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.
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.
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.
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.
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
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.
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.
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.
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!
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.
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.
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/
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.
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 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.)
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.
If anybody's interested in what the meta is for buying bicycle lights:
1. Reflectors are the best for baseline visibility, they cost nothing and are reliable.
2. The tail light is the important one.
a) Try to find ones with a pulsating mode, blinding other road users by shining bright rapidly blinking lights into their eyes is not good, actually.
b) If you've got the money, consider a radar instead.
3. For the front, decide if you *need* illumination strong enough for you to see where you're going in darkness, or if you can get away with just being seen by others.
a) For the former, make sure you get a front light that has a beam cutoff, preferably a lens-based solution. Then of course, install the light with a proper downwards angle, otherwise there's no point to the cutoff.
4. Side visibility is important, this is usually accomplished by some see-through parts at the business end of the light.
5. DO NOT use blinking modes at night. Blinking is for daytime, use only constant or at most, pulsating modes at night.
6. BTW, the 2nd most important piece of equipment after your tail light is a mirror (>=2inch diameter).
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.
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:
I wasn’t sure how to find out what an “LI????77” was […]. I would be interested to learn non-LLM ways to figure this out though. There must be a way.
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.
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.
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.
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.
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.
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!
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.
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.)
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.
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...
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.
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.