Originally Posted by Deadwood Dick
At my age, I need all the light I can get. So, I have installed not only the standard sealed beam passing lights, but also the HD engine guard-mounted 55 watt driving lights. I have them wired separately to only come on at low beam. My concern is with overloading the electrical system. Is there a possibility of doing some damage, or is the only risk running down the battery? For what it's worth, the tail light has been converted to LED's. Ideally, I would convert the headlight and passing lamps to LED's, but that's too expensive for me.
There are two mathematical formulas that may help here:
1.) Ohm's Law: Voltage = Amperes X Ohms
2.) Power: Watts = Voltage X Amperage
Your Lights are rated in Watts at 12 Volts.
Thus a 55 Watt Lamp will draw about 4.6 Amperes at 12 Volts. Since you added two 55 Watt Lamps that will be 4.6 Amperes for each bulb, or 9.2 Amperes (2 X 4.6 = 9.2).
Now do the same for your passing lamps and your headlight and add them all together. The LED Brake Lamp current draw will be so small it's not really even worth worrying about. The total you come up with adding everything together will be your total lamp draw. Now subtract the total Amperes for the Lamps from your Alternator Output. But remember that the Charging System still has to deal with other things as well. Like: The Ignition, The Fuel Injection System which includes the Electric Fuel Pump, the Radio if you have or use that, any Electrically Heated Gear you may have and something needs to be left over to Recharge the Battery from the last time you started the bike.
The other way is to take your alternator output . . . let's say let's 32 Amperes and then multiply that by 12 Volts. 12 X 32 = 384 Watts. Add up all your bulbs wattage's and subtract from 384. The number you come up with is what will be left over for running: The Ignition, The Fuel Injection System which includes the Electric Fuel Pump, the Radio if you have or use that, any Electrically Heated Gear you may have and something to Recharge the Battery from the last time you started the bike.
There is probably a better, or maybe a more practical way to do this. Put a Voltmeter on the Battery. The cheapest Digital Volt Meter you can get at Radio Shack for about $5 will be just fine for this. Read the voltage with everything turned off at idle and at 1,500-2,000 RPM. Write down the Voltage readings. It will probably be around 14.0 Volts +/- 0.5 Volt. Now turn everything on, Lights, Radio, Heated Gear, etc. and read the Voltage at idle and at 1,500 - 2,000 RPM. If the Voltage readings are the same with everything off and everything on you have not exceeded your Charging Systems ability to keep up with the maximum electrical load you can create on the bike. If the Voltage falls below 13.5 Volts at idle but goes back up above 13.5 Volts at 1,500 - 2,000 RPM you're kind of at the maximum electrical load your bike will comfortably handle. Remember that over the long run very little of your bikes actual running time is at an idle so as long as things are OK above 1,500 RPM you should be OK. However if the Voltage stays below 13.5 Volts, even at 1,500 RPM you have exceeded your Charging Systems capacity. Because it's not a zero sum thing, how far you are below 13.5 Volts and how often you ride with everything turned on can mean you may have a problem or you may not. Naturally it's nice to see more than 13.5 volts all the time but realistically that's not always possible and it doesn't mean that you WILL have a problem.