Why do you think it's not your battery?
If it's an original battery it's already past its usefull life, (4 years).
or...you may have just left a light on but the easy way to tell is jump it (the correct way, on the + post in the engine compartment)
2000 V70R Black, 114,000 miles (Wife's)
2000 V70R Silver, 36,200 miles!!! SOLD (I will regret this)!
2007 XC70 White, 118,000 miles, loaded and mint, mint, mint in all aspects,