sometimes, the battery starts the car sometimes the battery is dead. The car is a 2002 v70 xc. The battery hold a good charge. I am woundering if this is the battery telling me it's time to replace me. Or is this something that is putting some drain on the battery while the car is off.

 

Based on non-Volvo specific information, the answer is "yes". Here's what I mean. Either something is causing a near-constant drain on the battery or the battery itself is no longer any good. While a battery can show sufficient voltage to be "charged", unless you have a load charge test run on it, you don't really know. That tests the battery under stress to see if if it has
enough cranking amps to deliver the juice your starter needs. There's something called a "surface charge" which will show adequate volts, but there
s not enough kick in the battery to turn over the engine.

On the other hand, you need also to check when the car is off to see how much juice is flowing. Modern cars drain batteries. Alarms are a big culprit.
I don't know the "spec" for igition off battery drain, but I'll bet somebody here does.

The older the battery, the more likely it has lived its useful charge/discharge cycle life.

 

Check the voltage with a meter to see more clearly what the battery is doing. If it's the original battery (7+ years old) with the coming colder season, then a new battery is a great idea.