Correct me if I'm wrong, a flat battery when measure the voltmeter (between +ve and -ve) still shows readings between +12 to +16V. It is the ampere (current) that matters right ??
The term "flat battery" usually means low voltage. Meaning voltmeter reads 11+ volts then mechanic usually will tell you battery flat, need to change.
Low volt on battery means it is "flat", but does not mean the battery needs to be changed. Sometimes just get the mechanic to change it for you and it can still be used. For eg, if you go overseas for 2~3 weeks, the battery may go flat as there is no charging for that period.
The critical thing is cranking ability of the battery. A battery with normal readings on the voltmeter does not necessarily mean it can provide the ampere to crank the engine. That's why a lot of us get stranded in the carpark. The battery volt reading is normal but we just can't crank the engine.
The Battery Bug monitors the cranking ability in terms of %. When it detects that the cranking ability falls below 10%, it will sound an alarm
after you turn off the engine. This prevents you from driving the car around with an alarm ringing all the way.
When the alarm goes off, you can still continue driving for a few weeks but would need to start looking for the best deal to replace your battery as soon as possible as the battery would not be able to crank your engine soon.
You can read more by downloading this brochure below.
http://www.argusanalyzers.com/fileadmin/user_upload/support/monitors/Literature/BatteryBug_brochure.pdf