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View Full Version : OT Camera battery question. Why 3 contacts?



retusaf99
05-16-2008, 02:22 PM
The batteries for my Canon SD300 digital camera died after about 3 years of use. One was the original Canon and the other a Lenmar that cost about 1/4 a real Canon replacement, but worked just fine.

Can someone explain why they have 3 contacts (in layman's terms). I thought dc stuff was pretty straightforward: ya got a "plus" and ya got a "minus" or do we need a third contact to be PC?

Doug:confused:

Alistair Hosie
05-16-2008, 02:29 PM
Ask Evan:D he's a technobuff Alistair

Tony
05-16-2008, 02:37 PM
this is just a guess...
(how's that for an opener?)

3 'contacts' for DC usually imply a split +/- power supply..
that means 0V (GND), +V, -V.

this is usually done at the board (chip) level so I'd exclude
that from the battery. all thats left is some kind of "info"
contact -- perhaps a reference for the camera to measure
the battery level without putting additional drain on it.

i say this because, usually, (mine included), the battery
charger also has 3 contacts. i'd put my money on
it being a 'reference' or measurement terminal.

am interested to hear the real answer...

-Tony

Davek0974
05-16-2008, 03:12 PM
yes, i use a canon myself and one of the terminals is for the camera battery life meter and when cahrging it tells the charger the battery status.

Dave

A.K. Boomer
05-16-2008, 03:17 PM
Can be two separate cells, they might run them in series or parallel but the third wire can be a way of monitoring each individual cell, I think thats what their doing on my li-po's .

macona
05-16-2008, 03:27 PM
The third contact is usually a thermistor or a semiconductor temp sensor in NiCd and NiMH packs. In li-ion and lipos it may be a data line like I2C for communicating with the board in the battery pack.

Evan
05-16-2008, 04:22 PM
Yep, temperature sensor.

I2C requires 2 wires. There is a one wire standard though.

Carld
05-16-2008, 04:38 PM
The third contact in lithium batteries is for sensing of the charge rate, etc.. If it is a lithium battery it needs special attention during charging. If incorrectly charged it can burst into flames or explode. It can happen even with the correct charger if the battery is bad or damaged.

Is yours a lithium battery?

retusaf99
05-16-2008, 05:42 PM
The third contact in lithium batteries is for sensing of the charge rate, etc.. If it is a lithium battery it needs special attention during charging. If incorrectly charged it can burst into flames or explode. It can happen even with the correct charger if the battery is bad or damaged.

Is yours a lithium battery?
Yes, these are lithium-ion. So the third contact is actually a connection to some electronic component within the battery, rather than a + or - terminal?

Doug :confused:

john hobdeclipe
05-16-2008, 05:45 PM
The third wire is for sucking money out of your pocket.

retusaf99
05-16-2008, 06:07 PM
The third wire is for sucking money out of your pocket.Certainly true if you go with a Canon replacement @ $40. The Lenmars weren't too bad @ $10 off amazon.com. I saw no difference in performance or life between the two brands. ymmv.

Doug :D

andy_b
05-16-2008, 08:31 PM
like everything else, the minute you throw something away, that's when you need it.
i have a similar Canon camera, with i think the same batteries. for about 6 months one was laying in the center console in my car with the covers off, and my wife just cleaned the car out and threw the battery away. if i still had it i'd post a photo. anyway, yes, the third wire went to some small electronics inside the battery case. in cell phones the third contact is a thermistor to sense temperature. i'm guessing it's the same for these.

andy b.

rdfeil
05-17-2008, 12:11 AM
Like many have said above it is for a temperature sensor. Lithium batteries are extremely temperature sensitive, both while charging and actually before charging. If the battery is to cold the charger should not charge the battery until it warms up. If you try to charge a cold lithium ion battery without protection in the charger it will be damaged and/or destroyed, sometimes violently:eek: This was the explanation I was given by the battery system engineers for the radio batteries I use a lot.

Robin

Carld
05-17-2008, 12:56 PM
Some of the lithium packs have a connection to several points in the battery to sense if sections of the battery are charging faster than others.

The lithium battery can be a very dangerous item if mishandled or damaged. Because of the charge/discharge conditions of model airplanes the lithium batteries are dangerous and have to be handled with caution. They are charged/discharged in a metal box or some method to contain a fire or explosion if something goes wrong.

DFMiller
05-17-2008, 02:06 PM
Finally something I have some solid info on. The last 5 years I have been working with Large Li packs. up to 2 KW :-) single modules
Almost for certain it for a temperature sensor. As other have stated most Li based pack have this sensor for the charge circuit to monitor temp for under and over temp. If the pack is Ni based its used for Dt/dt or some other charge termination technique.

Swarf&Sparks
05-17-2008, 02:09 PM
I think it's time here, to draw the distinction between Lithium Polymer LiPo
And Lithium Ion Li ion cells
LiPo are generally high discharge for models and sich
LiIon are usually found in consumer electronics like mobile phones.


LiPo are prone to self-destruct (and create havoc about them) if treated with less than courtesy and consideration.

LiIon are generally, as safe as any consumer product can be.

DFMiller
05-17-2008, 02:14 PM
Swarf&Sparks
Glad to debate this off line. Not correct generalizations. Got it all backwards and twisted

Swarf&Sparks
05-17-2008, 02:19 PM
OK DF, be glad to hear if I've been mislead.
You can PM or email me

macona
05-17-2008, 02:38 PM
Actually the foil pack of the lipos makes them safer on one way where they can expand without exploding versus the metal can of standard prismatic li-ions. A lot of consumer devises use lipos, ipods, iphones, palm devices, bluetooth stuff, etc. Lipos are a lot thinner than prismatic cells so they can fit where the prismatics dont.

Almost every commercial lithium pack has battery management circuits in it. The monitor the voltage of each cell and will shut down the pack for overcharge and when it has reached a predetermined empty point. They will also disable the pack if any one cell drops below the minimum cell voltage at which the battery is considered to be damaged. Usually around 2.7 to 2.9v depending on chemistry.

Neither cell is prone to self destruct. The problems in the past few year have been due to manufacturing problems where there were impurities in the cell which could eventually internally short the cell.

The fires from model users is often caused by damaging the pack. Also most hobby packs do not have the management circuit in the pack so it is easy to over charge a cell with the simple lithium chargers on the market. Thats why they sell those ceramic jars for charging your packs.

tattoomike68
05-17-2008, 03:51 PM
The third wire is for sucking money out of your pocket.


LOL for $40 you got that right.. :)

Thats why I like a camera that takes "AA" batteries. Recargables work fine plus you can just buy some anywhere if needed in a pinch.