Soldering 18650 or not

[Benoit]

Hi,

Im planing my build with 12 x 18650 to create a 14.8v 9ah cell.

And of course i need to wire them up together !
My first idea was:
using 6 2X18650 holder from DX, easy to use, may use them for another build, no soldering on therminal, and i could recharge my cell one by one with my 18650 charger that a already own.
But i heard from another people that i will lose alot of power by doing that. What is your tought ?

My 2nd idea was:
The normal (but not my best) way to solder them all using big gauge wire !
That make me nervous like hell, and hard to clean after, using solder wick, have to use a dedicated battery charger

But recently i was tinking of anoter idea, pretty clever i think !
Make my solder on small rare earth magnet , like those we use for flat head battery so i will be able to re-use all my cell for another build or in flashlight !

What do you think could be the best solution !

 

[Packhorse]

I think you have listed your options from best to worst.

Use the battery holder if you have space. Upgrade the wires if you can/ need to.

Soldering to cells is frowned upon. But you can get cells with tabs welded on. These cost more for quality cells. Failing that if you must solder then use good resin core solder and a real hot iron and do it as quick as you can to reduce heat build up on the cells (this is what I do).

Using magnets will create a lot of power loss. Dont go there.

 

[Benoit]

So i will not lose power with all those spring if i use Holder ?

 

[hellokitty[hk]]

So i will not lose power with all those spring if i use Holder ?

Not if you make sure the connections are good and thick.
I think the last idea is terrible and could result in magnets sliding off and shorting somewhere, and would also result in high contact resistance.
IMO soldering with balance leads are best if you think you can do it.

 

[DIWdiver]

I'd be very careful of battery holders. I built a product using a 3AA holder from a very well known manufacturer. When it wouldn't work, I finally discovered that the 3-cell holder had a total contact resistance of one ohm! That's a lesson I won't soon forget. I'm sure it's why my first dive light kept giving low battery indications when I knew it still had plenty of juice.

The problem is in the springs themselves. The pack I had was made with conical coil springs, made of stainless steel. Cheaper ones are made with nickle plated music wire, AKA spring steel or piano wire. That's actually somewhat lower resistivity. There's a remarkable length of high resistance wire in those springs. Even the wire that ran diagonally across the pack to connect the opposite corners had noticable voltage drop at 1A. They just weren't made for ampere-level discharge rates.

Looking at those packs at DX, I'd wager your full battery pack would have 2-5 ohms just in the battery springs.

The type of contact you want is a leaf spring, which is stamped and bent from a flat piece of sheet metal. Most are spring steel, a few are phospor bronze, which is much lower resistivity. Because they are so much shorter and wider, the resistance is much lower, even for spring steel. Check out these babies: http://www.keyelco.com/products/18650-holders.asp I imagine the spring resistance would be in the tens of milliohms for 12 cells, based on my measurements of the phosphor bronze springs I finally ended up with, from this manufacturer.

 

[hellokitty[hk]]

Wow nice link to those holders they look pretty good.
I'd say something like that would be less of a hassle than soldering!
Of course soldering would give you the lowest resistance but IMO the difference would be negligible.

 

[DIWdiver]

Yes, soldering will provide the lowest resistance, if you use the right wire. Next would be welding. Next would be something like these. The differences would only be important if you were doing direct drive or some other design that had no overhead.

And these are SO much easier!

 

[demonic69]

You could build your own tab welder, a few decent tuts on the net

 

[tompijls]

Check out these babies: http://www.keyelco.com/products/18650-holders.asp I imagine the spring resistance would be in the tens of milliohms for 12 cells, based on my measurements of the phosphor bronze springs I finally ended up with, from this manufacturer.

Are these serial or parallel or both (referring to the 2 cell holders)? Interesting topic

 

[DIWdiver]

Neither. They're independent.

 

[Packhorse]

Another option is to buy a battery pack like this and remove the cells from it. Im guessing they will be tab welded.

 

[DIWdiver]

Great idea! Never thought of that.

 

[Packhorse]

If anyone buys this pack I would love to know if they are 2600ma Sanyo cells! I need to build a really big pack! (500wh+)

 

[350xfire]

Heck that pack is cheaper than buying the loose cells!

 

[Packhorse]

Yeah, I am guessing demand for that laptop battery pack is pretty low and that is why they are selling them off cheap.

But also consider that the pack may be a couple of years old and Li Ion cells dont last forever.

So perhaps not the deal it appears to be.

 

[350xfire]

True!

 

[demonic69]

Cheaper just to salvage the cells from duff laptop batteries, I've got dozens I've collected over the last year, all for free. Sometimes people even sell them as faulty on ebay!

 

[Packhorse]

Cheaper yes, but what are the condition of the cells? As I said before, Li Ions dont last for ever.

 

[SmokedCPU]

I got many laptop battery, and it seem that some board when detecting a fault condition, drain the cells to 0v to eleminate the risk of fire.
they are not made to be repaird .... just throw and buy an other. So i only take fresh 'suddent death' battery; cells are more likely to be in good condition.

 

[SemiMan]

I am trying to find it so I can take a picture for you, but I built a battery pack for a bike light a while back using approximately 3/4" ID plastic pipe. It fit 18650's almost perfectly. I had 4 in series. I DID use springs for the contacts at the end, but the trick was to solder the connection wire to near the top of the spring, not the bottom of the spring to eliminate as much resistance in the spring as possible. It can also be good to use contact enhancer.

Semiman