Wrecked a lipo this morning while soldering on a new connector.
I did the soldering and was shrinking the shrink tubing over the
solder joint when the pack burst briefly into flames.
I was holding the wire and it didn't feel too hot, but maybe the
damage was done while soldering and took a while to happen?

I thought I did a quick neat solder job, so what is the solution?
how can I stop the heat from reaching the pack? My only idea
is to strip more of the wire and clamp a pair of needle nose
pliers to bare copper between the solder joint and the battery
to act as a heat sink.

Rick

Couple of questions for the history book Rick as we haven't had a soldering accident reported before.

Brand and size of cells?
Number of cells?
Had the cells been charged before?
If they have been charged, do you know how much charge was in the cells at the time of the incident?
How far was the solder joint from the tab?

Thanks for reporting the incident - we learn a little more from each accident.

In answer to your question, modelers have used clamps to heat sink the tabs.

Also.........
What brand/type of solder
What wattage/brand of soldering iron/gun, what kind of tip
How long was heat applied, etc.

I use an aluimum heat sink clamp when soldering to the tabs when building packs and soldering leads to test cells but never when soldering connectors to battery leads. The time required to tin the wire and solder to a pre tinned connector should not heat the wire for more than an inch or so.

Charles

I thought maybe he was soldering directly to the battery tabs.

Originally Posted by rcplaneguy

I thought maybe he was soldering directly to the battery tabs.

Perhaps as he provided no details. Most LiPoly batteries have wire leads from battery tabs to the connector. If he was soldering a connector to the cells tabs then that is a no no. If he was soldering a lead(wire) to the cells tabs then he should make very sure he knows what precautions need to be taken.

Charles

The new connector might have had a short (jst?), or he might have cut across both wires from the battery at the same time with diagonal cutters, too.

When soldering connectors to wires I tend to use one of those little "extra hands" which has a clamp at each end of a bar. Some even come with magnifying glasses on a third arm. Usually put a connector into each clamp at the outset, pretin and then solder the male and female wires to their respective connectors. Of course this isn't a very good idea when soldering wires which at the other end already have a LiPo attached to them ;=)) Got a nice spark when I accidentally tried that once with a NiMH pack.

Answering the questions:

Battery: ThunderPower 730 mAh, 2 cell. battery had been used several
times before, and was fully charged.
Soldering: Radio shack 40 watt iron, radio shack solder. conical tip...

I was replacing the stock jst connector with powerpoles, so I cut it off,
cutting each wire individually and taping before cutting the next.
This may have been part of the problem, the amount of wire coming out
of the cell was skimpy - about 2 and 1/2 inches, so I was working pretty
close to the cell.
I was splicing in more wire to get a longer reach as well, so was doing
a wire-to-wire solder. Don't know exactly how long I applied heat with
the iron, but my technique has been improving and I was just congratulating
myself on what a nice quick, compact connection I had made when the
battery burst into flame.

Rick

Thanks Rick, that's a puzzler.

If the Lipo pack was constucted without a header pcb, It is possible that enough heat flowed along the wire to the Lipo tab. The wires look thin but even the 16G wire has a fairly heavy cross section of copper conductor to carry the heat. Rick states that he wasnt sure how long it took to solder the joint. Its a fairly small pack with less ability to dissipate the heat

If the wire has a film of surface corrosion, it can take quite a few seconds before sufficient heat is transfered to the wire to allow the solder flux to break down the surface film allowing the solder to bond to the wire. However at the same time it is flowing along the wire to the tabs.

I use resin cored solder, a non corosive liquid flux and a high powered Iron to solder power connectors and battery connections when making up packs. This results in much faster soldering and less time for the heat to travel. Supprisingly, I've found a lower power iron can cause more unwanted heating as it takes quite a bit longer to solder the joint. This also applies when soldering Nmhi cells together.

Ron...