Further to Gomers reply,
Most pilots who have flown 206s very quickly come to regonise a low battery.
pt: Your bus voltage will be 28 volts only if there is an external power source connected, or about 27.5v when the eng and generator are running.
The battery voltage should be about 24 volts when the engine/generator is not running.
2) As there is no ammeter or volt-meter, how does a pilot catch this?
3) If it was not caught, and the pilot attepted to engage the starter (assuming fully charged battery in good condition) would the start function?
If your battery has been drained a bit by leaving a landing light on then you may notice a slow acceleration in the n1 after you hit the start button.
The n1 may not reach an RPM that is satisfactory for a light off.
If you choose to continue the start a slow acceleration may occur and the dreaded hot start!
EDIT: Note the above symptoms may only be applicable to 206s fitted with lead/acid batteries.