the battery may actually be dead/dying, the alarm going off like that is one of the indicators. From what ive seen, unless the battery is 100% flat, that gauge is absolutely useless. Everyone's zed seems to behave differently too, some alarms go off, some have weird starting issues, it just seems to be a car that really hates having a low battery, though from what I understand the engine is a pig to turn over and takes a hefty chunk out of the battery every time so it does get a hard life.
If you can afford to id try a new battery (yes, assuming your window, boot and bonnet are properly closed first) but alternatively go to Halfords and get your battery tested with their machine, it checks each cell in the battery as if even 1 is failing then your battery is as good as dead, its a free test too.