The Naked Scientists

The Naked Scientists Forum

Author Topic: Why do lithium-ion batteries have three terminals?  (Read 17532 times)

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Rechargeable Li-ion batteries usually have three terminals. I think one must let electrons in and one must let electrons out, so is the third terminal allowing some other type of particle to nip around?
« Last Edit: 01/09/2011 20:26:44 by chris »

MikeS

  • Hero Member
  • *****
  • Posts: 1044
  • The Devils Advocate
    • View Profile
Re: Why do lithium-ion batteries have three terminals?
« Reply #1 on: 01/09/2011 08:50:29 »
Rechargeable Li-ion batteries usually have three terminals. I think one must let electrons in and one must let electrons out, so is the third terminal allowing some other type of particle to nip around?

It's a thermistor terminal to monitor the temperature of the battery.  You don't want your electrons nipping about too energetically do you.

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Re: Why do lithium-ion batteries have three terminals?
« Reply #2 on: 01/09/2011 17:36:37 »
Rechargeable Li-ion batteries usually have three terminals. I think one must let electrons in and one must let electrons out, so is the third terminal allowing some other type of particle to nip around?

It's a thermistor terminal to monitor the temperature of the battery.  You don't want your electrons nipping about too energetically do you.

Thanks! I wondered if "T" stood for "Telectrons", but "Thermistor" makes a lot more sense  :D

chris

  • Hero Member
  • *****
  • Posts: 4660
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Why do lithium-ion batteries have three terminals?
« Reply #3 on: 01/09/2011 20:28:16 »
Yes, the batteries are "intelligent" and have various monitoring functions built-in to prevent over-discharge and over-temperature; nothing, sadly, to prevent over-price, however...

SeanB

  • Hero Member
  • *****
  • Posts: 1101
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #4 on: 01/09/2011 21:43:40 »
On basic batteries this is a connection to a thermistor, used to prevent overcharging the cell, by allowing the rise in temperature to be accurately measured, as the voltage of the cell is dependent on temperature. In more sophisticated packs the terminal is connected to a charge controller inside the unit, that uses this connection to control the charger that is providing the power to charge, as well as info about cell charge percent, an well as other information like a serial number, max capacity, number of charge cycles and other info.

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #5 on: 02/09/2011 00:05:38 »

On basic batteries this is a connection to a thermistor, used to prevent overcharging the cell, by allowing the rise in temperature to be accurately measured, as the voltage of the cell is dependent on temperature.


If the voltage iof the cell is dependent on the temperature, why would you need a thermistor? You could just measure the voltage.

Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 7715
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #6 on: 02/09/2011 06:13:51 »
Because that's not the only thing it depends on.

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #7 on: 02/09/2011 18:11:35 »
Because that's not the only thing it depends on.

Gosh! Well, I'm impressed BC. That certainly clears things up a lot  ::)

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #8 on: 02/09/2011 18:59:22 »
BTW, nickel-cadmium rechargeable batteries can get pretty toasty too, but I don't think they have T terminals.

FuzzyUK

  • Full Member
  • ***
  • Posts: 206
    • View Profile
    • http://www.fusniak.co.uk
Why do lithium-ion batteries have three terminals?
« Reply #9 on: 03/09/2011 01:14:47 »
BTW, nickel-cadmium rechargeable batteries can get pretty toasty too, but I don't think they have T terminals.

FuzzyUK

  • Full Member
  • ***
  • Posts: 206
    • View Profile
    • http://www.fusniak.co.uk
Why do lithium-ion batteries have three terminals?
« Reply #10 on: 03/09/2011 01:33:07 »
A NiCad can reach 800 atmospheres internal pressure before the vent blows. Fast charging at 10C rate is a risk and hence the need to monitor the charge.

In earlier days a battery was fast charged by cycling for 900ms with an off charge period of 100ms. The voltage was monitored during the latter sequence. Batteries are usually switched to trickle charge once they reach 80% of full charge. The thermistor was really a last resort cut out device and would rarely kick in unless the battery was faulty.

I used to design battery chargers. One day a NiCad battery pack literally exploded in the laboratories so we chucked the smoldering wreck out of the window and the whole factory had to be evacuated. Scary.
« Last Edit: 03/09/2011 01:36:58 by FuzzyUK »

SeanB

  • Hero Member
  • *****
  • Posts: 1101
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #11 on: 03/09/2011 11:16:11 »
Lithium cells have a terminal voltage dependent on charge state and temperature. If the cell is too hot or too cold it will not charge properly. If charged to a point over 100% of capacity it will heat up, and generates gas that causes the cell to swell up, and eventually burst. The problem with these cells is the gas generated is flammable, and the materials that make the cell are both flammable and react with moisture. Thus you get the all too common laptop flaming battery.

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #12 on: 03/09/2011 17:12:04 »
Lithium cells have a terminal voltage dependent on charge state and temperature. If the cell is too hot or too cold it will not charge properly. If charged to a point over 100% of capacity it will heat up, and generates gas that causes the cell to swell up, and eventually burst. The problem with these cells is the gas generated is flammable, and the materials that make the cell are both flammable and react with moisture. Thus you get the all too common laptop flaming battery.

Ah! That makes sense.

So, to properly charge a Li-ion cell I would need to take account of voltage, temperature and (presumably) current?

Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 7715
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #13 on: 04/09/2011 08:55:16 »
And time, they commonly measure rate of change of temperature as an indicator of the state of the battery.

Geezer

  • Neilep Level Member
  • ******
  • Posts: 8416
  • "Vive la résistance!"
    • View Profile
Why do lithium-ion batteries have three terminals?
« Reply #14 on: 04/09/2011 16:21:49 »
And time, they commonly measure rate of change of temperature as an indicator of the state of the battery.

Good point BC.

 

SMF 2.0 | SMF © 2011, Simple Machines