I used to ride with battery lights, and built the battery pack myself, using 18650 cells I harvested from old laptop batteries. My main light was a 5 x Cree T6 monster. Now, a single Cree T6 chip outputs between 700 and 1200 lumen, depending on a number of factors, including the chip itself, external temperature, and more. I always simply use the lowest value to do a calculation, and in this case, my bike’s main light would output at least 3 500 lumen. Trust me, that’s a lot of light!
Most people don’t understand the difference between lumens and lux, as units of measurement for light. Lumens measures the total amount of light produced, while lux is the amount of lumens per square metre. This means that lux is a more accurate measure to use, and it the reason dynamo lights are rated in lux. After all, what matters most is how much light to focus on where you need it, regardless of how much light is produced.
Now here’s the problem: most battery lights make extremely poor use of the light they produce, scattering it in all directions, almost at a 180 degree field. When your light outputs enough lumen to make oncoming cars dip their headlights before they round a bend, you also get a lot of light reflected straight back at you.
This had the result of ruining my night vision to the point that I struggled to properly see things more than 20 metres ahead. Oh, of course I could see trees, and where the road went, but not potholes, debris, manhole covers or worst of all, well-mushed roadkill. As a result, I was forced to either cycle quite slowly, or add two more Cree T6 zoomable lights (yes, I added another 1 400 lumens of light to the 3 500 lumens I was using, taking me up to 4 900 lumen!). These two additional lights were zoomed to focus all their light far ahead of me, at two different focal distances, and means I could see potholes, or roadkill, in time to avoid it.
Several others tried to convince me of the benefits of dynamo lights, but I did the calculations and there simply is no way a dynamo can produce enough power to feed my monster light, never mind the two auxiliary lights, and so I’d pull my nose up at dynamo lights.
I do a fair bit of all-night cycling, and even used to organise an annual (COVID-permitting) all-night ride, called Darkmoor. During one such ride, the Exmouth Exodus, I started speaking to another cyclist. A member of the Audax Club Bristol, 100-odd mile rides to him are a warm-up, and as Audax riders pride themselves on being self-reliant, he used dynamo lights.
Cycling most of the way from Bath to Exmouth with him, I saw first-hand just how good dynamo lights really were, and not long after, I upgraded my bike.
Essentially, because I wanted to retain matching rims on my bike, I just bought a Shimano dynamo, then relaced my front wheel onto that hub. That was the power source sorted.
Next, I ordered a B&M front light. Dynamo lights use a different rating, and the light I bought was 70 lux, which produced far less light than my battery lights.
The difference is it’s a German dynamo light, and Germany has extremely tight laws about these lights. This means the beam is tightly focused just where you need it, and no light is wasted. Importantly, it also won’t blind oncoming road users, which my old lights often did.
For a few years, that was my setup, until my B&M front light started suffering problems caused by water ingress. I had good use out of it, so while not happy, I still believe it was a great headlight. So much so, that I replaced it with another B&M headlight, bought second-hand from a fellow South African in the UK.
My new headlight has two seriously cool features. The first, is that it has integrated USB charging, which means that on multi-day rides, during the day I rely on the dynamo to charge up a power bank, and at night, in my tent, I rely on the power bank to charge my phone and my Garmin.
The second feature is quite literally brilliant: the light has an integrated battery. When charged up (which doesn’t take very long) I can press a button to feed all the charge stored in that battery, along with the power from the dynamo, to the light, meaning it has a super-bright mode that can last around ten to fifteen minutes – extremely useful on nasty bits of road.
That doesn’t mean I’m done with battery lights, though! I also have a set of USB-rechargeable lights as emergency backup. When my previous dynamo light started flickering, then completely failed, it was around 03h00, and I still had another 15 miles of dark, rural roads before I got to Leamington Spa, along roads I was completely unfamiliar with, and the emergency backup lights meant a failed dynamo light was an annoyance, rather than a disaster.
Many people ask about the resistance dynamos create. After all, that electricity has to be generated somehow, right? Some clever people wondered just that, and did some lab tests, with surprising results.
The short version is is this: a dynamo slows you down far less that you will have guessed.
If you clicked the link above, and read the article, you will have read this bit: “The most efficient dynamo hubs when switched add between 3min 24sec and 5min 47sec to a flat ride over 100km when compared to a regular hub.”
I always factor the worst-case in when doing calculations, and in this case it means my dynamo can slow me down by as much as one minute per 10 miles. To me, that’s negligible, and given how much my dynamo benefits me, I’ll happily take that as a win.