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I have a LG Optimus One (P500). For sometime I am wondering why my phone's battery has four terminals. I am aware that two terminals are for the positive and negative points and the the third connects to the battery's internal thermistor to monitor its temperature. Still that leaves me puzzled with what the fourth terminal could be for?

Interestingly my phone has only three terminal where as the battery has four. I have searched for answers in Google but no useful results other than description about temperature monitoring comes up.

Also since another doubt also lies in the same image, what is the port that is marked as "A" in the following image? It appears to me as some sort connector and my guess is that it is a connector to a cable with which manufacturers can access something deep in hardware. The Nokia phones which I had also had this feature and I guess this should be common for all phone then.

Connectors

Update: I am more keen on knowing what the fourth terminal on the battery does rather than why there are three in phone and four in battery. As @Axeman said, this battery could be used in other phones (of LG) as well.

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I don't think this question is related to Android. –  roxan Aug 9 '12 at 10:08
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My guess is it's for NFC, my Galaxy Nexus has 4-pins and all 4-pins exist in both battery and phone. Even if the battery itself does not have NFC, I guess it's still going to be cheaper, manufacturing-wise, to only have a single form factor for both NFC-enabled battery and non-NFC battery. –  Lie Ryan Aug 9 '12 at 13:22
    
Ahaa, upon prodding the Google with this clue, I landed up in AndroidForums.com and 'binary visions' said: NFC circuitry in the main board will have the chunky battery between it and NFC readers and hence a RFID antenna is kept in battery and the connection from the circuitry goes through this 4th terminal. –  Narayanan Aug 9 '12 at 13:34
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The battery model is LGIP-400N which is used in the Optimus (GT540), Optimus One (P500), GM750, GW620. The GAlaxy Nexus has also 4 terminals, the fourth and another is used for the NFC antenna (see Why is the NFC in the Galaxy Nexus battery?). It could be that one of the models includes extra functionality like the GN and just the battery got reused for your model (not changing the 4th terminal). EDIT: all of the other models also only use 3 of the 4 terminals (maybe they got rid of the 4th for price reasons but didn't change the battery) –  ce4 Aug 10 '12 at 6:56
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I don't mind if it has to closed or removed. but to answer the above question, I have learned that the battery plays an important role with Nfc. Choosing a replacement battery without this thought might resultin loss off NFC capabilities. –  Narayanan Aug 21 '12 at 2:09

2 Answers 2

Connector A seems a U.FL-R-SMT-10 and probably is for an external/additional antenna.

U.FL, is a miniature coaxial RF connector for high-frequency signals up to 6 GHz manufactured by Hirose Electric Group in Japan.1 U.FL connectors are commonly used inside laptops and embedded systems to connect the Wi-Fi antenna to a Mini PCI card. Female U.FL connectors are not designed with reconnection in mind, and they are only rated for a few reconnects before replacement is needed. The female U.FL connectors are generally not sold separately, but rather as part of a pigtail with a high-quality 1.32 mm doubly shielded cable, which allows for a low-loss connection.

(from Wikipedia)

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+1 Thanks for the info on connector. Can't accept as answered as the battery terminal question is still not solved. –  Narayanan Aug 9 '12 at 9:46
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It's likely that you'll never have an answer to that question. Battery could be used in more than one phone model, with different mechanical connections, and that pin could simply be connected to one of the other battery pins. I can't be sure, but I bet my two cents on this :-) –  Axeman Aug 9 '12 at 9:55
up vote 10 down vote accepted

As @Axeman said, the connector is an Ultra Small Surface Mount Coaxial connector (quite a mouthful and still could not get how it was abbreviated as U.FL) which is a kind of expansion port which could be used for any radio communication, including GSM, WiFi, GPS.

And as @Lie Ryan said, the fourth terminal in the battery is for connecting the antennae in the battery to NFC circuitry on the motherboard. Few searches in Google confirmed this and gives raise to following information:

Those who have NFC in their phones, may lose the NFC capabilities if they try to replace with cheap and duplicate batteries which may not have the NFC antennae as said here.

I am summing up all the points to make this question as answered. I was unable to accept @Axeman's answer as complete as it did not answer the NFC part and @Lie Ryan had added as a comment only.

Kudos for them both!

Update: I happen to get hold of LG P500's service manual which confirms the RF connector with the following image tagged as SW1001. enter image description here

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