What is the 3rd terminal on a typical Li-ion battery used for? I am
planning to create a battery pack and therefore need to build a CV
charger.

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Clarification of Question by microi-ga on 28 Dec 2005 13:30 PST

The battery pack I plan to build will be constructed of two cell phone
batteries. These batteries typically have three terminals: one (+);
one (-); and a third unmarked.  The unmarked terminal is the subject
of this question.

Please do note: 
"
Warning
If you are not completely sure what you are doing, don't apply what is
described below. In any case www.electronics-lab.com does not have any
responsibility in any way to the consequences that this guide may
have. Read it at your own risk. Please don't continue reading if you
don't agree with the above.
.....


4. The thermistor (usually PTC) that measure the battery temperature
inside the pack. It's terminals are connected to the charger so it can
sense the temperature of the pack and control the charge current until
the battery it's full charged."
http://www.electronics-lab.com/articles/Li_Ion_reconstruct/



Of interest?
http://www.faqs.org/faqs/pc-hardware-faq/laptops/compaq-aero/section-67.html


bonus link
http://www.discovercircuits.com/B/batt-chrg.htm

comment: Good guess kottekoe. Just  add few  'links' to your knowledge
- and you got it made :-)

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Request for Answer Clarification by microi-ga on 29 Dec 2005 14:06 PST

Is a schematic of a charger which has a temperature sensing terminal available?

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Clarification of Answer by hedgie-ga on 29 Dec 2005 23:51 PST

Unless there is one in the bonus link, I do not know of any.

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Clarification of Answer by hedgie-ga on 30 Dec 2005 00:01 PST

Try search terms: charger battery thermistor

which leads to 
://www.google.com/search?hs=pTE&hl=en&lr=lang_en&safe=off&client=opera&rls=en&q=charger+battery+thermistor&btnG=Search

e.g

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1037,C1078,C1088,P8990&action=featur

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Clarification of Answer by hedgie-ga on 05 Jan 2006 13:50 PST

I am wondering what it would take to earn 4 stars from  microi-ga.

 I see this is your first question here, microi-ga,

so,  I welcome you to GA and want to give you few more links to look at:

http://answers.google.com/answers/pricing.html

http://answers.google.com/answers/faq.html

guide
http://www.christopherwu.net/google_answers/answer_guide.html




cheers

Hedgie

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Request for Answer Clarification by microi-ga on 06 Jan 2006 08:34 PST

Hedgie,

On reconsideration, your response is a 4 star response.  How do I
modify the rating?

Mack

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Clarification of Answer by hedgie-ga on 06 Jan 2006 17:07 PST

You can not. 
That's OK. Let's both do better next time.
Hedgie

Good answer, fairly throough with pointers to additional related material.

Two guesses:

1) Two power wires, plus a third terminal for a temperature sensor.
Lithium batteries can be very dangerous, so I think some of them have
a built in sensor to turn off the charger if they overheat.

2) A center tap? Some electronic systems require a ground reference
and both a positive and negative supply relative to that ground.
Perhaps the third wire is the ground connection. If so, it would
likely be colored green.


My guess is on answer #1, but it is only a guess.

Looks like a 5-star answer to me. Excellent work, Hedgie-ga.