You probably answered your own question in the first line. If you're car is 4 years old and only has 40,000 km on it, your problem is most likely the lack of use/recharging on the battery. You are averaging less than 7000 miles a year. Thats less than half of average vehicle usage.

A battery is one item that responds better if it is used/recharged routinely and never allowed to discharge. If it does begin to discharge and is not completely recharged it will sulfate. You typically will not notice this slow process because the battery will operate normally until the battery is completely ruined. Once the sulfation builds up - the battery is shot and cannot be recovered.

These cars have a very high current consumption level. If you typically drive short distances and/or stop and go traffic or dont drive for weeks at a time, the battery may not have the opportunity to recharge even though everything is working fine.

This process occurs a lot at new car dealers lots. If someone buys a car that has been setting on a dealers lot for more than 1 month, ask to see the battery maintanance record. If they cant show it to you the sulfation process is likey already begun. Demand a new battery. Volvo dealers are required to maintain the batteries. Some dont, ours does. Coincidentally the battery warranty claims for cars we sold is down significantly.

Suggest you buy a voltmeter and check your battery weekly (check it only after setting without consumption for at least 2 hours). If it drops below 12.4 volts (stand by voltage) it should be recharged. You can purchase a small trickle charger (max 2Amp output) and charge your battery when it gets low.

Larry.