2P 18650 pack went open circuit.

You're gonna have to be a LOT more descriptive about the 'pack'. passive/active/protected? What did it power?

Just a bare 2P 18650 pack powering an e-cig with a 1 Ohm coil.

The only electronics is a power MOSFET where there used to be a microswitch for the fire button.

Charging is controlled by a shunt regulator, there is a feed SB diode so the shunt failing short can't harm the battery.

That seems to contradict the statement the the only electronics is the MOSFET. I don't know what to make of that... And something has to control the MOSFET

You're saying that the shunt regulator failing open can't harm the battery? You're saying that the SB diode, guessing that it's a series diode from regulator to cells, can't fail short and harm the battery?

The 18650 cells they put in cordless drills can do 30 amps each. The cells you get on ebay for cheap may not like 4 amps at all.

Assuming this is a commercial battery pack and not something you cobbled together. Assuming that the cells are bare unprotected cells. Assuming that you're measuring the cells directly at the cells and they are open.

There's not much that can go wrong. There's typically a pressure switch inside the cell that opens if the pressure gets too high or the positive terminal gets too hot.

Some people believe that you can take random cells, solder wires on 'em and you're good to go. That dramatically increases the failure rate, assuming you don't set them on fire while soldering.

I'm not a smoker, but it's hard to imagine a successful e-cig that's as big as a pair of 18650's. Is there more to the story?

Here's how to fix it: Something may have shorted one or both cells.

If you're having problems charging cells which exhibit low terminal voltage (under about 2.5v), try this:

--
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

I came away with this clever tip: use small magnets to hold test leads to the ends of cells.

Yep. However, I suspect that may have been an accidental discovery. When one salvages 18650 cells from a laptop battery pack, the resulting cells do not have a projecting button top on the positive electrode. Getting such a cell to make a proper connection in a battery holder or flashlight is problematic. So, someone determined that a tiny magnet will stick to the positive terminal and provide a suitable button top contact (and test lead connection):

--
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

Yes - a tact switch salvaged from the front panel of a Sky box.

It helps if you read before replying.

A shunt regulator failing short circuit (the most likely fault) wouldn't do the battery any good at all - A SB diode in the feed makes that accident unlikely.

Every day is a school day.

I might have to rummage those cells out of the bin and do an autopsy - but conscious enough of potential hazards to not bother re using them though.

Plenty replacements to hand, I just assembled another battery pack - but that seems to answer my question; what happened.

Or you can get the ones not specified as battery extenders

for 3-cents each in hundreds.

If you get some square magnets, you can wrap a piece of battery tab material around them. Leave some sticking out so you can solder a wire or put a clip on it. Works for charging all types of batteries with magnet-attractive connection points. And the current doesn't go through the magnet or depend on the surface plating.

I wouldn't put a lot of faith in the assumption that your regulator won't fail open. I suggest that a bad solder joint might be more likely than an actual component failure.

The diode also solves the problem of disconnecting the power source while the battery is connected.

But it adds another failure mode. A shorted diode will cause the battery voltage to exceed the safe max voltage. I'd also worry about the forward voltage vs forward current curve for the exact diode you're using. Depending on the charging current and the capability of your diode heat sink, the junction temperature might get well above ambient and need to be considered. A diode conducts all the way to zero volts. It's easy to assume that the current is small enough to be ignored, but VERIFY that's the case in your application. Look at the curves for your diode over temperature. What's the maximum battery voltage if you left it on charge for a week.

If you want to charge it as quickly as possible to the highest capacity, typically 4.2V depending on chemistry, you might find that it's not possible to do safely with a diode inline. Backing the max voltage off 200mV to be safer seriously impacts run time, but it's probably a good idea.

Another alternative is to set your shunt regulator to 4.2V and put a resistor across the diode. You get quickly to 3.9V or so. Takes longer for full charge but can be much safer.

Lithium batteries are usually charged right up near the destruction limit. There isn't much margin for error. If you have a limited supply of paranoia, charging lithium batteries is a good place to allocate some.

That's another variation on my "Learn by Destroying". If you haven't destroyed it, you don't understand it. My skool education was very much like that. I would break something, and then take it apart to see how it works. The skool would frown upon me taking apart something that was still working, but offered little resistance if I took apart something that was broken.

Bin? Don't you have a local recycling facility that takes batteries?

It might answer what happened, but not why. I've had brand new battery packs, assembled by a reputable and experienced rebuilder, trip the protector for no obvious reason. I recently had one go open circuit during charging, which might offer a clue.

--
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

Nice price. However, 10mm wide is MUCH too large for a button top replacement. The eBay offering did not mention the size, but measuring the one's I'm using produced: 6mm diameter x 0.84 mm thick.

Measuring the normal size of the stock button top on a typical 18650 cell, I get: 6mm diameter at the base x 2.5 mm thick.

I don't have much of a problem with current going through the magnet. Perhaps if I were using the cells for a high discharge application, but not for the typical small flashlight (about 1 amp) or E cigarette application (about 2 amps max).

My ESR meter is in my office so I can't measure the resistance of the battery right now. I'll try to remember to check tomorrow.

--
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

click the size selector on that page.

Thanks. I missed that. Looks like 6mm x 1mm is available very cheap.

1pc for $1.08 and 100 pcs for $2.99. I just ordered 100 each of 6mm dia x 1mm and 2mm thick magnets. Total price was $7.01 (which was about what I paid for 10 magnets from the previous vendor).

I managed to lose the magnet that I was measuring. It's probably stuck to something on my desk, but I can't find it. Grumble.

--
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

The first powerbank I bought got destroyed by my curiosity - the second donated its PCB to an LED bicycle light that I converted to lithium. Just bought another to sit on the shelf just in case.

I suspect the charger - but it hasn't damaged any of the other batteries rotated through it.

The cell I peeled had what looked like a popper disc in the top, but it was malleable instead of springy - I suspect "resetting" it would compromise safety.

If the battery has a protection circuit built in, you can tell by the dark band at the negative terminal and a strip up the side to the positive it will trip at something under 3 volts and refuse a charge for safety sake.

You can put a very low current limited charge into the to get them up to around 3.2 - 3.4 volts then and only then will a lithium battery charger ( a proper one) attempt to charge it.

I don't think there's a problem with pushing current through the magnet if there is enough spring pressure on the contacts.

The 6mm dia x 1mm thick magnets show about 0.02 ohms measured with the original Bob Parker ESR meter. It was fairly difficult getting a good connection which required using two strips of nickel flat wire to get a decent connection. I also had to apply some pressure to get a reliable connection to the meter probes. Even so, the resistance never climbed over about 0.30 ohms.

I don't have any square magnets and most everything that I could find in the right size on eBay is round, but I'm sure they exist.

I'm (slowly) building a better spot welder suitable for welding tabs onto batteries. I plan to buy some flat nickel wire at about 10 mm width, and make replacement button tops in a small bench press, which would then be spot welded to the top of recycled batteries.

Or, I could be crude, and just spot weld one end of a nickel strip to the battery, and zig-zag the strip to simulate the button top. With luck, it might act as a spring.

Sigh... yet another project.

--
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

That was my concern. I could not get a reliable connection, as mesured by a VOM, with casual contact to the magnet. I had to use pointy probes and press very hard.

I'm not recommending square magnets for contact extenders in use. Problem I solved was the lack of battery holders for charging cells. If you wrap battery tab strips around the magnet, you can stick it on any cell connection that's magnetic. The square magnet self-centers in the rectangular cavity you created in the tab material. Also works on most of the power tool batteries that I have.

I'd be interested in your welder experiments. I have a small arbor press with a 1/2" arbor. Turns out that a 1/2" copper pipe cap fits nicely over the end. I soldered a circuit board to it that holds spring-loaded contacts made from square brass tubing and tips made of big copper wire. Gives some consistency to applied pressure.

Bigger problem was consistency of applied energy. I tried to use a microwave transformer. I have some tips for dramatically increasing the reliability of the weld from "useless" to marginally acceptable...sometimes, if anybody's interested.

I tried discharging caps. I could get some very nice welds, but the consistency was abysmal. Don't think I ever built a battery pack with 100% good welds. The voltage was just too low. Tiny variations in contact resistance created MAJOR variations in weld quality.

I tripped over a small CD spot welder on ebay for cheap. The thing delivers a pulse of energy, so it's much more tolerant of contact resistance. It can put 7000 amps into a milliohm. That terminated my work on the microwave transformer welder.

There are some relatively cheap CD spot welders on ebay. I looked into a few of them. Although I never got any real info, I surmised that they're still trying to switch relatively low voltages. A real CD spot welder switches 600V or so into a stepdown transformer. I think the magic is in the design of the transformer.

I plan to buy some flat nickel wire at about 10 mm

That's what I do. If the cell was removed from a pack, there's already enough tab left to fold over for a tip. You don't really need to zig-zag it, just fold it over once and put a blob of solder underneath to raise it to the desired height. Soldering the blob on both sides reduces the resistance, but risks overheating the cell top.

Zig-zag just gives it more opportunity to fold to the side and cause problems. If you need a spring, it's often part of the battery holder.

Hobby stores sell sheets of brass that can be cut to any size/shape. Very easy to weld. I wouldn't use 'em in high current applications, but they can work well at low current.

I did ok with points, the for higher currents, a flat surface with many points of contact is better. This is my test fixture for discharge testing various cells. The clamp is plastic so insulation is not a problem:

Ok, that should work. Where did you find a small enough square shaped magnet?

For power tools, I use the spot welder. I tried battery holders only once. The vibration from the tool caused are rather interesting form of intermittent connection.

It's nothing special. Cazapitor discharge systems are ok, but I think I can do as well with a LiIon (or LiPo) battery for power storage. My big worry is that as the battery discharges, the energy delivered to the weld will decrease. So, I'm trying to measure the current and adjust the duration accordingly. I don't have this part working yet.

Another nice thing about using a battery instead of CD is that I can spot weld as fast as I can push the foot switch. I don't need to wait for the cazapitor to charge. At this time, it's just a simple eBay timer board driving a automobile starter relay controlled by a foot switch. It took about 3 weeks to arrive. The starter relays I bought surplus many years ago. Get the biggest you can find. I'll post photos when I get something that's reliable.

I use two 1/4" copper rods mounted on drilled fiberglass insulators.

I think the secret is in the ESR or internal resistance of the power source. New quality LiIon cells run about 150-400 mohms (milli-ohms). Older cells and eBay crap batteries are much higher. The series connectors, switch, SCR, wire, probes, and relay all add to the series resistance.

I do ok with soldering to NiCd and NiMH cells. However, LiIon cells are stainless steel and difficult solder. With enough flux, I can do a tolerable job. However, I tend to overheat the cell trying to get a decent connection, which is both dangerous and irritating when the battery pack eventually fails.

--
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