Replace Li Ion battery

If that's the best you can do at specificity, best not risk setting yourself on fire.

Now, now. Many single-cell Lithium battery packs include a thermistor to help prevent overcharging, and dangerous overheating. 10k at 25 deg C is a common value.

I'd imagine that a big battery, replacing a small one, would experience reduced stress, because its charge / discharge rate would be lower. You do want thermal protection, be sure to use one with the same themistor.

--
 Thanks, 
    - Win

I decided to go with some plain Li Ion batteries(18650) instead of the LiPOs which are a little more dangerous.

They have more charge cycles as well.

Andy

Those two words suggest that YOU shouldn't be playing with lithium batteries either! Be Safe. You may not be the one injured when it catches fire.

that a big battery, replacing a

Looks like he is safe to me.

Andy

Sure he is. People who give random DANGEROUS advice in response to a question you haven't even asked don't have to worry about their safety...or yours.

To recap the thread... _____________________________ I am replacing a small bomb with a big bomb. What's the yellow wire for?

I 'imagine' you should use a thermistor. ___________________________

Am I the only one concerned that there is zero information about the bomb or its environment? Or that the bomb that you bought on the internet has the same generic label, but may have radically different characteristics that may or may not lead to surprise detonation?

Do not f*ck with lithium batteries unless you know EXACTLY the specs of both cells and intimate details of the charge/discharge cycle. If you knew that, you wouldn't have asked about the yellow wire.

This thread contains ZERO information beyond the color of the wire.

Be safe. Don't do it, or get a LOT more informed about exactly what you're doing. You can't do that if your question isn't more specific than wire color.

Life is a series of calculations of the risk/reward ratios of your actions. Say there's a 99% chance that you'll be ok with your project... and 1% chance that you'll burn your house down, or that your kid will have an eye put out when it explodes. Do you like that risk/reward ratio? I sure don't!

I don't believe you've given a reasonable synopsis. Exactly what did Winfield (actually) *write* that you disagree with?

But you object to Winfield's post. His statement sounded correct to me, though perhaps not a full answer.

What does that have with Win's answer?

What does anything above have to do with what Win actually wrote?

You need to chill out.

If you are not happy unless you can be 100% safe on anything, then you are a consistently unhappy and wrong person.

Foul language is also not necessary.

Andy

Whoa, firestorm! It's time for me to step in and add to my remarks. First, I specified an answer for a single-cell battery, the most common form.

My comments are incomplete and not relevant for a multi-cell battery, as some of the thread seems to assume. (I agree that modifying such a battery is a serious engineering exercise.)

We can assume the single cell charging system in question is safe, having appropriate voltage and current-limiting features, and is equipped with a cell temperature sensor (the O.P. will verify this using an ohm-meter on his existing cell). In such a case I think it's safe to move to a larger cell, incorporating the same range thermistor. Naturally, we assume our O.P. has relevant electronics skills.

--
 Thanks, 
    - Win

LiPo can handle higher charge and discharge currents than LiIon cells. If your application requires high currents, such as powering a drone, you should continue using LiPo. This article offers a comparison: "Lion vs LiPo? Which better, or are they just different?"

Perhaps it might be useful if you disclose what device you're powering, the number of cells, and what you're using to charge the battery?

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

There you go. Look at all the assumptions you made. Based on the composition of the original question, there's no basis for any of them. You may have also assumed that the OP knew better than to solder directly to the cells. Or that he used eye protection when dealing with things that might vent. How could you assume that the OP knew about "appropriate voltage and current-limiting features...?" or that the random lithium cell from the internet is unlikely to perform anywhere near expectations. Or that a lithium cell designed for a laptop computer would perform poorly and be dangerous in a power tool or drone. A cell with the highest possible claimed amp-hours is exactly what you don't want in a drone.

If you hang around the internet for very long, you learn that people have ambitions that far exceed their current capabilities. Nothing wrong with that. That's how we learn.

The problem happens when well-meaning tutors forget that and give them vague advice based on their own assumptions, not the (often unstated) design parameters and skills of the OP. Questioners have no way to determine whether the advice they're getting is good advice. When dealing with dangerous devices, it's best to err of the conservative side and work up from there.

One of the most important characteristics of a mentor is empathy. The ability to assume the mindset of the student and communicate in HIS terms, not yours.

You can learn a lot about the mindset of the student by the specificity of the problem statement and the words used to do it. Over the years, dealing with very smart engineers, I observed that there is a serious inverse relationship between smarts and empathy. I've managed very smart engineers who had very poor communication skills. Others were just too dumb to understand their precision offerings, and they were proud of it.

The most relevant thing you've said in this thread was, "(I agree that modifying such a battery is a serious engineering exercise.)"

My assumption, based on the format of the original question, is that the OP is not up for this exercise. If you want to guide him through this, that's great. I've got some experience here and would be glad to help. I object to vague statements that it'll be OK based on YOUR optimistic view of the OP's skills. This project is a minefield. Even if you think you know what you're doing, you might still blow yourself up.

I'm NOT dissing the OP. Asking questions is how we learn. I'm dissing the mentors who dismiss the significant personal risks in doing this without proper skills and equipment.

Here are more details on my project.

I bought a bike tail light.

It has a built in LiPo battery(about 800 Mah) but it only lasts a day before needing to be charged.

I plan on removing the battery and soldering wires to the board and run them to a set of 2 external protected 18650s.

Combined capacity will be about 5000 Mah.

Andy

Now it is you who is making the assumptions. Go away so that you don't discourage people.

Maker and model?

One "day" is how many hours. Do you run your bicycle tail light during daylight hours? Tail lights have various modes (continuous, slow flash, fast flash, random flash, power save flash). Which mode do you normally run yours and what's the approximate duty cycle? How many hours do you need, want, expect, or dream that the tail light will operate before demanding a recharge? How quickly do you expect to be charging from nearly dead to full charge?

Presumably, the two external 18650 cells are in parallel. You can charge two identical cells in parallel without much risk. It's when one cell starts to deteriorate faster than the other that the problems begin. The result is usually that the dead cell hogs most of the current which it then converts to heat, while the other good cell never charges. Instead of using two in parallel, I suggest either a really good single 18650 or single 26850 cell which greatly simplified charging and safety.

Careful here. The rated capacity of a LiIon cell is tested at a current drain equal to 20% of rated capacity (0.2C). For example: is tested at 0.65A, which is 20% of 3400ma (see Section 5). Discharging the cell at a higher rate, as you would in a bicycle headlight, results in lower capacity. Different manufacturers cells have different discharge curves and characteristics: You also run the risk of purchasing counterfeit or inferior cells: Here's my results testing SkyWolfEye 5800 ma-hr and an Ultrafire 5000 cells. Neither tests even close to their spectacularly inflated specs resulting in 890 ma-hrs and 1030 ma-hr at 1.5A respectively. The best cells I've tested usually test show about 2200 ma-hr at 1.5A.

If you plan to pursue this design or modding effort, I suggest that you:

1. Read the aforementioned Sanyo 18650 data sheet which is full of warnings and limitations on the use of their cells.
2. Dive into: (etc...) and do some reading. See what others have done to in areas that apply to your problem. Look at competitive products.
3. Do the calculations. If it doesn't work on paper, it's not going to work when you build it. The math is not that difficult if you make some simplifying assumptions, such as the "typical" white LED will produce about 90 lumens/watt (ignoring losses in lenses and reflectors).
4. You'll need some way to charge the cell. I suggest one of the numerous cheap TP4056 charger boards found on eBay such as:
5. Plan on sacrificing a few cells and tail lights to the gods of electronics. Burnt offerings are best, but other forms of immolation and destruction are acceptable. I have scorch marks on my Formica workbench to remind me of what can go wrong.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

This charger version is better: because it has overcurrent protection.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

See how much more relevant help you get when you SPECIFY what you're doin'. Your current plan sounds safe, as long as you don't solder wires directly to the cells or try to use the existing charger without first verifying that it terminates charge before exceeding the recommended maximum charge voltage of your new cells. Not a safety issue, but if it takes 8 hours to charge

800mAh, it would take 50 hours for the 5000mAh cells. Might want a new charger for that reason alone...then you have more stuff to validate for safety.

Lot to consider.

I bought UBL batteries and their charger. Is it a decent charger?

Got burned in the past with Ultrafire batteries.

I am doing more reading. I love learning. :-)

Andy

The ebay link you gave is for LiPO, unless it also works with 18650.

It takes 5 hrs to charge 800 mah battery using 500 mah charger.

EBL charger is much faster. Takes about 5 hrs for 2 batteries.

Andy

Is that better than this ?

I found a 5V switching adaptor. Could that be used for the 5V input?

Andy