Replace Li Ion battery

Let's go back to specifying what you want. Currently, you have a light that does not meet your requirements. But you haven't stated those requirements. I'm assuming that your existing light is not defective. The simplest solution is to buy a light that does meet your needs. You might find that the cost of all the parts you need and replacing the parts that didn't work well in your experiments far exceeds the cost of getting the right light in the first place. But you haven't stated those needs an a way that lets anybody help.

How bright does the light have to be? Does it have to be on all the time? Flashing can use considerably less power. How long does it have to run between charges? How long can you wait for it to charge? Those are NUMBERS. Vague comments can't help. MATH is required and that takes numbers. My experience working with customers suggests that customers always ask for WAY more than they really need. That can dramatically affect the work needed. Do you ride in the rain? Do you charge it in the rain? Do you park where someone can steal the light assembly? What are your expectations for behavior at end of discharge? Is it acceptable to just shut off without warning? cuz that's what protected cells do. What are your options if that happens?

It makes no sense to modify a light to run 5 times as long if it takes longer than you can wait to recharge it. Vibration is a killer. You can't have a bunch of stuff hanging off the seat. And you have to keep the rain out of it, especially the charge connector. And you don't want it to be attractive/easy to steal.

I once made a bike headlight that used 4x 18650's in series. They fit nicely in a 3/4" plastic pipe and the pipe caps keep the water out. It clamps nicely to the vertical pipe that the seat goes into. I made another one with a shorter 2-1/2" pipe that fit nicely in a water bottle holder. And yes, I do have a battery tab welder. They were a pain in the ass to charge safely, so I dumped the whole thing and bought a headlight with a removable battery that I can replace on the road in a few seconds. Ditto for the taillight.

The specifications on batteries and chargers on EBAY look like they've been translated through a few different languages and become gibberish. For example, what the heck is a 500 mah charger? You might think it's optimized for charging 500 mAh batteries, but that is useless for design calculations. Charger spec says 450mA. Is that each cell or total for both? It's virtually impossible to know what you're getting. If you just assume you understand what they are trying to say, you'll likely be disappointed.

As with most projects, the real-world physical limitations require far more design work than the simple task of putting electrons into a battery.

Planning is the key to success. Measure twice...cut once.

Thanks.

I already have most parts I need.

Buying another lite makes no sense.

I looked into larger lites.

They are brighter, thus they use more current and the built in battery does not last long.

And built in batts are not replaceable which may be by design.:-)

Learning is a process and mistakes will be made along the way. That is one way I learn.

For ex. Many years ago I decided to salvage parts from an analog TV. TV had not been used in at least 5 yrs.

But I did use insulated gloves and cut the lead going to the picture tube. Found out about capacitors. :-)

I try to ask a lot of questions in the process.

Have a great day.

Andy

Better in what way? Price, size, charge time, ease of use? It all depends on how you want to charge your tail light. If you want to remove the cells, such a charger as the EBL thing is probably a better idea. If you want to weld tabs to the batteries and make them a permanent part of some package, then an internal charger powered by a common 5V cell phone charger with a Micro SD connector is probably better.

I've had tolerable luck with other EBL chargers. The one you found MIGHT be junk. The problem is that if you take it apart, you'll find that the two cells are charged in parallel. You can do that if you have identical cells. However, the cells are different age, quality, manufactory, or even temperature, one cell will hog all the current possibly getting it hot, while the other cell just sits that at a lower voltage and not charge. The clue is the number of charge LED's on the charger. If there's only one, it's probably charging the cells in parallel. If there are two charge LED's, then there's a chance that they're separately charged. I can't tell for sure from the photo of the EBL charger but it looks like one LED. There's a huge review of different chargers found here: If you want to shop by brand, look at Xtar chargers.

On the small TP4056 boards I mentioned, the Micro USB connector is the

An 18650 battery holder, some wire, and soldering iron. Watch out if the holder will take an 18650 with a button positive terminal, or one without a button positive such as found in cells salvaged from laptop batteries.

Now, my turn. I've asked you twice as has Mike for the maker and model number of your LED tail light. You've failed to produce even a hint of what you are working with. I wanted to calculate how long the battery or batteries will last based upon the estimated current drain of your tail light. If you have a no-name Chinese piece of deleted expletive, then post a photo somewhere showing the type and number of LED's. I can guess the numbers. That's all you're going to get from me until you deliver a maker and model number.

Charger has two charging lights.

Thanks for the light info. It's not my job to tell you what to do. But I can relate my experience.

From the point of view of the bike rider and the car driver not wanting to run over a biker, I find that a flashing light is WAY more attention-getting than a fixed light. When driving the car, I find that a dot of light in the distance makes me tend to veer toward it as I try to figger out what it is. If it's flashing, I expect that it's the back end of something that is moving...perhaps a biker.

I do have a problem with a flashing bike headlight. Great in the daytime to get attention. At night, the flash rate of my headlight confuses my brain.

The advertised run-time differences for your light are HUGE. If it were me, I'd flash it and be happy for 29 hours.

There's another obvious option to charge your light without removing the battery. You can get cheap boxes designed to charge cellphones. They typically contain one 18650 and have a usb output. It won't be the most energy efficient solution, but you can stick one in your backpack and be ready for anything. Or bolt one to the frame. Get two and swap it every day. Depending on what you use for a headlight, it might also work as a fix for an emergency light outage for that too.

They come in all shapes and sizes with and without battery.

Thanks, I have a powerbank.

I will see if it works.

The light may think it's being charged and not flash.

Will post back.

Andy

Thanks for that great idea. Saved me from having to solder inside the tail light.

Andy

Next project is to make my head and tail lights harder to steal.