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Why do laptop batteries require so many terminals (upwards of 10 or more!)? Why not just a positive, negative, and a data pin?

All the phones I've used only have three terminals, and I would think these these are more or less comparable devices, separated by scale. All other devices I can think of using or taking apart only use two to three connections/terminals. Likewise, you remove the laptop battery, it runs fine with just the positive + negative of the power adapter, and maybe a data pin.

It seems as if the number of terminals has been growing, too. Is there a reason for the terminal-wagging competition amongst laptop makers?

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    \$\begingroup\$ For cell balancing, I imagine, but I don't have a source for that. \$\endgroup\$ – Hearth Nov 24 '19 at 11:28
  • \$\begingroup\$ Current carrying capacity? If you can find a reverse engineered pinout that would help. \$\endgroup\$ – pjc50 Nov 24 '19 at 11:53
  • \$\begingroup\$ for current capability, for prognostic information. Typically 4pins for power, A few for an I2C interface \$\endgroup\$ – JonRB Nov 24 '19 at 11:59
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Google "laptop battery pinout".

Typically you'll have:

  • Plus and Minus power terminals (maybe several contacts each for higher current)
  • Some form of communication like I2C
  • Temperature sensing (for example a thermistor)

Your typical laptop battery has several cells in series, so it requires balancing which is usually implemented in the battery management PCB inside the battery. This also handles protection for conditions like short, overcharge, overdischarge, etc. The laptop itself doesn't need to know the details about internal battery chemistry, that's the battery management PCB's job.

Also important is reporting state of charge accurately. This is difficult to do with voltage alone on Lithium battery, so you might find something like a coulomb counter, which implies values stored in a RAM which must be in the battery. The I2C bus also allows communicating useful information like battery capacity, maximum charge current for the charging circuit, or authentification for higher profit margins...

A cellphone runs on one cell, thus it does not require balancing or other complex functions. It will most likely just have plus, minus and temperature. If the battery is removable then it should have a protection, at least against shorts, in case the user stores a battery along with metal objects like keys. If it is internal to the phone, then it probably won't have a protection PCB.

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