I found that some laptops use a 3 wire CMOS coin cell. The construction is simple, 1 wire is spot welded to the positive side of the CR2032 and the other 2 to the negative. There is nothing like a sensor of any kind. Why is that so? I can't see the purpose.

enter image description here

  • \$\begingroup\$ Perhaps so that if you insert the connector the wrong way around, you don't apply -3 V to the circuit? Normally you make connections with the power off, so there's no harm in trying it both ways, but a battery is always live. Or, perhaps the third wire is for charging, and a non-rechargable battery would leave it out. \$\endgroup\$
    – tomnexus
    Sep 14 '17 at 2:54
  • 2
    \$\begingroup\$ @tomnexus It could have been designed to prevent reverse connection but from the photo clearly is not. There is definitely harm in trying it both ways but fortunately that connector is polarized. \$\endgroup\$
    – RoyC
    Sep 14 '17 at 5:13

The extra terminal would allow the CPU to sense if such a battery is plugged into the connector. I don't know if that's what it's used for, but that would make some sense.

Alternatively, perhaps the wire is connected to the + terminal if a rechargeable battery is used, and the trickle charge is just shunted away for the primary cell setup, similar to what @tomnexus is suggesting in a comment.

If it was for reverse polarity protection, the red wire would go to the middle terminal, so it isn't that.

  • \$\begingroup\$ I have seen other of such batteries, and they white wire is on the negative terminal, so I think that this might be used for detection. \$\endgroup\$
    – next-hack
    Sep 14 '17 at 8:38

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