# Li-ion / lipo battery pack building and soldering 123



## -LightOn- (Jan 6, 2009)

Hi,
I have been reading this great forum for a while, but now I just had to jump in to ask few things I couldn't find answers for :wave:

I have a multimode bike/ski/run light project which is based around one P7 dbin led and a r2 drop-in. I am building the driver around a LM3401 and d2flex, but this is just useless background information.

My real concern is the battery packs. I was planning on building one light weight 2s pack and one larger 2s3p pack for those long rides. I have ordered some trustfire 2500mah 18650 cells for the packs with protection circuits.

I haven't seen too many instructions or experiences on soldering li-ion cells, but this is how I planned to assemble the packs:
First glue the cells together with epoxy. Then balance the charge on individual cells by placing a 10Ohm resistor between the terminals. After this pre-solder the battery terminals and connection tabs with a powerful soldering iron at 380C for about 2-3 seconds using flux. Do the same for the main and balancing leads. Solder the wires, tabs and terminals together for 2-3 seconds. Then top-off the cell by shrink wrapping it and giving it a nice neoprene or cordura casing.

Is this just doomed to fail?

Additionally, I bought a bantam bc-5 balancing charger and the manual says I shouldn't charge protected cells. I have understood that you can charge protected cells just fine?

I would be grateful for any advice, tips, experiences before I go and blow my head off while burning down my house


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## steve6690 (Jan 6, 2009)

I would recommend that you buy 18650's with tabs. They didn't need pre-balancing because they are shipped in a "storage" state of charge. They are unprotected cells so you will need a pcb to control overcharge/discharge and cell balancing. The charger manual is, I believe, telling that you shouldn't balance charge protected cells in a pack.


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## Fallingwater (Jan 6, 2009)

-LightOn- said:


> I haven't seen too many instructions or experiences on soldering li-ion cells, but this is how I planned to assemble the packs:
> First glue the cells together with epoxy. Then balance the charge on individual cells by placing a 10Ohm resistor between the terminals. After this pre-solder the battery terminals and connection tabs with a powerful soldering iron at 380C for about 2-3 seconds using flux. Do the same for the main and balancing leads. Solder the wires, tabs and terminals together for 2-3 seconds. Then top-off the cell by shrink wrapping it and giving it a nice neoprene or cordura casing.


You might find my guide helpful. 

Do note the disclaimer and safety reminder though.


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## -LightOn- (Jan 6, 2009)

Fallingwater said:


> You might find my guide helpful.
> 
> Do note the disclaimer and safety reminder though.



Thanks for the instructions. Exactly the confirmation I was looking for. It seems that I was on the right track anyway. Have you tried soldering packs consisting of protected li-ions? Any problems?


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## SilverFox (Jan 6, 2009)

Hello -LightOn-,

Welcome to CPF.

The main problem is that the high transient temperature during soldering may destroy the protection circuit. The protection circuit is a small chip that sits on the - end of the cell. You would be soldering directly to it.

You may be able to remove the protection circuits from the cells and approach the project as if you were using bare cells. Then you could simply add a global protection circuit for the whole pack. 

Keep in mind that soldering Li-Ion cells without tabs can be very dangerous.

Tom


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## Fallingwater (Jan 6, 2009)

-LightOn- said:


> Thanks for the instructions. Exactly the confirmation I was looking for. It seems that I was on the right track anyway. Have you tried soldering packs consisting of protected li-ions? Any problems?


I have as of yet not soldered directly to protected cells, so I can't tell you how the circuit takes this treatment.
I do have two bare protection boards from when I needed unprotected 18650s and only had protected ones; one I'll keep because I need it, but as soon as I get home I'll experiment on the other board by soldering a wire to the terminal. I'll then report on my findings.


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## -LightOn- (Jan 6, 2009)

How would a fried protection circuit be verified? Is it an open or short (hope not) circuit? At which end (terminal) is the circuit usually located and is it easy to remove?


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## SilverFox (Jan 6, 2009)

Hello -LightOn-,

I have had protection circuits fail both ways, but it is more common for them to fail open. You can get a voltage reading, but as soon as you apply a load it drops to 0.

The protection circuit is located at the - end of the cell. If you search you should be able to find a thread showing this. I seem to remember that member LuxLuthor was showing how the protection circuit was attached to the cell.

Removal is easy. Remove the outer layer of shrink wrap. This will expose the circuit and the ribbon wire running up the side of the cell to the + terminal. Now you have to disconnect the spot weld connections from the circuit board to the cell being careful not to short things out. Now you simply need to re-shrink wrap the cell and you end up with a bare cell.

Tom


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## Fallingwater (Jan 6, 2009)

I want to stress the danger of shorting the metal strip connecting the positive terminal to the board. When I removed the protection from my 18650s I found the metal strip to be insulated from the body with a strip of paper-like stuff only marginally wider. In my experience the best way avoid shorts is not to remove the whole plastic sleeve around the cell and then remove the circuit; rather, to cut only the lower part of the sleeve, cut/rip off the negative tab, then grab the circuit firmly and tug. This way the strip itself will tear open the sleeve, and since you're pulling away from the cell the risk of it contacting the negative body is minimized.


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## -LightOn- (Jan 7, 2009)

Hmm... this is getting more complicated than I thought. I had planned of just using protected cells in the pack to be safe, but this would mean I have parallel protection circuits. Now if one fails and the others don't I could be in for trouble. Would you rather just use individual protected cells or remove the protection circuits, solder the cells and then assemble 2 separate protection circuits to the balancing leads.


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## Fallingwater (Jan 7, 2009)

I'm sorry, I cannot give you a decisive answer before I try soldering to a circuit, and this will have to wait a few days.

One thing that will work is solder the unprotected cells in parallel, then connect the parallel pack to a single protection circuit extracted from a previously protected cell, which can then be connected in series with circuits from other packs done in the same way. Once the circuit is removed you can trace the connections from the terminals, so you can solder to the board directly and avoid damaging components.

Note that this only works if the current draw is low enough that a single circuit won't give you trouble - if, in other words, your light would work from a single cell, and you're using a parallel pack just for runtime, not for getting more current out of them.

Also note that the cells need to have the same resting voltage before you put them in parallel, or damage can occur.


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## -LightOn- (Jan 7, 2009)

Fallingwater said:


> Note that this only works if the current draw is low enough that a single circuit won't give you trouble - if, in other words, your light would work from a single cell, and you're using a parallel pack just for runtime, not for getting more current out of them.
> 
> Also note that the cells need to have the same resting voltage before you put them in parallel, or damage can occur.



Yup. Thanks for the advice. I'm not sure if the 2.8 Amps rated for the P7 is exactly low, but in the lower capacity pack the p circuits will have to handle 2.8A anyway so single protection circuit should do in the parallel pack also.

I was planning on evening the resting voltages with a resistor.

I'm sure this one has been answered earlier elsewhere but can the over discharge protection be left to the protection circuits? The driver circuit itself has no low input voltage protection or cut-off.


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## LuxLuthor (Jan 7, 2009)

Here is my thread showing PTC's. 

I don't think it would be reliable (as in damaging the PTC) to solder to the bottom of PTC. Note in these pics that the cell connections are made to the cell -/+ ends with welded nickel contact strips, which in turn are solder to contacts on PCB. The contact strips and PCB are insulated from cell. It won't take very much transferred heat from soldering PTC ends to melt thin insulator from + contact strip running down outside, and dead short against battery can.

For this reason, and the deleterious effect it MAY have on the actual PCB/components/solder points, I could never recommend soldering to a protected Lithium cell. I have made Lithium packs with individual AW protected cells using my battery pack welder, using thinner nickel strips and on reduced weld energy settings. There have been others who at their own peril, have soldered to unprotected Lithium Cobalt cells, but personally, I would never do that.


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## Fallingwater (Jan 7, 2009)

-LightOn- said:


> I was planning on evening the resting voltages with a resistor.


No need.
Charge the pack fully, then set the charger to single cell (or use a single-cell charger) and hook it with small magnets on each cell on the pack, starting a new charge every time.
This will insure all cells are charged to full capacity, and shouldn't take long because the cells will be close to that point already.



> I'm sure this one has been answered earlier elsewhere but can the over discharge protection be left to the protection circuits? The driver circuit itself has no low input voltage protection or cut-off.


I assumed that was why you wanted the protection circuits in the first place.


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## -LightOn- (Jan 10, 2009)

Ok, I got most of the kit already so I can start this project.

I got the bantam bc-5 and converted an old atx-power to give 12V and enough amps for it. (Conversion was a fun little project itself )

First thing was to measure the resting voltages of all the cells:

1 3.985
2 3.797
3 3.796
4 3.981
5 3.796
6 3.983
7 3.802
8 3.976

So all the cells were over the nominal 3.7V. First I took the first cell and charged it alone. End result 4.20V and 610mAh into the cell. Then I took the cells 2 and 3 since they had equal resting voltages and charged them together. This time the result was 4.20V and 4.22V measured with a DMM. And the charge put to the two cells was 3100mAh which is 1550mAh per cell. There is a huge difference in the amount of charge put to the cells between 1, 2 and 3 even though the difference in resting voltage was only 200mV. Do the amounts put to the cells sound correct? Additionally I am concerned with the 4.22V of the third cell. The manual said that the li-ion will explode if the voltage of the cell reaches 4.25V which isn't too far away from the last figure.


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## -LightOn- (Jan 10, 2009)

Update: I found that by doing a constant current charge and constant voltage charge for a pair to the point where the charge current has dropped to 1A work great. After this the cells are around 4.1V. Then top off the cells individually to 4.2V. 

It seems that the first cell (from currents perspective) gets more charge because the minute resistance of the charge leads between the cells is enough to cause a difference in loading currents. (Kirchoff's laws)


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## Packhorse (Jan 10, 2009)

If you get protected cells and remove the PCB you can then solder onto the Nickel strip that was used for the PCB. Then use a single PCB externally.


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## waddup (Jan 10, 2009)

i connected 3 x rcr123s together with 'painters tape' (just rolled em up together like a cigarette) yesterday, to make 1 long 11.1v cell, charged it on my 'intelligent' trition charger, no problems, its works well.

why solder?


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## -LightOn- (Jan 10, 2009)

waddup said:


> why solder?



Well... I guess the electrical connection is a LITTLE bit more reliable than just bare tape.


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## Fallingwater (Jan 10, 2009)

Tape will give you a very unreliable connection.


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## waddup (Jan 11, 2009)

-LightOn- said:


> Well... I guess the electrical connection is a LITTLE bit more reliable than just bare tape.





Fallingwater said:


> Tape will give you a very unreliable connection.



the pressure from the spring/s keep the cells together in the body of the light, the tape only allows me to remove and charge 3 x 123 as 1 cell= easy.


and i can remove the tape and reconfigure cells easily.


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## -LightOn- (Jan 11, 2009)

waddup said:


> the pressure from the spring/s keep the cells together in the body of the light, the tape only allows me to remove and charge 3 x 123 as 1 cell= easy.
> 
> 
> and i can remove the tape and reconfigure cells easily.



Aah... okay. I see you haven't read the whole thread. I'm not going to have any springs. The intent was to build a custom battery pack that can be put the back of my bike riding shirt or straight to a bike. The form factor of the pack is going to be 65mm x 36mm x 54 mm. So all the cell are side-by-side in 2x3 configuration.


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## Fallingwater (Jan 11, 2009)

waddup said:


> the pressure from the spring/s keep the cells together in the body of the light, the tape only allows me to remove and charge 3 x 123 as 1 cell= easy.


So why use tape at all then?


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## -LightOn- (Jan 15, 2009)

I'm happy to report that the first 2s-pack with protected trustfire cells was a success! I hooked up the battery to my sscp7 driver and drove 2.8A to led with no problems.

So no  ... at least this time.


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## Fallingwater (Jan 15, 2009)

So you soldered to the negative (protected) terminal and nothing went wrong?
Might want to check that the protection circuit hasn't stuck closed though. Discharge a cell to its limit and see if the circuit kicks in.


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## -LightOn- (Jan 15, 2009)

Fallingwater said:


> So you soldered to the negative (protected) terminal and nothing went wrong?
> Might want to check that the protection circuit hasn't stuck closed though. Discharge a cell to its limit and see if the circuit kicks in.



Yup. When the protection kicks in, is the voltage read-out then 0V or 5.5V? I would assume that there is a transistor switch that controls the cell so even when its closed you get the 5.5V which is the low limit for the 2s-pack. If you stick a 100 Ohm resistor between the terminals you should get 0V due to voltage division?


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## Fallingwater (Jan 15, 2009)

It goes open circuit.


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## -LightOn- (Jan 16, 2009)

And the 2s3p-pack with protected cells is done and ok. So I guess there is a big fuzz about nothing when it comes to soldering li-ion packs (even protected)


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## LuxLuthor (Jan 17, 2009)

-LightOn- said:


> And the 2s3p-pack with protected cells is done and ok. So I guess there is a big fuzz about nothing when it comes to soldering li-ion packs (even protected)



Yeah, right. Nothing to worry about. :shakehead Fallingwater will be right there by your side cheering you on. You see now how lightly they take all this soldering advice?

Oh well, I give up on trying to bring some rationality into not soldering Lithium Cobalt Ion cells--with protection circuit to boot! OMG!


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## paulr (Jan 17, 2009)

I don't understand why even bother with a homemade packs for those configurations. 2s and 2s3p 18650 packs made for camcorders are very common and cheap. Sony NP-F550 and NP-F970 and off brand versions are easy to find.


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## Fallingwater (Jan 17, 2009)

LuxLuthor said:


> Yeah, right. Nothing to worry about. Fallingwater will be right there by your side cheering you on. You see now how lightly they take all this soldering advice?


Would you please stop behaving like I'm encouraging everyone to put a loaded gun to their head and pull the trigger?
The guide which, I presume, he followed to solder his pack clearly states that soldering to LiIon cells can be dangerous, and that you should be prepared to deal with a lithium fire in the unlikely event it occurs. There's also a big fat disclaimer. If he reads it and chooses to proceed anyway, he takes the responsibility of his own actions.



> Oh well, I give up on trying to bring some rationality into not soldering Lithium Cobalt Ion cells--with protection circuit to boot! OMG!


I agree on the protection circuit. Since he's done it already, he should test it to make sure it works before using the battery.
I'm going to add a paragraph or two about protection circuits to my guide.
Edit: done.

Paulr: those batteries (even off-brand ones) tend to cost a lot more than two 18650s, and then you have Sony's InfoLithium circuits that can complicate things.

What we really need is 18650 holders.


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## paulr (Jan 17, 2009)

There are any number of off brand vendors selling NP-F550 (i.e. 2x18650) packs in the $10-15 range and NP-F970's (6x18650) for $30-ish. You can also get an 8x18650 (4p2s) EEE PC laptop battery pack from Deal extreme for $50. That's not much premium per cell for a preassembled pack with all kinds of monitoring stuff and separate charge contacts per cell.


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## -LightOn- (Jan 17, 2009)

Fallingwater said:


> Would you please stop behaving like I'm encouraging everyone to put a loaded gun to their head and pull the trigger?
> The guide which, I presume, he followed to solder his pack clearly states that soldering to LiIon cells can be dangerous, and that you should be prepared to deal with a lithium fire in the unlikely event it occurs. There's also a big fat disclaimer. If he reads it and chooses to proceed anyway, he takes the responsibility of his own actions.



Exactly so. I took the risk - did the soldering with some good equipment and succeeded.



Fallingwater said:


> I agree on the protection circuit. Since he's done it already, he should test it to make sure it works before using the battery.



Yeah. I have to be careful for the first couple charge/discharge cycles. I went outside for a bike ride for 1,5 hours with the 2s-pack and did about half an hour with the 2.8A mode on since I wanted to sure that I would over discharge the pack now. At home the pack was at 7.4V so still plenty left. I am currently discharging some more to find out weather the low voltage circuit works.

At which voltage point should I get worried/abort? The cells state 2.75V low so I'm guessing 5.5V at this point.


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## -LightOn- (Jan 17, 2009)

Okay,
something happened with the battery pack. I had the light on and the last measurement was 7.4V. I thought the battery would still last a long while, but after 45min or so I noticed the light turned off. I measured the voltage across the 2s-pack and got 1.8V?? I thought that was pretty weird. Then I disconnected the battery and measured again, now 3.5V. 

I started charging the pack with my balancing charger with 0.3A current. The charger showed that the cells were at 3.4V and 3.6V in the beginning. So I guess I'm all good with this pack?  Now, the pack is charging and both cells are at 4.12V.

cheers.


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## bretti_kivi (Jan 17, 2009)

yeah, you got the low voltage rating under load, and the 3.5 was without load. 

Bret


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## -LightOn- (Jan 17, 2009)

bretti_kivi said:


> yeah, you got the low voltage rating under load, and the 3.5 was without load.
> 
> Bret



This is kinda what i figured too. But immediatly when I started to charge, the cell showed much higher voltage.


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## LuxLuthor (Jan 17, 2009)

Fallingwater said:


> Would you please stop behaving like I'm encouraging everyone to put a loaded gun to their head and pull the trigger?
> 
> The guide which, I presume, he followed to solder his pack clearly states that soldering to LiIon cells can be dangerous, and that you should be prepared to deal with a lithium fire in the unlikely event it occurs. There's also a big fat disclaimer. If he reads it and chooses to proceed anyway, he takes the responsibility of his own actions.
> 
> ...



Fallingwater, the problem with typing responses and forgetting to use smilies is obvious from your (understandable) reply. I apologize. I actually do respect how you have gone out of your way to provide warnings, so I meant that in a "Larry David - Curb Your Enthusiasm TV Show" kind of humorous way.

I was drawing attention to -LightOn-'s incorrect assumption of: "So I guess there is a big fuzz about nothing when it comes to soldering li-ion packs (even protected)." There is a tendency for people wanting to do this to view it in an overly optimistic manner. Where I am coming from when I post my warnings, I am picturing the first person doing such soldering, and the explosive reaction putting his eye out, or worse.

There can be immediate effects when soldering Lithium Cobalt Ion cells (especially with protection circuits), but if you get through that apparent hurdle, it does not mean the heat damaged cell and PTC are going to be safe later.


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## -LightOn- (Jan 18, 2009)

LuxLuthor said:


> I was drawing attention to -LightOn-'s incorrect assumption of: "So I guess there is a big fuzz about nothing when it comes to soldering li-ion packs (even protected)." There is a tendency for people wanting to do this to view it in an overly optimistic manner. Where I am coming from when I post my warnings, I am picturing the first person doing such soldering, and the explosive reaction putting his eye out, or worse.
> 
> There can be immediate effects when soldering Lithium Cobalt Ion cells (especially with protection circuits), but if you get through that apparent hurdle, it does not mean the heat damaged cell and PTC are going to be safe later.



Perhaps I shouldn't have said that. Certain precautions must be taken when soldering (like wearing goggles and having fire extinguisher ready) and I don't want to dismiss the risks that are involved (that's what the smiley was for). 

I'm not sure what you meant by the last sentence. "...it does not mean the heat damaged cell and PTC are going to be safe later."


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## LuxLuthor (Jan 18, 2009)

-LightOn- said:


> Perhaps I shouldn't have said that. Certain precautions must be taken when soldering (like wearing goggles and having fire extinguisher ready) and I don't want to dismiss the risks that are involved (that's what the smiley was for).
> 
> I'm not sure what you meant by the last sentence. "...it does not mean the heat damaged cell and PTC are going to be safe later."



Lighton, same respect is accorded to you. My intention to point out safety issues is noble, if not adequately diplomatic.

Most fire extinguishers will have no effect on a Lithium metal fire...some will actually worsen it. Some of that is covered in the PC Pitstop video I linked last post. The force of a pressure release venting explosion with flames would blow through a typical set of lab goggles. There are other videos where they blew through the sides of steel metal ammo cans.

My last sentence relates to the excessive heat of soldering goes beyond design tolerances of the internal lithium plastic layer separators that eliminate shorts, also the plastic temp/pressure PTC. The insulation of the metal protection strip shorting against the negative can as it runs down the side of battery only consists of a layer of shrink wrap or 1 mil Kapton tape which is very easy to melt through. 

Partial damage can be done to the PCB protection circuit components leading to an early failure. There are also the issues beyond low voltage protection, namely current limit, overcharge protection, and overheating protection. Any of those systems may have been partially or fully damaged from the soldering heat leading to abrupt catastrophic failure at an unknown time later.


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## Fallingwater (Jan 18, 2009)

LuxLuthor said:


> Fallingwater, the problem with typing responses and forgetting to use smilies is obvious from your (understandable) reply. I apologize. I actually do respect how you have gone out of your way to provide warnings, so I meant that in a "Larry David - Curb Your Enthusiasm TV Show" kind of humorous way.


Ok then, no problem 

As for the protection circuits, if I were in LightOn's shoes I'd uncover them and carefully check that all components are where they should be, and that there aren't any broken solder joints. I would also disconnect them, run a full test with my bench power supply and resolder them to the strips after I'm convinced they work right. He probably doesn't have a bench power supply though.


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## -LightOn- (Jan 18, 2009)

Fallingwater said:


> Ok then, no problem
> 
> As for the protection circuits, if I were in LightOn's shoes I'd uncover them and carefully check that all components are where they should be, and that there aren't any broken solder joints. I would also disconnect them, run a full test with my bench power supply and resolder them to the strips after I'm convinced they work right. He probably doesn't have a bench power supply though.



Actually... I do, and a whole bunch of other electronic measuring equipment at work :twothumbs

But, I have glued the cell together and shrink wrapped them so I am very reluctant to open up the cells.


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## bretti_kivi (Jan 18, 2009)

Running with this theme: I can get (via conrad.de) 18650 protected 2300mAh cells. Allegedly Samsung ones.
Now, I want to run a 3d and 2d mag or two with LEDs (not sure precisely which ones yet, it's not really relevant here). 

If I were to get some of these cells (they have flying leads), how do I make sure that they stay balanced within the pack when I've built it myself? Do I / Can I assume that the charger will take care of this? The 3 hour recharge time with the Ansmann chargers I can find / the cell specs is kinda OK, but I also then don't really see the advantage over 2700mAh sanyos bunched together. OK, they're lighter, but....that's about it.

I understand it's slightly offtopic, but I think it's relevant.

Bret


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## -LightOn- (Jan 18, 2009)

Assuming you are going for two/three cells in series, you need balancing lines out from the pack to the middle nodes of the pack. Then you can use a balancing charger to charge the pack. (Charger monitors individual cell voltages and evens them if they are different)


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## Fallingwater (Jan 18, 2009)

-LightOn- said:


> Assuming you are going for two/three cells in series, you need balancing lines out from the pack to the middle nodes of the pack. Then you can use a balancing charger to charge the pack. (Charger monitors individual cell voltages and evens them if they are different)


Don't need a balancing charger. If you have an outboard charger, you first let it charge the pack to full power, then set a slow charge on every cell through the balancing leads. This will top them off and make sure they're full.
Granted, ona 6s pack this would get old pretty soon... but 2s or 3s isn't really a problem.


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## LuxLuthor (Jan 18, 2009)

The best procedure is to have separate balance tap leads, and use a balance charger. Probably the easiest, "relatively affordable," ready to plug in and use is the eStation BC6.


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## Fallingwater (Jan 18, 2009)

(whoops, misread a post, ignore)


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## bretti_kivi (Jan 19, 2009)

ok, so from the join between each cell, I have a cable out; 

1 - black -
2 - cell 1 +, cell 2 -
3 - cell 2 +, cell 3 -
4 - cell 3 +

and I could snake that out to a four pin connector on the base of the mag, so I don't have to open it up.

Right?

thanks!

Bret


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## Mr Happy (Jan 19, 2009)

-LightOn- said:


> Okay,
> something happened with the battery pack. I had the light on and the last measurement was 7.4V. I thought the battery would still last a long while, but after 45min or so I noticed the light turned off. I measured the voltage across the 2s-pack and got 1.8V?? I thought that was pretty weird. Then I disconnected the battery and measured again, now 3.5V.
> 
> I started charging the pack with my balancing charger with 0.3A current. The charger showed that the cells were at 3.4V and 3.6V in the beginning. So I guess I'm all good with this pack?  Now, the pack is charging and both cells are at 4.12V.


If the 2s pack was at 7.4 V that means 3.7 V per cell. By normal reckoning that means the pack was completely discharged. What led you to think it would still last a long while after that?

So you discharged the 2s pack down to 0.9 V per cell. Which suggests the protection circuit did not cut out in time.

Also, 0.9 V on a lithium cobalt cell is in the danger zone. That is where internal changes can happen to the cell that may cause it to explode when recharged.

I also do want to comment on the "I did it and nothing bad happened so it must be safe" line of thought. That is completely false logic. It is like saying "I lit a cigarette in a gas station while fueling my car and I did not get blown up, so all this banning of lighted matches in gas stations must be a big fuss about nothing". Just because nothing bad happened so far does not mean that there is no risk of something bad happening in future.


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## -LightOn- (Jan 19, 2009)

Mr Happy said:


> If the 2s pack was at 7.4 V that means 3.7 V per cell. By normal reckoning that means the pack was completely discharged. What led you to think it would still last a long while after that?
> 
> So you discharged the 2s pack down to 0.9 V per cell. Which suggests the protection circuit did not cut out in time.
> 
> ...



Hmm...you got me there. Although I already took away that "...a big fuss about nothing" statement. It can be that I damaged the cells. But, if I improved my driver instead to include a low-voltage protection circuit there? I could use the dim-input to shut off the light when voltage goes bellow 7V? This would be fun.. :devil:

Are you sure that my cells were discharged to 0.9V each? Wouldn't you get a low voltage at the output due to voltage division when the protection switches open? Or should I get 0V reading instead?


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## -LightOn- (Jan 25, 2009)

I did a discharge measurement for the 2s-pack. The pack was not fully loaded so the starting voltage is lower. Discharge current was about 200mA.

Here is what I got.






The measument ended right after the last 5.8V reading by the LED turning off. Then I measured 1.85V across the battery as before and 3.5V from a disconnected battery. Now the battery has been resting for 1-2h and the voltage was 3.63V.

My guess is that the protection circuit works because according to spec the d2flex and LM3401-based driver can operate under 4.5V so I should see the battery voltage falling even lower if the protection circuit would not cut in?

Now I just have to test that my larger battery (2s3p) does the same.


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## Fallingwater (Jan 25, 2009)

The circuit certainly seems to be working.


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## rpage53 (Jan 25, 2009)

Mr Happy said:


> So you discharged the 2s pack down to 0.9 V per cell. Which suggests the protection circuit did not cut out in time.


Not likely. Many circuits bypass the batteries so that the measured voltage doesn't represent the actual cell voltage. You have to put the pack on the charger to reset the circuit. Some chargers, however, won't recognize a pack that measures this low.

Sounds like the OP successfully achieved exactly what he wanted despite the naysayers and incorrect advice. Larger battery manufacturers won't sell individual lithium ion cells because the risk to a naive user is too great but one of the values of this board is to provide the knowledge that modders require.

I salvage protection circuits from old packs and use them with new batteries so that they can be charged on dumb, non-balancing chargers.

Rick.


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## -LightOn- (Jan 28, 2009)

-LightOn- said:


> Now I just have to test that my larger battery (2s3p) does the same.



Testing of the larger pack done and everything went as with the smaller 2s-pack. I got kind of worried though because the battery voltage kept falling and falling down to 5.3V before the protection cut in. I thought about this and it seems that the smaller the discharge current is, the lower the cut-off voltage is because internal resistance does not affect the voltage as much.



rpage53 said:


> Sounds like the OP successfully achieved exactly what he wanted despite the naysayers and incorrect advice. Larger battery manufacturers won't sell individual lithium ion cells because the risk to a naive user is too great but one of the values of this board is to provide the knowledge that modders require.
> 
> Rick.



Couldn't agree more :thumbsup: That's what this place is all about! To test, experiment and then share that knowledge with others so that they can do it better.


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## Fallingwater (Jan 28, 2009)

-LightOn- said:


> I thought about this and it seems that the smaller the discharge current is, the lower the cut-off voltage is because internal resistance does not affect the voltage as much.


This is exactly the opposite of what should happen.



rpage53 said:


> Sounds like the OP successfully achieved exactly what he wanted despite the naysayers and incorrect advice.


SilverFox and LuxLuthor are just trying to prevent people from hurting themselves. You may or may not agree with their opinions, but they are not naysayers.


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## -LightOn- (Jan 28, 2009)

Fallingwater said:


> This is exactly the opposite of what should happen.



What I wrote was wrong. Of course the external cell voltage is lower at higher currents due to resistance. What I meant was that without knowing the electronics behind the cut-off circuit I am guessing that with lower current the cut-off voltage can be allowed to be lower.


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## Mr Happy (Jan 28, 2009)

-LightOn- said:


> What I wrote was wrong. Of course the external cell voltage is lower at higher currents due to resistance. What I meant was that without knowing the electronics behind the cut-off circuit I am guessing that with lower current the cut-off voltage can be allowed to be lower.


No, it is exactly the opposite of this. At higher currents the cut-off voltage could be allowed to be lower and at low currents the cut-off voltage should be higher. However, I don't believe any existing protection circuits are this sophisticated. They just have a single cut-off voltage regardless of the applied load.


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## -LightOn- (Jan 28, 2009)

Mr Happy said:


> No, it is exactly the opposite of this. At higher currents the cut-off voltage could be allowed to be lower and at low currents the cut-off voltage should be higher. However, I don't believe any existing protection circuits are this sophisticated. They just have a single cut-off voltage regardless of the applied load.



Ok, then I'm just plain wrong . Well, I just reported what happened.

Another thing... I've been charging the 2s and 2s3p-packs with 0.5-0.8C (2Amps for 2s and 4Amps for 2s3p). When the charge is complete the battery reads out about 8.35V, so I restart with 0.1-0.3A and the charger completes in about an hour and voltage reads out around 8.39-8.4V. Now I understand that NiMh-chemistry benefits from the trickle charge at the end, but is the situation the same with Li-ions as well?


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## Fallingwater (Jan 28, 2009)

As long as voltage doesn't exceed 4.2V per cell no harm is being done.


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## jambraun (Oct 14, 2015)

Hey all, new user on the forums. I couldn't find a definitive answer for my PCB wiring so I thought I'd reopen this topic.

I'm trying to wire together a 14.8v Li-Ion battery pack (easy part) and properly connect it to a PCB (questionable). Here's what I'm using:

http://www.batteryspace.com/pcbfor148vli-ionbatterypack5alimitwithfuelguagesocket.aspx
http://www.batteryspace.com/prod-specs/2778.pdf

My question is, how do I properly wire my battery pack to the PCB? In my first attempt I enlisted the help of an experienced solder tech that gave me the basics and helped me build my pack but I don't believe we connected it to the PCB properly. The pack was built using 4 li-ion 3.4v batteries in series to up the voltage to 14.8v, and then wires were soldered from the pack at each point to the PCB. The PCB registered voltage on the fuel gauge so I know the PCB was was able to detect power properly, but I feared since the battery pack was already wired in serial and we only had wires coming off the pack at the designated points, that there was actually no protection being provided. I've since dismantled the battery pack because I wasn't comfortable with the configuration.

Can someone offer suggestions on how the battery pack should actually be constructed for use with a PCB so that the protection is offered during charge and discharge? This should be true for all PCB's and not just this one so it may be useful to others as well. Thank you!

--Jarred


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## Gauss163 (Oct 14, 2015)

@jambraun Click on "Download Spec" on the PCB page and you will find a diagram showing how to connect the batteries to the PCB. Ah, I just noticed that is your 2nd link. Is there something that is not clear in that diagram? Why do you think that the protection wasn't working? Was the voltage measured between P+, P- correct?


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