At the moment I can't afford one of those new LED headlights. However, I found that I have a 80/100 watt H-4 bulb and I installed it. My bike is an 07 softail Heritage. I rewired it so the passing lamps stay on with the head light on high beam. I have no other accessories running (unless I use the H-d heated gloves and a Grebbings heated jacket liner). The question is, will the alternator handle the headlamp, and if needed the jacket liner? Another thread said they increased the alternator output to 40 amps in 2007. I want the headlight so hopefully I won't end up hitting a deer. I have enough venison already. Thanks.

 

Watts = Voltage X Amperes (P=E*I)
My owners manual says that my 2010 Softail is rated for 38 Amperes. But it gives two other specifications in Watts. One says that it is rated at 439 Watts at 13 Volts at 2,000 Engine RPM which is 33.8 Amperes. The other says that it is rated at 489 Watts Max Power at 13 Volts which is 37.6 Amperes. No RPM value is given for this second specification.
The Harley-Davidson Electrical Diagnostic Manual (Service Manual) says several things which do not exactly agree with the owners manual; but may help: The Alternator Maximum Output Test calls for 35 to 50 amperes output at 13 volts at 3,000 RPM.
The Computer will set one of three different Diagnostic Trouble Codes if the system voltage falls below 12.2 volts (P0562), goes over 15.0 volts (P0563), or if it goes over 16.0 Volts (B0563).

So what to make of all this? First, Charging System Voltage should be between 13.0 and 15.0 Volts when the engine is running. Second, the Engine/Alternator can not be expected to produce it's rated output unless it is spinning above 2,000-3,000 RPM. Third, I wouldn't count on anything more than 33.8 Amperes.

Lastly, that 80/100 Bulb is rated at 12.0 Volts. On High Beam at 12 Volts it will draw 8.33 Amperes. But at 13 Volts it will draw 9.03 Amperes.
At 13 volts the Gerbing Jacket will draw 6.93 Amperes, the Gloves 2.38 Amperes and all the other lights on the bike will be drawing another 5.8 Amperes. The the Computers will probably draw about 3-5 Amperes and the Fuel Pump another 5 Amperes or so. Adding all this up comes to 32 to 34 Amperes and that's not leaving anything to charge the battery. This also assumes that the engine stays above 2,000 RPM all the time.

This sort of a load calculation would indicate a charging system that is maxed out and an alternator that will have to run at nearly 100% of it's capability almost all of the time. While do-able, this is also likely to result in a fairly short charging system life span.

 

Thanks for the detailed response. My 07 manual shows 35-50 amps @ 3600 rpm, so I probably have the same system as your manual shows. I'll ask the mechanics at my dealership what they think, and it looks like they will concur with your info. I usually carry back up bulbs when I travel, so in the case of having to use all the stuff in a jam, I could just reduce the headlamp bulb to the standard. I also understand the police bikes have alternators that have more output, so I suppose that's an option, however the LED headlamp probably draws (?) less wattage so the cost of that would counter the cost of a more powerful alternator. I have no plans of adding more accessories such as CDplayers and don't carry a passenger. Thanking about this, I wonder what a rider and passenger would do if they BOTH had heated suits??? Trailer with a generator????

 

My Alternator quit at 35000 miles. All I have is the passing lamps, and I run them all the time. The stator went bad I was told. It cost 1g to get it fixed ! I had opted for the extra warranty. Missed that bullit. BTW I didn't get a fault code at all. Just a dead battery 5 miles from the house.

 

I did some research and I also just got my Iron Butt magazine (I completed the 1000 miles in 24 hours Saddlesore 1000). They had an article on lighting that was extensive. It seems the cost effecient for me to do and cut the wattage down is to get a Philips X-Treme Power H4 bulb that will put out probably as much with 60 watts as the higher wattage (100w) bulb. They said the PIAA Xtreme white series and the Sylvania Osram Silver Star series and the Philips series are outstanding bulbs, but the first one I mentioned is the best of this type.

 

[QUOTE=Bluehighways;7927407]On High Beam at 12 Volts it will draw 8.33 Amperes. But at 13 Volts it will draw 9.03 Amperes.
QUOTE]

12 volts will draw 8.33 amps (100 / 12)
13 volts will draw 7.69 amps (100 / 13)

 

[quote=silvrbill;7941647]The power output of the bulb at 13 volts will be greater than at 12 volts. You can't just use the same wattage for 13 volts that you did for 12. Power (watts) is a dependent variable in the equations. Power = Voltage * Current and Current = Voltage / Resistance
You are defying the laws of physics by assuming the Power Output of the bulb remained constant as you raised the Voltage.
The Watt/Power rating is specified at 12 Volts NOT 13 Volts. The resistance must be calculated at the 12 Volt value and then that resistance value can be used at the new voltage to calculate the new current. The higher temperature of the filament at the higher applied voltage and resultant current and power output may have a slight impact on the overall values but it is almost certainly very minor.

 

[QUOTE=Bluehighways;7941818]Well put but doesn't the resistance stay the same regardless of voltage?.
I guess I didn't put enough thought into it.
I see now that the wattage is increased with the higher voltage so that has to be taken into consideration.