L-ion Cell Configuration

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Unless you know what you are doing, it might not be a good idea to try to replace individual cells. As you probably know, lithium ion batteries can overheat if not charged properly, and if damaged, can catch fire.

Li-Ion chargers monitor temperature of each cell as well as voltage and current, and temperature monitoring requires a physical (thermal) contact with the cells.

I would strongly recommend reading up on Li-Ion charging.

http://www.homemade-circuits.com/2013/03/simplest-safest-li-ion-battery-charger.html

http://www.nxp.com/files/microcontrollers/doc/ref_manual/DRM043.pdf

I would advise caution. The battery chemistry of lithium ion being what it is, these cells are prone to all sorts of real nasty reactions when exposed to charging routines that are not just "right" and to discharge cycles that may also be not quite "right".

Mixing and matching cells together to form a new battery... may just be the impetus for a tragedy.

I'll second that. I store my Li-x batteries in a steel ammo box. They are fine when half charged for storage, but anything over OR under the proper voltage and they come apart...sometimes violently.

Shot in the dark....

http://www.ebikeschool.com/how-to-build-a-diy-electric-bicycle-lithium-battery-from-18650-cells/

I have some experience with this. DO NOT SOLDER! Spot weld the tabs only. It's the only way to attach the nickel strips without overheating the negative terminal of the LiIon battery. I made a simple spot welder for this from a microwave oven transformer and a variac. Works like a charm and I can upgrade the cells in my Ryobi 18v batteries with 10000maH LiIon 18650s.

Cool!! Of course just as with NICAD packs, tabs are usually spot welded at the factory. However, on this particular pack low temp solder for PC work is used. Cheers!

A spot welder from a microwave oven. Can you provide a picture, technical details and a schematic of your creation ? This necessity is the mother of invention, could be the start of a new blog.

I can give you a description.

The Transformer is opened up by carefully cutting away the fine wire high voltage windings and the 5v winding leaving the primary 120v winding in place. Then wrapping about 6 turns of #8 stranded and insulated wire through the opening to create a high current low voltage secondary. (I used the 8gauge cable from a 220v double oven for mine, but cheap jumper cables will work too). The more turns, the higher the voltage and less current. Mine can push 300A at 6.5vAC for about 5sec before it starts to heat up. That's enough to spot weld two quarters together!

Terminate the ends of the secondary with copper lugs and attach whatever cables and probes you want to those lugs for the welding end of the device. (I used 3/8" copper pipe and ground the ends at an angle to make probes and soldered them on the wire. Then I cut the heads off of a couple of hard concrete nails and crimped them into the cut probe end of the copper probe. Solder it in and viola! sharp spot probes)

The primary is wired through a foot switch and then to the output of the variac. (auto transformer for testing TVs and the like). The variac is used to dial up or down the voltage supplied to the transformer you just rewound.

Turn on the Variac and set it to 0, plug in the welder, make sure the probes aren't touching each other or anything else, dial up a bit of voltage, set your probes where you want to make your spots, and step on the switch. Turn it up if it ain't welding and try again.

That's it in a nut shell. I clamped my probes together with wood and insulated screws so the probe tips are close but not touching. The gap is about 1/8" and that is fine for spotting two spots at once on a 18650 battery.

Yeah, this style is familiar and ,obviously ,fundamentally like mine. However, the form is quite different. Thanks for that "shot".

You make a very solid point! Thanks

Whilst what you say is correct for NiCd and some other chemistry cells , it doesn't necessarily apply to Lithium based ones.

As many Electric vehicle nuts have found to their consternation, series connected Lis do not self balance, and it is very easy for one cell in the string to become either over or under charged while the entire pack still exhibits its correct terminal voltage - ie for a 12.8v 4 cell pack, one cell could be at 4.5v while the other 3 are at 2.8v, or alternatively one could be at 1.5v while the others are at 3.8v - in both cases the odd one out will likely be destroyed while the others have remained within safe operating parameters and are still quite viable.

The problem may only become evident when the internal resistance of the dead cell becomes sufficient to prevent adequate tool function, longer than normal charge times are a good general indication of high resistance in one or more cells.

This can occur just as easily with a new pack that is not correctly top balanced as with an older pack that has simply lost its balance.

Many of the cheaper chargers are temperature based - they measure the temperature rise of the pack to determine when the charge should be terminated. Many users kill their packs by re-inserting a fully or nearly fully charged pack - the pack becomes over charged before the device detects a sufficient temperature rise to stop the charge.

Not such a big deal replacing the entire pack when we are referring to smaller packs with cells like your 18650s, but can become a very expensive issue tossing perfectly good cells when we get up into some of the larger Prismatics.

Yes 2 different animals...

http://batteryuniversity.com/learn/article/low_voltage_cut_off