My first YouTube video, failure analysis of a power tool battery meltddown.

[Carl_Smith]

Dave has nearly 800 videos online.  Now I have one.   

My first video is about how I rebuilt a power tool battery with cells I bought online, and about a year later it was still working great until it decided to turn itself into a heater, partly melting the case and killing several cells inside.  I take it apart to figure out why.



Of course, if I have a YouTube channel, I have to have a blog to go with it.

http://blog.megamicrowatt.com/

[Tomorokoshi]

Looks like a DeWalt 18V?

I rebuilt one once. It stopped taking charge in a fairly short time. Maybe I'll open it up. I don't think it ever got hot.

How about a 1/4" high strip of heat shrink tubing at the top and bottom to space them apart? If space permits then there would be some air clearance for heat flow.

[G7PSK]

The plastic wrapping was quite likely damaged originally when the tags were soldered, I have done that myself but spotted it and slipped some tape underneath. The original card tubes will most likely fit if the plastic sleeve is removed first.

[SeanB]

14.4V pack. I have the same one...... You need to put the card sleeves back on from the old cells, or make new ones if damaged, to keep those cases apart. Slitting and using some tape or contact adhesive to hold the card will work. Afterwards use a spot of glue on each sleeve where they touch ( as tape will not go back in the housing) will hold them in place.

When doing those solder joints make some extra cardboard washers to cover the battery top, you will have enough space to fit it under the positive pip, so there is reinforced insulation there so a cell will not short out if it is dropped and punches through the thin sleeve. If spot welding this also works by placing before spot welding, though there you can get by with a small rectangle you glue down to the tab afterwards placed under it.

[Carl_Smith]

The plastic wrapping was quite likely damaged originally when the tags were soldered, I have done that myself but spotted it and slipped some tape underneath. The original card tubes will most likely fit if the plastic sleeve is removed first.

Never thought about damaging the plastic when soldering.  I looked at the pack again and the spots where it shorted weren't lined up with the direction of the tabs.  But I suppose I could have bumped the wrapper with the soldering gun.  That's something I'll have to keep in mind if I rebuild any more.  And maybe I should look under the tabs in the other packs I rebuilt.  I plan on opening them up when I have a chance to put some sort of insulator between the cells.

Afterwards use a spot of glue on each sleeve where they touch ( as tape will not go back in the housing) will hold them in place.

Glue is a good idea.  A couple I rebuilt had some hot glue holding the pack together.  I think the failure happened to due rubbing of the cells because they had room to move.  Glue would help prevent that.

When doing those solder joints make some extra cardboard washers to cover the battery top, you will have enough space to fit it under the positive pip, so there is reinforced insulation there so a cell will not short out if it is dropped and punches through the thin sleeve. If spot welding this also works by placing before spot welding, though there you can get by with a small rectangle you glue down to the tab afterwards placed under it.

Not sure I'd have room for any extra cardboard on top of the cells.  I don't know if it is because the cells are taller, or because my method of folding one tab back over the top of the cell takes up space, but I couldn't even close up the pack all the way with the original cardboard cards on the top and bottom of the pack.  I didn't think they were really necessary since they were on top of everything so they didn't really insulate anything.   But maybe I'd have enough room to slip some heavy paper under each link.

They are Dewalt 14.4V batteries.  I didn't mention that in the video since the failure was no fault of theirs.

[HighVoltage]

Another way of insulation is to use kapton tape (polyimide), especially when you do not have much space for thick cardboard sleeves.