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In the Netherlands many people use these kinds of detachable bike lights. I've never came across soldered batteries in the dozen or so I've replaced batteries for. Not always cheaper, but just tossing the whole thing seems like such a waste.
Well done!
There are also dynamo powered LED lights which only provide light while riding the bike, and about a minute afterwards (I assume from an internal capacitor). They are not detachable but why would that be necessary? From what I can tell they also don't require regular replacement.
These are fine for bike commuters, and I’ve had them before. But they are in general not fine for recreational bicyclists; these people would gladly pay a hundred dollars for a new tire to shave off three watts and are not about to add any unnecessary human-powered accessories.
An exception is bikepackers, who may prize reliability and runtime over brightness and weight. But then the dynamo we're talking about is $200-$400 and that's not including the light.
Though you can get Shimano Dynohubs pretty cheaply:
https://www.sjscycles.co.uk/hubs-front/shimano-nexus-dhc3000...
Dynohub history rabbit hole for those interested:
https://www.sheldonbrown.com/dynohubs.html
No, it's about a tenth of that, light included.
Amazon has the Shimano dynohub for $125. Then you have to build a wheel. You might be able to re-use your rim if it's got 36 holes.
I explored dynohubs in the past, but LEDs and LiPo batteries have really narrowed the gap. In my garage, I've got a super long USB cable that we all use for charging our lights. I've used cheap lights from AliExpress, and they are actually quite durable.
Nonsense. At $20 you'll be very hard-pressed to get even a cheap dynohub alone, and that's before the wheelbuild and the light. And bikepackers aren't going to be satisfied with ultra-low-end wheels, anyway.
bikepackers aren't an homogeneous population. Some are more instagrammers than bikepackers and are using expensive bikes and equipment, others bikepack on a shoestring budget with stuff from the thrift stores and you could mistake them for homeless. And finally some are actually homeless. And you get any kind of budget and requirement/vanity level inbetween these 2 extremes.
This summer while I was cycling I came accross an ukranian guy who was bikepacking accross europe with an old mtb from the 90's, using an old kids trailer to carry his gear and water supply. He was stranded in a climb in the middle of nowhere with a broken wheel, 3 or 4 spokes had given up the ghost, and the tire was flat you could see the casing instead of the tread on both wheels. We could only communicate using google translate on my phone but he told me straight nomoney. His tent, which he had setup on the side of the road to have a bit of shade could have come from a dumpster judging by its state and discoloration, that tent was 20 to 30y old. Since my daughter has a folding bike with 20" wheels and I was 20 minutes from home I convinced him to give me his wheel, filled his empty water bottle with my own, gave him some snacks and went back home to check if spokes length matched any of my daughter's bike wheel and relaced quickly his wheel using 4 spokes from my daughter's. Then I climbed back that climb with the repaired wheel, 2 tires from my daughter's bike, a veggie sandwich and 4L of water (having myself a bike that is bikepack ready made it easier) so he could eat something and set off. He paid me with what he had available: a big hug and proposed to help me with my back (I understood he was some kind of chiropractor) which I declined.
Wow, that was very charitable of you. I don’t have the wheelsmithing skills to even attempt that repair.
I also met a number of homeless people while on a long bike pack (C&O + GAP). They were usually pretty hard up: they lived in junky tents off the side of the trail and used their bikes to commute to work. It’s hard for me not to think that we have failed in an important way if somebody is dragging themselves to work while living out of a tent.
I remember my mum telling me about going climbing in the 1960s with people who would saw the handle off their toothbrush to save 10g or so in their backpack.
Seems a little performative to me, that's about the weight of all the raindrops on your jacket.
A reliable dynamo hub setup can be very cool for the right situation, but the up front investment to rebuild your wheel around a dynamo hub is expensive. The power output is also on the low side compared to the light output you can get from the battery units.
Not having to charge anything or worry about batteries running out in the dark is a real benefit.
It's all on the very opposite end of the cost spectrum than the cheap bike light in this post though
The old-school dynamos cost 10-20 bucks: https://axabikesecurity.com/product/axa-8201-right/
They're just generic AC motors held against the rim, as simple and cheap as it gets. Nowadays I'd add a basic voltage regulation circuitry to get a stable 5V DC out of it.
That's why I said "reliable" in my post.
You'll spend more time futzing with one of or dealing with weather-related failures that you would have saved more time charging a pile of batteries before every ride.
That overstates their unreliability. Such dynamos used to be universal before modern rechargeable batteries and worked well enough, I don't recall ever having to replace or maintain one. For dirt roads where mud gets everywhere, non-mechanical options are of course better, but simple dynamos perfectly are fine on regular city streets, country roads and dry paths. They're especially good for kids who can't be bothered to keep batteries charged.
I don't remember a period of my life where bottle dynamos have not been unreliable when it rains, and it rains a lot here (or it used to, anyway).
I have rigged all kinds of rubber contraptions to make them work better, I have made my own dynamos from old electric motors as a kid, and yet I have never in all those decades found a way to be sure they would work when I need them, never.
Aren't those dynamos going to wear down your rim quite fast ?
Aren't those dynamos going to wear down your tire quite fast ?
I recently bought a dynamo from what should be a good brand (dunno the name off the top of my head). Cost about 45 Euro's, not cheap. Just after a couple weeks, whenever I cycle through the rain my dynamo wheel would slip and I'd be fully without light. I thought it was not properly pressing the rubber of the tire. But I found that the wheel was just cheap plastic garbage, totally worn flat already. I guess capitalism flew by, enshittifying once more what was once good.
The old-school dynamo's were undestroyable iron casings, with an iron wheel with deep grooves. Now I am looking if I can find one of those again, to last me a lifetime.
After 15 years of dynamo hub, I’m about to switch to a power bank connected to usb light. Dynamo hubs are quite fragile and not easily maintained. Cheap front hubs are found everywhere and they less drag. I have a few Deore hubs that I found on the street on abandoned bicycles.
Depends which one you buy! I bought a SON about 10 years ago, have done around 20k miles in all weathers, and have had zero issues at all with it.
I broke two Deore and one SP PL-1. My cheap Nexus one seems to hold better but drags a lot. There’s a market for a hub that has serviceable bearings, especially with brake disks where it’s the only part of the wheel that’s supposed to experience wear. I’d love to be able to pop two new bearings in place and ride 10k more.
Shimano hubs are a bit heavy but quite reliable. No maintenance required, survive frost, rain, heat, any weather.
Love this. I have a pile of nice Cygolites (both front and tail) that my whole family uses that are maybe 8 or 9 years old. Their capacities have largely dropped to the point that I need to charge them every ride or two. Maybe get about 90 minutes of use, down from 3-4 hours previously.
They're not user-serviceable by design but I've been pondering cracking them open and having a go at swapping the batteries. This post may inspire me.
I've got a pile of new 18650s lying around and hopefully the front lights use those!
Depending on your lifestyle, IMO the USB-C cygolites are worth the expense vs dealing with old ones
Talk about burying the lead. This was a MEC bike light. It should have a replacable battery!
https://www.mec.ca/en/explore/sustainability-innovation
Ah, memories. I miss the old MEC, before they overextended on real estate and had to sell the farm to private equity. I have a pile of MEC branded gear I'm still happily using 20 years on. I have some restrained hopes for the new ownership group to bring it back, but I'll always miss the co-op.
These lights would have predated the private equity era but I'm guessing sourcing options were somewhat more limited back in 2016.
The best motorcycle sunglasses i every bought were mec, from the vancouver store in around 2005. Flat sided (for helmets) with interchangable lenses, and seriously dark lenses too. But when I walked in, in full sportbike leathers, i got more than a few dirty looks.
I had a membership since 1994. They basically erased it.
Between that, and selling out to a foreign-private-equity-held holding, and the incident during which an innocent shopper who came to return an item was attacked by a security guard, and the idiotic Vancouver location with no connection between underground parking and the store room, I decided never to set foot there again.
Fuck MEC, the Mountain Equipment Coo^H^Hompany! Not the MEC you knew.
Did we not just see the battery replaced? ;)
I don’t know what it is with Julia’s writing but it always makes me happy. It reminded me of replacing the soldered battery in Super Metroid so I could save my progress again. As a cheapskate I used a regular battery and connected the wires mechanically.
I wouldn't worry about matching the exact model number of the battery. Instead, it's more important to get roughly the same chemistry and a capacity equal or maybe a touch greater (if technology improved in the last 10 years, capacity could be greater with modern parts).
Specific batteries go in and out of production. But as long as it's the same CHEMISTRY, voltage, and capacity, it's likely compatible. Chemistry tends to set the voltage, size sets the capacity.
Charge/discharge controllers and algorithms are based on these three bits of info.
In some cases, some chemistries are actually compatible with other chemistries. NiHM cells are largely backwards compatible with Ni-Cd chargers (but not vice versa). It's not good for the NiHM but they will actually take the charge.
The chemistry set the voltage and the voltage and the size set the capacity. Everything else equal, a larger capacity battery is often compatible with an original smaller capacity battery in a given device because the charge current will be lower in relation to the capacity.
The corollary to this is that if you choose a smaller capacity battery, you run the risk of charging at too high of a current possibly leading to fire.
I know it's slightly offtopic, but the brand I am using - ExposureLights - offers a battery diagnosis/replacement service for their bicycle lights.
Those are on the expensive side, but still, most brands would rather sell you a new one.
Thanks for sharing this. It's nice to see something not immediately turned into landfill junk. I ran into a similar issue awhile ago, and was able to replace the battery too. Not trivial, but very satisfying. Here's that journey: https://www.mooshwerks.com/2024/11/09/bike-light-hack/
Love to see MEC gear - but I wish it were more repair friendly!
I recently had an even easier time changing the rechargeable battery in some mini Christmas lights. The unit was a solar panel in a box held by 4 screws. Took them out, and it was a regular AA nimh just slotted in there. Changed it out and the thing worked again.
I wonder how many people just chucked the unit when the battery wore out.
I just bought the Steam Frame, valve's new VR headset. After hunting for a USB port on the paddles i was shocked to find how eaily it is to replace thier batteries. They are Duracell AA.
On the subject of bike lights. Most of the current lights appear to use some LEDs which makes sense, of course. However the brightness of output light looks like completely unregulated.
On so many occasions I would get blinded by overly bright lights of an incoming cyclist during late evening ride.
That seems like an intentional design choice as truly unregulated LEDs without even ballast resistors will burn out.
It's probably this way because companies design products to cater to their customers, who would likely prefer a Damn Bright Light to feel like they got their money's worth.
They mean there's no policy regulating how bike lights can be designed, not that the LEDs themselves aren't electronically regulated.
Probably depends on jurisdiction. E.g. in the UK there is no formal limit (and car headlights are more powerful than bikes') but light must not dazzle or cause glare to other road users, hence why headlights on full beam must only be used in certain circumstances, and beams in general must have specific orientation.
So if you are blinded by an incoming cyclist this is not legal... of course enforcement is another matter.
It's not a brightness thing, after all car headlights are probably 1000x brighter than anything you may have on your bike.
It's because 99% of bike lights are basically just flashlights, spraying light everywhere. The solution is the same as for cars — lights with horizontal cut-offs.
I do some optical engineering, and once took a course on lighting design. Indeed, bike lights are unregulated, and therefore, un-engineered. During the winter on my commute home from work, I usually have to cover my eyes on the bike path to avoid being blinded by oncoming cyclists.
I think Germany has regulations on this. I've bought lights on AliExpress that have a cut-off.
It might be complicated.
It is indeed unregulated and often blinding. But there is also no one limited use case as it can be for car lights:
- these lights are sold internationally so they don't assume one specific country in the first place
- it's not just for road use, people will use them in rougher bike paths where there might be absolutely no other light.
- seeing the path forward isn't often enough. You don't have a windshield nor can assume a cleared path, so knowing what's at head level can also be crucial. Even on dedicated bike paths there will be branches and unmaintained/unexpected bits that need avoiding.
- being seen can be as important as seeing yourself, and often back lights are small and a pain to recharge. Many customers will want a wide spread front light that reflect all around, even if it mean charging it more often etc.
All in all, it might be that biking itself is wholly unregulated so lights will have a hard time being single purpose and regulated as well.
I also hate flashing lights. Is it just me or do flashing bike lights cause some kind of strange mild disorientation? In addition, they are distracting and stop me seeing other things.
It's not just you. Objects that blink are hard to track (our brains aren't good at it), and working to process it anyway creates an unnecessary distraction as a secondary effect. And as a tertiary effect, this added distraction leads to a reduction in safety.
Having some movement can be good, though. This is why I went out of my way to mount reflectors on the back side of my aftermarket pedals.
Along with a constant red tail light. And a fixed red reflector.
(A grown-ass man with reflectors on his bike? You betcha. The dork disc is still intact, too. I don't like crippling surprises, and I'll look way better with a safer bike than I ever can in the hospital.)
I think flashing makes more sense at day time or twilight where they aren't nearly as contrast with the background brightness. You get massively better battery life blinking than solid on. But of course nothing stops a incompetent user using flashing lights pointed upwards at night.
Much of that (I'd like to say most) is due to a bad beam pattern and/or improper aiming.
For instance: Round beams suck. The first LED front light I ever bought was nothing more than a flashlight that took 3 AAA cells and attached to a handlebar mount.
It produced a round beam. If I aimed it up/far enough ahead that I could see the road ahead in any meaningful capacity, then it was also blinding to other riders and people in cars.
I could aim it down some and limit others' exposure, but then it was useless at letting me see what I needed to see, myself.
The most recent front light I have came from Walmart, and was sold in Schwinn packaging. It's rechargeable. With this light, other people have zero direct exposure to the LED because it is occluded by the housing.
It uses optics to shape the light. A curved mirror, and AFAICT a lens are inside. The beam it produces is shaped to evenly illuminate my path and it has a very sharp horizontal cutoff.
It's a very sensible design, in that when I aim it at the road so I can best see, other people's eyes are above the cutoff so they aren't blinded by glare. Everyone wins.
But not every bike light has a sensible design. And not every rider has sensible intent.
(The Germans have standards for this stuff enshrined in StVZO, wherein: Stuff like beam shape and cutoff is governed by law. Searching for StVZO lights tends to yield good results.)
Ok, so only loosely related - I find new matrix headlamps on cars to be pretty intriguing. Basically they have an array of lights that are focused like pixels, and they can turn off the pixels that would blind others.
Wonder if this kind of stuff can trickle down to bicycles and motorcycles and you only light what's important in front of you. This could be to light more stuff so you can see it, or less stuff and save power while seeing what's important.
I've experienced the same problem with newer car headlights, which despite being regulated are now far too bright and concentrated, creating a very bright and very dark region with a sharp line between them instead of the dimmer but more diffuse gradual cutoff they used to have. Eyes have dark adaptation so you can see farther with the latter, and it's also better for oncoming traffic.
In some European countries there are regulations for bike lights, some even require cut-off beam like Germany. However in US the front light is dual purpose - it's illuminating the road at night and keeps salmoning bikers/runners/electric motors enthusiasts from running into you so a cut-off beam is counter productive for this purpose.
If it killed the first battery, won't it just do it again to the new one?
They were 10 years old. Batteries are wear items from time and cycles.
If they kill a battery even every 5 years, that’s still fine IMO and replacing the battery is way better than tossing the whole thing.
It didn't. The battery killed itself. Modern rechargeable batteries don't like being left discharged in storage.
If anybody's interested in what the meta is for buying bicycle lights:
Some recommendations:
• If biking casually or without a need for strong front light, honestly just get the cheapest lights you can get + some reflectors. Having any light at all gets you 99% there.
• Magicshine lights in general, I myself have the EVO 1300 for front and the SeeMee 300 for back. Proper lensed beam cutoff for the front, pulsing modes for the back, and good side visibility for both.
• Bontrager Flare RT for the back — small but powerful thanks to its focused beam. They also seem to have a front positional version but I haven't tried it.
I’d replace #5 by “don’t use battery-driven lights, use a dynamo hub” (except for e-bikes, of course, which also won’t have strobing lights).
" b) If you've got the money, consider a radar instead."
Can you explain this? What is this and how does it replace a taillight?
A bike radar alerts you about vehicles behind you. It does not replace a taillight, rather they always (I think?) have taillight integrated. For more details:
https://www.cyclingweekly.com/group-tests/best-radar-bike-li...
Radars don't save you nor do they tell you which motorist will run you over and which will not. So it doesn't help you take decisions on wether to take evasive actions such as jumping in the field next to the road.
Radars don't push cars away from cyclists.
They are more a placebo than anything really.
Neither does a car's mirrors save you being run over by a truck
They show the approaching car's speed and direction, which is quite handy to take an evasive action. Some also integrate an alarm and blink at highest intensity in case of an imminent danger. That won't stop the usual terrorist cab but might catch the attention of a distracted driver.
Note that in some (at least European) countries, only steady light is legal.
Actually my own recommedation is different.
Having steady rear lights mounted at the socks/ankles is the way to go. The movement of the feets makes you instantly recognisable and standing out from very far away. There is a (australian I believe?) company that sell pedals with integrated lights too but I can't remember the name.
I am still using a regular rear light powered by a dynamo hub though, as batteries can always die without warning.
Happy to discover those two ideas, thanks!
There are also reflector pedals (see above for reliability etc)
I use https://redshiftsports.com/products/arclight-pedals - originally bought them on Kickstarter.
They're not super cheap, but I'm very happy with them. You can change the mode from always on to pulsing. They automatically turn off when not in use, and they sense the direction of the pedal so that the front light is always white and the back is always red.
Seconding point 3a. I think many people assume brighter is better and also assume they can't possibly be dangerously blinding bright compared to a car's headlights or even deliberately want to fight back against headlights. But despite how bright and poorly aimed a lot of headlights are, a small light with literally no cutoff can be even worse. You could make it so a pedestrian can't see a car, or make it so a driver can't see a pedestrian.
Plus it's annoying. I was in a French city a while ago with a very nice pedestrian area full of outdoor tables. It's so nice to sit for dinner and look out at the surrounding old buildings, canal, people going by. Unfortunately every 30s or so you get blinded by the lights of a cheap scooter.
Ive been using Costco headlamps as bike lights with the headstrap bracket ziptied to the handlebars. The usb-rechargeable light snaps into the bracket at whatever angle desired. When the 3-pack goes on sale they're $7 each, which is a small fraction of the equivalent made-for-bike lights I've seen around. Haven't found a use for all the headbands yet.
Reading https://en.wikipedia.org/wiki/Button_cell#Type_designation would have indicated a solution: The third character must be an R (for rechargeable), the “77” would be the height in tenths of millimeters (which makes for a good plausibility check), and then you can measure the cell’s diameter to obtain the remaining digits in the middle.
Word of warning: batteries from AliExpress might not be the size it says on the tin.
Also: not all lights require soldering to change the battery. Some brands, like Fenix, offer lights where you can change the battery on the go.
I had a light from a lesser known Taiwanese brand - an INFINI Lava 500. At some point it stopped working, so I took it apart with my iFixit kit and inside was a 102660 battery (1700mAh instead of the advertised 2600mAh), so I bought a 102560 unit - 10mm thick, 25mm wide and 60mm long (supposedly).
It was a good few millimetres longer and didn't fit. What's worse - AliExpress stopped shipping batteries to my address, because due to safety regulations you can't ship batteries via airmail, it's just that one vendor who chanced it, so now I couldn't even buy a smaller replacement.
Local vendors didn't have anything nearly the same format, so I gave up and bought a LEZYNE LED Macro Drive 1300XL. Inside there are two 18650s wrapped in a pack that has a plug, so should they die, you can replace the battery without soldering.
Was it a protected cell? They have a little circuit board on one side, sometimes you can see it through the shrinkwrap.
Both were protected - it's just that the battery I bought was genuinely the wrong size - it wasn't 60mm in any axis.
Yeah, I really like that Fenix lights just take a Li-ion 21700 or 18650 (and they have a little caddy to adapt the latter for the former).
However, the manual is very specific about only using their specific batteries and not just arbitrary batteries of matching size and chemistry (generic 18650s). I'm currently trying to get some information out of the manufacturer about whether there is risk of damage or they're just trying to avoid people using shitty batteries and blaming them for a bad experience.
(The manufacturer is fenixlight.com, but the seller is fenixlighting.com. It's kind of confusing.)
OK, got a response back, although it was possibly slightly AI flavored? But they said no, a good quality 18650 shouldn't harm the light. It just might not power the top modes.
The button-top vs flat-top distinction trips a lot of people up here: many bike lights ship with protected button-top 18650s, and a bare flat-top cell is a few mm shorter, so it can rattle or lose contact over bumps even though it's nominally the same battery. The other gotcha is cells with welded solder tabs vs spring contacts - if the original pack is tab-welded, a drop-in replacement sometimes needs a shim to make reliable contact. Worth checking both before ordering the replacement cell.
The wider point stands though: a bike light is a battery, an LED, and a switch, and the battery dying first is the most repairable failure mode there is.
Back in my days I simply googled my way through the nomenclature of batteries. I feel pretty old, somehow.
I'm pretty sure that's what most people would do now and the AI summary would still be wrong half the time anyway.
I don't think much has changed at all. The LLMs don't magically have new information that didn't already exist out there.
In recent months (I don't use it enough to confidently state when the change occurred) Google's LLM summary more often answers the wrong question rather than confidently answering the question wrong.
If it answers and the answer is for what I asked, it is right much more often than not. With that said, "trust but verify" is more like "don't trust and also verify" anyway, which is the same metric that should be used for most websites.
he specifically asked for non-LLM, tho :)
Back in my day, Google wasn’t an LLM. And we liked it!
Julia = she
Very irritating tendency of people here to assume everyone on the internet is “he”.
To be fair, the grandparent comment being replied to has a username beginning with Mr.
I went the obvious path, back in the days.
I prefer "they". It's just too much work trying to find out someone's preferred pronouns on the Internet, unless they have somewhere specifically asked not to be mentioned as "they" on the Internet, and that is also too much work, but anyway, in that case I may also indeed have to answer to fellow Internet strangers. But in this case the title says "Julia" so should have easily been "she" (in my defence I am from a region where these kind of names are not very common, so I may be wrong).
Could've been a trip to a local library!
This guy is funny, he spent~$20 and at least 10 hours to fix a product that can be purchased with $5.
So what?
I replaced a lezyne light battery too. There was no point buying a complete light, send this one to waste when I could just look up for the actual reference in a search engine, find the correct one and replace it.
It is not about money, it ia about not sending unneeded stuff to waste management, which we pay for actually.
I would say $15 is cheap for the learning, growth and experience. Interacted with humans too along the way. People pay much more than this for schooling which teaches you less.
It's weird that rechargeable bike lights almost universally use soldered batteries. Conventional flashlights with onboard charging often use standardized cylindrical Li-ion cells that are field-replaceable by unscrewing the tailcap. They're usually 14500, 18350, 18650, or 21700 size, sometimes with a protection circuit stuck on the end (making it semi-standard, but easier to handle safely).
The light in the link uses an absolutely tiny 0.5Wh battery, which strikes me as insufficient for cycling. I use a light with a 21700 cell (20Wh).
It’s more of a “be seen” light rather than one that lights the way ahead.
My jurisdiction requires a solid white light facing forward at night, but you can have a flasher in addition to that (cuz drivers still won’t see you with a solid light…).
Also a good backup it your main light does to stay legal.
I'd love to see an actual standard for user replaceable batteries. Raw lithium cells are just far too dangerous considering they can explode from being short circuited. So products have the choice of either non replaceable, or proprietary packs.
Given the EU is mandating replaceable batteries very soon we might see some momentum on this.
I did something similar recently. One thing you don't get to check before buying a light is the age of the battery within it. In two cases now expensive lights I have purchased had 8 year old LiPo puch cells in them, and died within a year of use.
It was a surprisingly repairable light except for one catch: I could not find a pouch cell LiPo with this kind of connector nor shape anywhere. Not even in part catalogs let alone for sale. So I have snipped the leads of the connector and will revisit this when a cell that suits shows up.
I would be surprised if they rolled their own but perhaps their factories have a B2B supplier that was able to.
Would a normal makerspace do?
For removing soldered-in batteries like this - I do it on synthesizers a lot - instead of risking heating it up too much just cut the legs off and pick them out individually.
Same for any multi-legged part with soldered-down mounting tabs like pots, chop 'em up and remove 'em a leg at a time. You'll put far less heat into the PCB, drastically reducing the risk of damaging the pads.
When you come to solder the new battery in, you'll have trouble getting the solder to stick. Get a blob of blutac, stick the battery to your bench, tin the legs very lightly, and let it cool for a few minutes while you clean up the pads on the board. Then solder it in while it's warm but not totally hot.
By doing it the slow and complicated way, you'll be done so much faster than if you tried it the quick and easy way.
Does anyone know of a back light that turns off after 5 minutes of no movement? I'm tired of backlight being out of juice just because I forgot to turn it off ... I'm tempted to make a circuit and maybe even the sensor myself but I don't have the time for it and it probably already exists.
If you use a bike computer: the Garmin Varia tail lights auto shutoff shortly after disconnecting from the computer, and the computers themselves have auto shutoff after a period of inactivity.
The Ravemen CL200 does. And it's cheap. Doing a 2000km ride with it currently.
Thanks!