Need help identifying component in laptop battery

[Phaseseeker]

I'm rebuilding a battery pack for an old ThinkPad 380XD by changing the dead cells. After the replacement, the raw battery voltage is now 10.42V, which should be in the correct range for a 3S2P pack, but the voltage isn't getting to the battery's connector. The protection circuitry controls two FETs on the positive side, which are both turning on as they should, so the problem doesn't seem to be there; the negative side isn't directly connected to the output connector, instead it goes through the component in the picture:

Left terminal goes to battery negative. Right terminal goes to the output connector. Center terminal goes to the central pin of a SOT89 SMD deviced marked "PH 7Y" (no clue about that one either). Resistance between the center and the right pins reads 40 ohm, every other combination reads open.
I've tried googling the markings, no result. It was touching a couple of the cells, but it wasn't glued to them. I've thought about it being a thermistor/thermal fuse but I've already found one of each of those so...

[robert.rozee]

i believe it is what is called a "chemical fuse". the middle pin is a trigger input. the outer pins go (as you have observed) to source and load. when a voltage is applied to the trigger, the fuse goes permanently open circuit to disable the battery pack when a non-recoverable defect it detected. you can likely just bridge it out - although i am sure others will warn you of the sky falling down if you do this!

type chemical fuse into google for more information. i looked into them recently when i was replacing an internal (fixed) netbook battery. in my case, the fuse had not been triggered.


cheers,
rob   :-)

[fzabkar]

See page 21:

http://www.mouser.com/ds/2/405/bq3055-194327.pdf

... and page 8:

http://www.mouser.com/ds/2/256/MAX1906-102698.pdf

UMI = Uchihashi

https://www.google.com/search?q=uchihashi+fuse&tbs=isz:m,itp:photo&tbm=isch

"400" = 40 x 10^0 = 40 ohm heater resistor?
"T4" = 4A ?

Sony tech note and datasheet:

https://www.datasheetarchive.com/pdf/download.php?id=e8b8a294716ab0c305d3d4fcabd4b44a195b33&type=P&term=SFD-145B

I suspect that the device is mounted off-board to prevent thermal stress during manufacturing.

[Phaseseeker]

Well, thanks to both. I guess I'll have to get a new one before closing the battery pack. Not sure what was the issue with the old cells that made it blew, but I bridged it for testing and there's no issue with the new ones (I checked the voltage on the control pin and the control circuitry isn't trying to fuse it).

[mikerj]

These laptop battery protection circuits can blow the fuse if cell voltages drop below some threshold, since this can compromise the safety of the cells.  Disconnecting the cells in the incorrect order can trigger this.  Note that just replacing the fuse may not make the pack functional, the BMS IC may also have set an internal non-volatile flag which will not allow it to turn the MOSFETs on for charge/discharge.

[fzabkar]

I suggest you search for "self control protector" or "self control fuse" in addition to "chemical fuse".