Battery/electric charge warning light

[Greg15]

I've got a C4 GP 2009 which we've had since 2013. Warning light re battery/charge has been coming on and off - not every journey but quite a bit. Today car wouldn't start - so jumped and all fine.Took to garage for free battery check and told battery life zero (not surprised as we've never changed it so is at least 7yrs old and prob 12). New battery and checked alternator - all fine. So why is warning light still appearing?

[GiveMeABreak]

Because your alternator may not be charging the battery, which is likely why your other one went flat. If this is the case, the same will soon happen to this one, so I would get the belt and the alternator checked out to see if it actually generating anything.