Monday, September 5, 2016

The weird fuses in laptop batteries

I called them "externally triggerable fuse" in my first post. The datasheets call them "Combined Thermal Fuse/Resistor" or "Fuse-Resistance protector".
Chances are your first time seeing one of these will be in a smart battery, it sure was for me.

As an over-current protection device (ie. regular fuse) they're probably only going to trigger during catastrophic short-circuit scenarios. Their main purpose is to provide a way for the microcontroller to physically break the circuit if it detects a potentially dangerous condition like the overheating of the cells and can't stop it by shutting the FETs off. The problem is some firmwares will count the sudden disappearance/reappearance of cell voltages amongst the list of conditions that warrant doing this so if you want to re-cell a pack you might end up with a blown fuse.
Replacing or jumping them is a pain so it's a good idea to connect the Reset pin of the controller to ground before doing anything of the sort to keep it from overreacting.

The rectangular one

The Cyntec 12AH3 / 12AG3 (datasheet) or similar devices will seem like large capacitors on first sight, though the fact that they have 4 terminals might make you think "current sense resistor". Nope!

The black plastic cap attaches to the ceramic body with glue and you can tease it off with a pair or side-cutters, pliers or tweezers. Here's what you see under it:

Left: intact, Right: blown

What you have is a rectangle of "solder" (probably some special alloy) with a resistor/heating element under it in this configuration:

As you can see on the images above, when the device is triggered the thin layer of solder heats up, flows onto the center pad and breaks the connection. The heater is about a 10 ohm resistance so with a 12.6v battery voltage that's 14 watts dissipated on that tiny surface area (or more if the charger voltage is used)

To check this fuse you'd look for continuity on longer sides and for the heater resistance between either contact and Pin 4.
To jump it carefully dab older onto the broken connections. Use low temperature and thin solder. You can probably shave away at it afterwards and the fuse MAY be able to blow again.. unreliably.. and completely of spec.
The best course of action is of course to replace it with a new one once the repair is confirmed.

If the controller is blowing the fuse repeatedly you have an issue in your data or a hardware fault in the controller board or your cell connections.

The 3-legged one

The SEFUSE D6X / D6T (datasheet) is a weird looking thing that doesn't really resemble any other component.

It's the same deal with a different physical construction.

To jump it you break off the cap (if there is one) to find a resin coated square. Then you carefully scrape away at the coating roughly in the middle until you find a tiny via. This is the center point you see in the diagram above. Now you just need to reconnect it to the two leads where the fusible material broke the connection so tin the via and connect it to both leads (ignore the lead for the heater).

Here's a pretty terrible example:

As with the previous one, it's best to get a new one once the repair is confirmed.


  1. Thanks for the great writeups on the battery stuff... I think I need to pull my "saleae" logic analyzer clone (fx2lp) out of my toolbox and try out your smbusb firmware.

    I was just tearing apart some chin-bay cheapo battery pack for a T61 Thinkpad, which I never wound up using, and these re-manned battery packs seem like a true fire hazard.
    1. it's a 3S2P - says capacity is 5200mAh, but when I pull it apart its using FST 2000mAh cells ... Ha they rounded 4000 up to 5200... nice trick, but if that was it, it wouldn't be so bad....
    2. looking over the battery monitoring board (has this bq8030 ic, which TI doesn't talk about - but your blog does, which lead me here ;)... I noticed that the board has a nice big wire jumper bodged over the top of the fusible link you describe.. The wire gauge looks to be on the order of 18ga... like a clipped lead from a power diode... So this pack must have had this link blown in a previous life, and they just do this quick repair (like the old trick of putting a penny in a fuse holder to bypass the blown fuse)...
    I'm sure this jumper will not be the first thing to go when/if over current condition hits this pack....

    1. Hey, glad you found it interesting.

      Wow, that's pretty bad... I guess at least they're still a bit better than the ones with the home-cooked chinese controller board that doesn't even do any thermal monitoring. Unless the controller just keeps the heater on indefinitely when the fuse doesn't blow... that could get messy :-)

  2. Thank you very much for the writeup! I've been dealing with 12AH3 already.. Have you ever heard about fuse fu**ing up the firmware when it blows? On x220T battery pack replaced the fuse but still 0v, cells are good and have power, tried to wake it up but no luck..

    1. Glad you found it useful!

      It's the other way around. If the controller blew the fuse then it's guaranteed to have also set the permanent failure flag in it's data flash and will refuse to work until that's cleared. For more info on that see related posts section here: