This morning I decided to make my 3s1p pack of 1500mHa li-po's from Vazquez Ramirez.
Everything went smoothly and I soldered the pack up and connected the relevant plugs. I insulated the tags on the cells with elecrical tape and then shunk wrapped the pack. On shrinking the heat shrink, it pushed the tabs in two of the cells together (see pic) and must have cut through the tape shorting out the cells. It made a hissing and sparking noise and the cell conected to the negative terminal on the pack inflated expelling gas, (bearing in mind this was in the bathroom where the hairdryer is kept for heat shrinking). I then ran out into the garden, chucked it on the floor after it burned my hand with the extreme heat. Then I watched as it grew bigger as the shortage between the batterys was still going on, so I had to force them apart with some pliers which were handy.
The house stinks of burning plastic and the cells cost 26€ each! Needless to say I was swearing for about half an hour(at the top of my voice).
I just go's to show that li-po's are not toys!

Can I still use the cell connected to the positive terminal of the pack? it doesent seem to have blown up like the other two.

another view

That sucks. I'll make my own nicd or nimh packs all day long but I'm afraid of making my own lipos.

Mario

Don't think for a minute that the same thing can't happen to NiMH or NiCd. I made up a pack of HECells for a micro car. After a race, I plugged them into the charger, but the charger wouldn't start. Curious, I pulled them from the car. They were hissing and the pack was getting HOT. I tossed them out into the middle of the empty parking lot, and they proceeded to disintegrate before my eyes.

mmm, high power batteries = FIRE. Scary stuff.
I love this hobby.

Let's put the proper perspective on this. There's nothing at all mysterious about extermal shorts. They can cause smoke, fire and destruction with any type of cell.

However, it is worth noting that Li-poly cells are more susceptible to shorting out when applying shrink wrap. Due to the proximity and construction of the tabs, they are much more vulnerable to shorting than any NiCd or NiMH cell. Much greater care needs to be taken in protecting the tabs of Li-poly cells before shrink-wrapping the pack.

In many cases, it is the simple difference in construction between the Li-poly "soft pouches" and the NiCd/NiMH "hard cylinders" that leads to problems. Just because you know how to build a safe NiCd/NiMH pack doesn't mean that you can do the same with Li-poly unless you first study the differences and learn the proper methods for building a Li-poly pack.

obviously, you don't want to depend on just a layer or two of standard electrical tape applied to thin, sharp edged terminals to be the only safety insulation.....there's other very tough,thin and liteweight insulating materials available to use before the outer tape or terminal sized shrink tubing is applied and then finished by the final shrink wrap.... also, attention should be paid to the direction of how outer shrink wrap is cut, as some of this stuff shrinks a lot more in one direction than the other, thus elimanating needless pushing and disturbing pressures on the terminals..... plan the assemblies very carefully with plenty of over-kill insulation and build it like your home depends on it, and with a good, proper charger we should be able to safely use this wonderful new technology without burning the world down................ kw

Very true!
But I can assure you now, that I have learned ALOT from my first delve into li-po's.
I cant belive how careful I was, but not even that was enough. The best thing that can come of this is someone out there leaning from this. I hope that they don't "assume" like me that it would be enough.
Also I was very suprised at the SPEED at which it happend. No kidding within 3 seconds it was hissing, inflating and heating up incredibly.

this is why I buy all my stuff pre assembled from bishop. I don't have the skills to do a competent job myself, and even if I did, when the inevitable mistake happens, at least it will most likely happen in production at the manufacturers site rather than in my house

oh..and by the way...I know you save money doing it yourself, but when you consider that generally the connector, wire and shrink wrap is included, it's not a huge overhead.

Well, darn. The first thing I meant to say in my previous message was to thank twinstarchris for sharing his experience. We do all benefit greatly from each experience recounted here, and I don't recall a previous one where the cells shorted while be shrink-wrapped.

Personally, I've always been impressed with the premium grade pack construction at LightFlight RC. This makes good reading:

http://www.lightflightrc.com/HTML/packGRADES.htm

Originally posted by Dave Hederich

Personally, I've always been impressed with the premium grade pack construction at LightFlight RC. This makes good reading:

http://www.lightflightrc.com/HTML/packGRADES.htm

Yep, his packs are great, and among the lightest for each type cell.
Marcus

Originally posted by Dave Hederich
[B]Personally, I've always been impressed with the premium grade pack construction at LightFlight RC. This makes good reading:

Agreed. I buy nothing but Dave's Premium packs. If he doesn't build it, I don't buy it.
don

Just shows you theres a better way! Also I might invest in a metal box. What scares me too, is the fact that i have been charging my li-ion's in an expensive car and not even properly looking after them!

For the record, I have checked all the voltages of the cells and they are all 3.7v apart from the one at the bottom (see pic) which was 3.6v. They were all balanced at 3.7 before the process, so I am wondering if I could use the two above the 3.6 cell again? Does anyone know how badly shorting out cells damages a li-po battery? I would be grateful if anyone knows.