If you are running the diesel 24/7, you shouldn't have to worry about running the batteries too low. They may loose water faster, so watch that as well. Have you load tested the batteries?
If you crank the motor over, meter the voltage as you are cranking. If it drops below 18V, then you should consider servicing/replacing them.
Keep an eye on the amperage gauge of the engine. If it is a voltage gauge, then it is not accurate at reading the load on the 'dynamo'.
12V lamps would be better to use, but may not be available in your area. Halogen or HID would give you more lumens, footcandles,etc per watt of input.
