300hrWannaB: I fly an earlier version of the DR400. It has a generator.
The low volt warning light comes on at low revs. What it indicates to me is that the plane should never have had a low volt warning light retro-fitted.
It means you (we) have got low volts according to the lamp.
In reality, I know if I've got low volts, because the engine didn't start. After that there's plenty of rotation to keep enough life for a flight.
It probably means that the battery is not in great condition.
I think the low volt warning is a good idea. For example it will let you know of a charging failure (broken drive belt etc) once airborne thus giving you a warning you will lose electrical power within a shortish period of time allowing you to plan to land without losing any critical items (e.g. radios if in controlled airspace).
The light comes on at low revs as at low RPM a generator cannot produce enough output to match the electrical load on the battery. It doesn't mean the battery is not in great condition.
I was told that Maintenance need to do a battery capacity check annually (totally drain and full recharge). On a lead acid battery this is the fastest way to kill them that I know. They are supposed to be kept fully topped up.
A battery capacity check doesn't totally drain the battery nor does it kill them. The battery is discharged at a predetermined rate till the voltage reaches 1.85 volts per cell, (so 11.1 volts for a 12 volt six cell battery) and the time is noted. The time taken is checked against specs for the discharge rate used to determine the health of the battery.
A test discharge can in fact be good for batteries that don't get drained or used often. The test discharge helps prevent sulphation on the plates