Any garage should be able to diagnose it for u, its not a hard job.
Need to check battery state and charge output from alternator.
I assume ur having problem with flat battery/poor starting?
I had the same problem as you! I bought a new battery, for a week or so, the warning didnt come up, then it started to appear again! Touchwood! it hasnt happened for a while though! I searched a bit , and apparently its a pretty regularish! fault!- happening more in the winter months (as you use more power eg heaters heated seats etc!) in the colder months!
( I know its not winter now! but mine was showing the same fault a few weeks back! but as I say its been ok for a while now!).