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Old Sep 9, 2003, 10:47 AM   #1
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Default Help/Advice in building a simple external battery pack.

Hi, I am interested in building a simple external battery pack for my Canon Powershot S20.

I would like advice on if the concept will work and have a few technical questions for which I can not find answers for.

The camera uses a 6v 650mAh Ni-MH battery (NB-5H) .

Now, I understand that these battery packs are built using 5 AAA Ni-MH cells, can some one confirm this?

I want to construct an external battery pack that will use 5 AA Ni-MH cells ( perhaps 2200mAh -> much cheaper and better )

Is this a feasable idea?

Now with my camera came an adapter which plugs into the charger and has an battery pack shaped plug which is put into the camera as if it were the battery.

My idea is to use this adapter and plug it into my external battery pack instead of the charger.

The unanswered questions I have relate to the third terminal on the battery packs and adapter.

From what I can determin, the third terminal is for a thermistor. If I check my battery packs, this terminal is connected to the negative terminal of the battery pack with approx 10K ohm resistance.

I initially thought this was used for charging only, but there is a spring loaded terminal in the top of the battery compartment of the camera which contacts this third thermistor terminal, so I assume that it plays some role with regard to the camera and is not only for charging.

If I check the same terminal on the power charger adapter mentioned above, there is no resistance between the negative terminal and the third terminal.

What does this mean? how will this impact my attempt to create an external battery pack? and can I use this adapter without modifying it?

Any help/suggestions/ideas would be great.

Cheers,
personX
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Old Sep 10, 2003, 7:11 AM   #2
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Some guesses: In a real battery pack the third wire usually connects to a thermal sensor as you say and is used to detect either end of charge on simple chargers, or can be used as an overcharge safety device.

My guess is the resistor is only there to simulate a flat battery, so the charger/power unit delivers full output to power the camera.

The AA batts will work, but 5 is an odd number as most chargers charge in pairs. You might get away with 4 and they could last longer. You'd need to try.

If 6V is what you want and you need plenty of juice, and a reliable capacity indicator (but you'd need a different type charger) then look at some of the small sealed lead acids for alarms etc. They should be float charged when not used.

http://www.mdsbattery.com/shop/produ...uctGroupID=556

In all cases, fit a safety fuse as close to one battery terminal as you can get -TO INCLUDE LEAD/PLUG & CAMERA
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Old Sep 11, 2003, 1:19 AM   #3
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Thanks voxmagna,

Your tip on the fuse is a great idea, infact since I am building this into a 6 battery holder, I will fit a fuse in where the 6th battery would normally be .


As for 5 being an odd number, I was also concerned about this, but I was considering using 10 batteries, 5 at a time, this will give me an even number for charging :idea: (?)

My only real issue that still stands is the third terminal, I initially thought that it was for charging only but there are a few things that lead me to think otherwise.

1) There is a spring loaded contact in the camera battery compartment which contacts this third terminal.

2) The charger to camera power adapter also has this third terminal (if it wasn't required then why would it be there? since this can not be charged or put into the charger).

3) I have read on a couple of forums that a user had an issue with an PowerShot S10 where he could only get about 10 - 15 shots out of his battery, but he discovered that by covering this third terminal (with tape) he could get more like 150 shots. Now, I don't intend on doing this as it is probably not a good idea, but it does show that it does play a role in the camera, but what that role is is unknown to me!

One guess I have made is that the camera uses this 10K ohm load as a dummy load to determine the batteries charge before performing some action like extending the lens just to find that there is not enough power to retract it and shut down (?) but this to me is just 1 possibility amongst many.

I just don't feel comfortable attaching my home made battery pack to my camera until I can determin exactly what this does in the camera.

Thanks again for you thoughts.
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Old Sep 11, 2003, 5:39 AM   #4
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In simple situations the 3rd terminal connects to a thermistor. On intelligent batts like the Sony's, they put in a microchip and firmware which the cam talks to and uses to monitor charge - and you're stuffed.

However, the workaround is to use the external supply input on the cam. But you're cam no longer gives a batt status indication. No probs if you use sealed lead acid, 'cos the battery charge monitor circuits based on terminal voltage change,you can build or buy from Tandy/Maplin (UK), do a far better job with a few led's. VOX
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Old Sep 13, 2003, 2:31 AM   #5
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Thanks again voxmagna,

But I think you are missing a few points.

1) I want to use Ni-Mh AA cells and not lead acid

2) The S20 camera does not have an external power supply input, instead they use a battery shaped plug (looks just like the supplied battery pack) which gets inserted into the camera's battery compartment and therefore acts just like a battery. the battery shaped plug then plugs into the side of the charger via a cable.

3) If you read my previous post carefully you will see why I am concerned regarding this third terminal.

I fully understand its use with regard to the thermistor and charging. My question still remains, I need to know what role this thermistor plays in the camera. I am sure it does play a role as explaind in my previous post.

Cheers,
PersonX
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Old Sep 15, 2003, 6:49 AM   #6
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To understand the role of the thermistor you need to do your own experiments. That means measuring what you've got if you think it really is a thermistor and not a 2 terminal intelligent device, replace it with a potentiometer, see what results you get and share them with us. This is what I've had to do with mobile phone batts.
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Old Sep 25, 2003, 6:38 PM   #7
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or...

You could just buy a Digipower DPS9000 which will give you hours
of use and screws onto your tripod mount!

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