple) trick around this for the future. Once it is running again remember this. 1 Have the new battery and jumper cables ready, 2 Loosen the battery terminals on the dead battery but DON'T take them off. 3 Connect jumpers to the car battery cables and to the new battery, like you were jump starting from just the new battery. 4 Remove the terminal cables from the dead one in the car and remove dead battery (be careful not to let them come off the cables OR the good battery) it's OK if it disconnects briefly, but reconnect it A.S.A.P. 5 Put the new battery in and connect the terminals BEFORE removing jumper cables. Once they're on the terminals just take away the jumpers and tighten the nuts. That's how you change a battery without letting the car ever lose power. Hope that helps Jose. Good luck, safe driving. Grimm

If the battery is completely dead, you'll need to slow charge it for a few hours before trying to use a jumper battery. And you could have something shorted to ground in the car that drained the battery in the first place.
Try disconnecting the battery cables and using a battery charger to bring the battery back to life first.

might be due to the battery being bad and may need replacement but if the battery is holding a charge then it might be due to a short out in some place

Do you have any sources of power drain, such as amplifers, powered speakers, that could be draining you battery?

unlock doors with the key from drivers door

sound like a fault in dvd player.its running battery down.have dealership check it out.sound like a electrical problem.

Could be a poor connection on one of the battery terminals - be sure they are tight.
Could also be a starter solenoid that is sticking.

It sounds like the problem is the voltage regulator. I would check that out first. Hope this helped.