A good friend of mine has a year 2000 VOLVO S70 AWD with a mere 35,000 miles on it. I am posting this here because the VOLVO board is not very active and the problem is probably something you folks know about.
My friends live here (Pittsburgh) but spent 6 months in Florida, hence the low miles. While they are in Fla each year the Volvo sits in their garage here. My job is to start it and drive it occasionally. In 2003, the battery died in spite of me driving it. I put in a DIEHARD for them. Again two weeks ago, I went to start the car to prepare for their return and, in spite of starting and driving all winter thru some bitter cold spells, the battery was dead and would not recharge after a jump and drive.
Is there something in that car that drains the battery when it sits for, say, 2 weeks without being driven? And is there any reason NOT to disconnect the positive terminal next winter and perhaps move the battery into the warmer house? I also hear there is a "switch" that accomplishes the same thing as disconnecting the terminal. What about a trickle charger? Any danger of leaving one of those on with no one home and maybe for 2 weeks without inspection? Any ideas/thoughts on this relatively minor but annoying problem would be appreciated.
My guess is the alternator. My mother had an 86 Thunderbird that did the same thing, it killed many batteries prematurely. That was one of many things to go wrong with that POS, but I digress...
Ironically, your friend also has a car affiliated with Ford, so I wouldn't be surprised if this was the problem.
The other option is a bad ground somewhere, especially on the solenoid switch, which connects battery to starter (that also had to be replaced several times on the T-turd as well).
There are 3 explanations to the Volvo's dead battery.
First, the answer to your question about whether there is something in the car that drains the battery when it sits for 2 weeks without being driven is yes, and this is the most likely reason the Volvo's battery is dead after 2 weeks. A typical car has a clock, radio memory, and various computer memories (EFI, transmission, supplemental restraint systems, ABS, traction control, etc.) to power. Although the current draw is pretty low, it will eventually drain the battery, especially if the battery is not fully charged. You can clear computer trouble codes by disconnecting the battery or pulling fuses because the computer requires power to retain its memory. Clocks need power to keep time, radios use power to remember radio station settings, etc. Add to this a dome light or glove box light that is stuck on and you have a quicker drain.
The second explanation is that you are not driving it enough to fully recharge the battery and so the normal drain doesn't take as long to deplete the battery. If you drive at low speeds, the alternator may not fully charge the battery. If you are going to drive the vehicle to charge it, do so at highway speeds for at least half an hour.
The third explanation is that there is something wrong with the charging system. A loose or worn alternator belt, bad alternator, voltage regulator, cables, or battery can be the cause and the components need to be tested to confirm or eliminate the charging system as the cause.
If a battery sits in a discharged state for any length of time, it become sulfated and loses its ability to hold a charge. Do this several times for a week or two and you'll have a bad battery in no time.
My guess is a combination of the normal parasitic load and a sulfated battery is the culprit.
You can remedy this by trickle-charging the battery. Modern chargers have automatic on-off features so it won't overcharge the battery and will start charging when it senses a voltage drop. You can purchase an on-board trickle charger that mounts in the engine compartment for under $40.00 U.S. that is permanently hooked up and all you have to do is pull out its power cord and plug it in when the car is going to sit idle for a long time. I've seen such chargers at Sears automotive departments. Or, you can purchase a conventional charger with an automatic trickle charger but then you have to disconnect it when you want to drive the car. With an automatic trickle charger, you won't have to start the car to keep the battery fully charged although starting and driving the car has other benefits, like keeping things lubed and loose, keeping tires from getting flat spotted, and letting the car know that it is receiving TLC.
A modern car does have devices that draw power even when the car is off. However, 2 weeks seems much too short. I have a 2nd car that routinely sits for a week or more without problem. I suspect that there might be something wrong, which can be detected by attaching an ammeter, and diagnosed by pulling fuses.
Disconnecting the battery will help, but it will reset various computers, which might be annoying. A trickle charger is better, but I'm also uncomfortable about leaving it unattended for weeks. Driving the car has other benefits, but make sure it's driven long enough to warm up and charge the battery, maybe 20 minutes.
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