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.

Comments

[panstar1]

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 !!

[V-Man737]

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!

[LinuxH4x0r]

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

[killerjackalope]

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...