My Volvo S40 -00 with a 12v battery has been standing still in a cold garage since Thursday, it has been around -6 at night and -4 and -7 degrees celsius each day, how often do i need to drive to keep my battery at an okay charge?
A battery that is in good condition should last weeks or more out in the cold at the temps you mention. However a charge will never do any harm. To save venturing out in the cold for possibly unnecessary journeys a trickle or smart charger may be a consideration.
Batteries in poor health are often killed off as the temperature outside drops as they just can't stand up to the extra load necessary to crank a cold engine. So as they age it's necessary to keep a bit more of an eye on them. If there is a concern over the batteries condition have it load tested to make sure its still ok.
If you have to keep charging it, its likely time for a replacement battery as that cold morning will eventually come when you turn the key.. the engine will try to roll over slowly, and then you'll hear the starter clicking as the battery takes its final breath.
If however the battery and alternator are still in good order and you just want to keep it charged up, 20 minutes at just above idle or a short 20 min run each week or two should be enough to keep things in order. In very cold weather though lasting weeks or more at a time a trickle/smart type charger can be a good investment.
You are asking the wrong question.
The question should be: for how long should you drive the car, once you have started it?
A battery in proper condition should last for at least a month in a car that doesn't have a smart key system (your -00 model almost surely doesn't). A smart key system means the car detects the key in your pocket, and thus, you don't have to take the key out of your pocket to press a button or worse, turn the lock cylinder.
Of course, if your 12V battery is also manufactured in year 2000, it may be starting to get too old, and thus, it may not be able to supply the idle current of the car for a month. In this case, the proper fix is not to drive the car often; the proper fix is to replace the battery, because eventually the day will come that the battery may not crank the car even after a day of standing.
And the answer to the question you should have asked is: drive the car to some direction until the coolant is at operating temperature. Then turn back. So essentially you drive for 2X the coolant warmup time. This should ensure not only the coolant but also the oil is hot, removing any fuel residues from the oil. Such a long drive is plenty to charge a battery in proper operating condition. If the battery is in the engine bay, it should also mean the engine bay will be warm, and thus, the battery in the end should accept charge really well.
If you drive for half a kilometer every day at cold temperatures, the short drive may not charge the battery enough especially if it's cold, so the battery will have depleted charge eventually. So, it's not how often you drive it, it's how long you drive it. Remember, it's cranking a cold engine that really exercises the battery, not leaving the car unused.