Just curious. How do you know your charging voltage? Do you have a gauge hooked up in the dash or are you using a voltmeter? Say you put your voltmeter leads across the battery terminals, with the engine running you have around 13.5 volts, than you turn on the headlights and it drops to 9, and belt starts squeeling. Does your charging light come on. It should. I don't think your problem is with the headlights. No matter how much power is being used by the lights the alternator should be able to keep up. Unfortunately you can't check you alternator amp output with a regular multimeter. You need to take it to a shop to do that. But you can check voltage drops across all the connections in the charging system first to make sure there are no problems there. Don't forget the battery either. I ran into a condition once where the alternator wasn't charging 'cause the battery was bad and wouldn't take a charge.