It could be that the engine idle speed at stop is set too low with the consequence being that the amount of charge coming from the alternator is below the level needed to re-charge the batteries. When I start the engine on my Bav 36, the alarms sound, but as soon as I move the throttle lever away from the stop position the alarm stops. Sorry, I've not actually noted what the idle speed on my engine is, other than it being significantly below 1000 rpm, once I've brought it up to 1000 the alarm stops. Hope that helps.