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My OEM motorcycle headlight is listed as 12V 60/55W (STANLEY). When swapping to a different style of headlight assembly and globe with lower power requirements, for example 35W instead of 55W does this mean the regulator rectifier is going to work harder to get rid of the extra power flowing through that the new headlight isn't using?

I see lots of videos of people changing to LED and videos about changing headlights in general but almost no one talks about the mismatch in power consumption between the new and old headlights and how this will affect the electrical system.

Do people who swap headlights hope for the best (or know from experience) that their regulator rectifier etc will work with no issues?, this is option 1. Option 2 is only swap to a headlight which matches the original headlight power requirements. Option 3 is calculate the difference in power consumption and draw that amount of power so that the regulator rectifier isn't overworking.

Please forgive my ignorance on the correct terminology. I'm learning this now and the swap is because I have a used motorcycle with an ugly custom headlight which I would like to swap to a different style (not specifically the original OEM headlight) without damaging the electrical system. My exact question is does it have a negative impact to the motorcycle electrical system (regulator rectifier, stator etc) by not replacing a headlight / headlight assembly with the same specs as the original headlight?

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    Is it a modern motorcycle with a modern electrical system? Or an antique? Don't laugh, I had a Craftsman/Husqvarna riding mower which had peculiar requirements for the headlights since the electrical system was so very cost-optimized. I was hoping to add aux lights and LED the headlights. Feb 26 at 3:54
  • Watts is a result of load, not what determines load. If you install something that uses less watts, the whole system "provides" less watts. If you install something that uses more watts, the whole system "provides" more watts - even if the conductors or supply aren't rated to do it. This is why its dangerous to go over, but almost never dangerous to go under.
    – James T
    Feb 26 at 15:20
  • @JamesT That is not correct. Motorcycles don't work like that. The charging/electrical system is designed in such a way to feed through 100% of power requirements whether that power is used by the lights or not. If the power is not used, the regulator is then worked harder to get rid of this power. The manufacturer knows how much power is required and designs the system to produce that much power. Changing to LED/lower powered lights means the regulator is worked harder than what is possible new out of the factory.
    – John Doe
    Mar 3 at 1:36
  • @Harper-ReinstateMonica It's not antique or a niche bike. I was hoping for an answer to apply to most bikes. Thanks
    – John Doe
    Mar 3 at 1:47

2 Answers 2

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A lower wattage lamp will draw less power. There is no "unused power flowing through the headlight that has to be got rid of."

There should only be a problem if you install a lamp with a higher wattage than rated, drawing more power than the circuits expect. Then you might be overworking other components.

Consider what is happening when the lamp is turned off: there is no unused power to get rid of. None is even drawn from the battery.


Added after comments about the different circuitry in a motorcycle:

Countries have different "construction and use" regulations, and some require the headlight to be always on (UK does not), which may mean there is no 'off' switch, and manufacturers release slightly different versions to comply with local laws.

It would seem unlikely that they fit a different heatsink for versions that have no 'off' switch, and far more likely that the heat sink will be capable off dissipating all the unused power when the circuits are under no load.

After all, if the headlamp bulb blows, they would not want the circuitry wrecked by an overload of unused power.

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    See, and you even get an upvote for it :o) Feb 25 at 18:20
  • To expand on this, in most vehicles including motorcycles, rather than using a linear regulator or DC-DC regulator on the output of the alternator, the power is regulated using alternator field excitation. This modulates the power supplied to the rotor to regulate the total power generated by the stator to match the load. This means that the alternator adjusts the amount of mechanical energy converted to electrical energy, generating less energy and imposing less load on the engine if less power is drawn. Feb 26 at 23:02
  • It's just like your house - there's no "extra power to get rid of" at night when you turn off all the lights, or is there anything extra when you turn on the oven, water heater or dryer. (TBF, there are situations when everyone turns up their AC in the afternoon/evening, but that's a total power consumption issue that the PoCo deals with, nothing in the individual house.)
    – FreeMan
    Feb 27 at 13:04
  • @user1937198 Incorrect. What you say is correct for cars. Motorcycles, ATVs and such use a permanent magnet generator, called a stator. That is followed by a rectifier and shunt regulator, if you looked at a motorcycle regulator, you'd note that it has large heatsinks. The generator is producing 100% power all the time, any excess is dumped as heat. This is because motorcycles have small electrical systems, and engine components are weight, volume, and cost sensitive.
    – user71659
    Feb 27 at 18:37
  • Regardless, the circuitry will be designed to handle any power use from zero up to the maximum rating. Individual circuits will be fused to prevent overload. Feb 27 at 18:40
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I asked the original question, I've found out some further information after researching.

There are different types of regulators which act differently based on:

  • How you ride (racing can generate more electrical power with higher RPM).
  • The amount of power drawn by your bulb also affects the regulator (if you use a lower power bulb, LED or switch off the bulb).

Some regulator examples include MOFSET and series regulators.

If you change to a lower power bulb, it doesn't just draw less power, it causes that unused power to work the regulator harder. So you need to be careful to match your power requirements and regulator type with a lower power bulb.

This youtube video describes it exactly at 10:58. https://www.youtube.com/watch?v=AXbZZZWLtzY

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