First, good point about the trailer-work, Iceman, I hadn't thought of that, and as an ex-caravaner, I agree, it would be a problem without stone guards.
Second, on the subject of whether your alternator can keep up with your headlights so your battery is not discharging when you run the lights, we need more information to resolve it. What is the wattage of the globes you have in those lights? Let's say they are ordinary car turn-signal globes, which are usually 18 Watts each I think, but may be a bit more in some cases. Two 18 Watt globes is a total of 36 Watts power consumption. 36 Watts divided by 12 Volts is 3 Amps consumption. Your alternator has to be cranking out 3 Amps to just break even on maintaining the battery's charge. However, you only get 3 Amps when the engine is running full scream, so realistically, you will be losing ground on battery charge any time you run a pair of 18 Watt globes. See why I recommended using the AC line? You'll get dim headlights, but the battery will actually be charging all the time, not discharging slightly.
I think that mower, like most, was designed to use DC for the lights because it was expected to only have the lights on occasionally for short periods, so the battery could recharge the rest of the time, when the lights were not on. Briggs provided the AC output as well, to cater for people who wanted to use the lights a lot.
