Introduction: Repair a Thermally-Fused 9.6v Battery Pack

Why doesn't your 9.6v battery pack work? Maybe it's thermally fused. Those are not resettable, and break the current to or from your pack if they get hot. This will show how to remove a broken fuse and reassemble the pack so you can keep rockin' on your merry way.

This procedure has been tested twice on a NiCd pack. I don't know if NimH or Li ion packs (if those exist) have thermal fuses, and if so, whether they are configured differently.

I think using thermal fuses is a form of designed obsolescence, considering most 9.6v NiCd chargers use higher amperage these days. I purchased this pack from eBay, and purchased a charger from RadioShack. The new charger wrought havoc on the old packs, and my DMM could read no current from them at all. I was at first convinced that the charger had destroyed my packs, but the little devilish statement on the pack, "Thermal Fuse Protected," raised my eyebrow. I decided to rip apart my packs, and what ensued is this, my first Instructable.

Step 1: Remove Cover

Find the groove where the wires enter the pack, and slip your scissors into that groove. The cover should come off pretty easy, but be careful if it's one of those not-so-flexible plastic sheaths like the one in the picture. It sat under the pressure of my scissors for a minute, then BAM! The scissors shot through all angry-like.
Once the cover is off, you'll find some tape in your way. I scissor'd my way through that as well.

Step 2: Locate and Destroy

Find the Thermal fuse. If it's melted pretty bad, the batteries might be damaged. This one isn't noticeably marred in any way(except that electricity can no longer flow through it, durr).
Bend the battery pack in half to break the glue as shown in the picture, then pull apart the sides so that the thermal fuse detaches. Clip it off from the contacts (I used scissors, not really recommended unless you don't care about your scissors).

Step 3: Rebuild the Pack

Since the fuse basically jumped one end of the battery to the other, you're going to want to connect those same ends again. Three ways you can do this:
1) put a jumper wire where the fuse was
2) flip half of the pack around to eliminate the need for any jumping
3) put another fuse (evil) where the first one was

I chose the second method. What ends up happening (if you flipped it correctly) is you'll find you now have some wire slack. You can tuck this into the pack.

If you chose the third method, may God have mercy on your soul.

Pick how you'll bridge that gap where the fuse was (I kept the ribbon on one cell, and took it off the other). I needed to shorten the ribbon so it didn't bridge over to the farther cell.

Solder the contacts together. I soldered the ribbon that was already attached to the one cell to the next cell. That was a bit difficult, but I managed it after a large glob of solder. I chose to do it directly onto the cell for the sake of space and keeping the pack profile low.

Tape the pack up real good. I used clear tape, since I had no electrical tape immediately handy and I can get pretty lazy sometimes. But if you've got it, you'll probably want to use electrical tape.

Step 4: Epilogue and Disclaimer

Your battery pack should read full speed ahead! You can charge it as freely as you want. However, keep in mind that without the thermal fuse, you'll probably want to babysit your pack while it's charging to make sure it doesn't get too hot or a'splode. In my opinion, you'd only really want to do this if you don't have any sentimental attachment to the pack, since getting your cells too hot could ruin them forever.

Comments

author
RangerJ (author)2012-11-24

On a similar note, I have repaired a DeWalt charger by removing the thermal circuit breaker and replacing it with a panel-mount fuse holder and a small 3AG fuse, The breaker was mounted on the transformer, and covered with the cloth tape covering the winds.

author
panstar1 (author)2008-09-09

I have a dead li-ion cell phone battery ,so I did what you did I opened it up. I found a small board attached to li-ion pack I removed the board and it came to life. I am not sure what the small board did I figure it was a voltage regular & temp cutoff. I knew the board was bad b/c it would draw 4.3 ma without any load ! but I still am going to put a thermal fuse in for safety I would rather not have a li-ion battery go into thermal run away and start a 3000 F fire burning though the my floor and melting it self into a puddle of lava !!

author
V-Man737 (author)panstar12008-09-10

Right. The circuit board in Li-ion packs regulates the level of each individual cell, to make sure one doesn't run out before the rest (which could lead to the meltdown you fear). Be extra careful using the pack now that it is "headless," you'll have to manage the even charging manually now!

author
LinuxH4x0r (author)2008-07-05

Nice! I never really liked those fuses, although I might use them in my car. I charge alkalines, so that shows how much I care about battery safety

author
killerjackalope (author)2008-07-04

It's true that thermal fuses are pretty obsolete, a super simple thermistor setup could kill charging, even a thermistor and resistor in series, once the basic setup is gained it would self regulate to the safe temperatures...

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