Is there a benefit to turning off dynamo-powered lights?

Question

I have a bicycle with lights and a hub Dynamo. I am looking for generic answers that apply to more setups but if that's not possible, the bicycle I have is a Stevens Randonneur 2021.

My bicycle lights can be turned off using a switch on the back of the front light. I was wondering if there is any benefit to turning them off during daytime. Two possible ones that I could think of (but don't know if are actually true):

1. More efficient riding, as no power is redirected to keep the lights on. Probably tiny, even if true.
2. Less 'wear' on components. The light and other components maybe last longer if not turned on unnecessarily.

Are the above true? Are there any other benefits?

Answer 1

Yes, there is an efficiency gain by turning the lights off.

Cyclingabout made a hub dynamo comparison based on an experiment by skjegg, where power consumption was measured for different dynamos attached to different lights.

To give you ballpark figures, let's use the example of the Busch & Muller IQ-X light. The first graphic shows pedaling drag (dashed line) and output electrical power (solid line) for when the light is on.

The second graphic shows pedaling drag with lights disconnected, which is similar to turning the light off.

By having an hub dynamo installed, you constantly loose around 1.5 - 5 W (at speeds above 15 km/h - lights off scenario). However, depending on light and dynamo, by keeping the lights off you can save around 8 - 11 watts.

An important follow-up question would be: does it matter in practice?

According to bikecalculator, for a 100 kg rider + bike system, flat, no wind, one needs about 115 W to cruise at 25 km/h and 39 W to ride at 15 km/h. Let's see how faster one could ride if this extra drag were to be converted into forward movement. I will use data from the Shimano 3D32 (the one I see most often) and assume that the calculator does not take into account hub dynamos.

So:

• 15 km/h (39 W):

  • +2.5 W (41.5 W total effort - hub dynamo & lights off) would equal 15.5 km/h, i.e. approx. 0.5 km/h faster;
  • +13.0 W (52 W total effort - hub dynamo & lights on) would equal 17.42 km/h, i.e. approx. 2.4 km/h faster.

• 25 km/h (115 W):

  • +3.5 W (118.5 W total effort - hub dynamo & lights off) would equal 25.24 km/h, i.e. approx. 0.2 km/h faster;
  • +10.5 W (125.5 W total effort - hub dynamo & lights on) would equal 25.85 km/h, i.e. approx. 0.8 km/h faster.

Note that these values are for instant speed. The average speed difference would be smaller.

EDIT: I originally made a mistake on interpreting the graphic that skewed the results significantly to the draggier side, thanks @jpa.

Answer 2

In general I leave mine on all day (SP-PD8 and Axa luxx 70) but it does take a little extra pedalling effort. A few Watts is likely to be a couple of percent of your effort, but if you were into marginal gains you probably wouldn't be riding with a dynamo in the first place. I do save the effort in bright conditions when the lights are of very little help, if I remember, and especially uphill.

The main reason to turn the lights off in my case is that then I can charge my phone from the dynamo as my light has a 5V USB socket.

Inside the dynamo, nothing will wear faster for drawing a current and the electrical parts don't move against parts they touch. LED lifetimes are at least tens of thousands of hours. Judging by mine the plastic lens will degrade before then, from little scratches.

Answer 3

Many answers already state that there can be efficiency gains, but: There can as well be efficiency loss from turning off the light. This depends on the circuitry involved.

For example, if you turn off only the front light (many rear lights can not be turned off individually), the voltage generated by the dynamo will be higher. Instead of the typical 6V (RMS), it may produce 15V. The rear light may have a protection device (TVS diodes), which clamps the voltage at 12V. Now, as soon as the voltage would exceed 12V, much current will flow through the TVS diodes. This is probably less power than what the front light would consume, but it may be more power.

There are also dynamos with integrated TVS diodes. These are known to make more drag when not connected than when shorted.

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Years ago I have made experiments with a Shimano 3D32. Of couse I can not link references for these experiments.

This model generates a kind of sine wave when under normal load and speed. In an open circuit it adds a sharp spike in the middle of each half wave. It generates an RMS voltage of about 1V per km/h, so at a typical speed of 20km/h it generates 20V RMS, which is much more than the rated 6V. The peak voltage is about 4V per km/h, so 80V at 20km/h. If you touch the wires of the unloaded dynamo at this speed, it actually hurts a lot!

The Shimano 3D32 is able to deliver much current if it is allowed to reach this peak voltage. I connected a 24V TVS diode, and at 20 km/h it burned about 15W. Therefore: If you have this dynamo, a rear light with 24V TVS diodes, and turn off your front light, you should actually notice the efficiency loss.

Answer 4

This depends on the electronic design of your light and dynamo but as of the time of writing it is rather inefficient and the drag is not non-existent.

A bicycle dynamo historically produced between 1.2 and 2.4 W so the lamp could not be more powerful. LEDs are typically at least ten times more efficient (6 W LED shines like 60 W incandescent bulb, bright enough as a primary beam of a car). It would kind of be logical to expect the modern dynamo LED light to draw 0.1 W at moderate brightness but nope, looks like the majority of the recent dynamo lights still draw up to 10 W of power instead. They should shine like 100 W light often seen as too bright for a living room, what are they doing with this power?

This is likely tied to dumb/naive solutions that are used to convert the varying voltage of dynamo into fixed voltage required for the LED. While the efficient converter can be easily built, a simplest naive stabilizer is highly inefficient and looks like exactly these are still mostly used.

Hence your dynamo light may be creating up to 10 W of drag resistance that depending on your fitness and overall situation may make it unreasonable to keep it always on.