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Old Sep 23, 2012, 7:37 PM   #1
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Default Homemade battery pack considerations?

Hello,

I want to embark on a project and I want to check with some electronics gurus to see if I have the basic idea right.

I have an Olympus E-PL3 and the battery always seems to be low. The battery is rated at 7.2v. There are 3 terminals (1),(T),(+). Fully charged, I measure around 8.2v. I want to build a battery pack consisting of AAA or AA batteries.

What I want to do is to run a small gauge wire out of the battery compartment and run the camera's battery in parallel to the AAA batteries. I saw several posts saying this is possible so I am excited to try. However, I have some questions:

1) Use 6 (7.2v) or 7 (8.4v) cells?
2) What's the purpose of the (T) terminal? Should I run the (-) from the battery pack to the (-) or (T) on the camera's battery terminal? In measuring with a multimeter, both (1) and (T) seem to be ground.
3) Do I need to run an inline fuse? If so, how do I determine the amperage?
4) What do I get out of this setup? If each AAA is 800mAh at 1.2v, I still get 800mAh when I run them in serial right?

Assumptions:
1) I don't need to charge with the batteries in place. I will always have a 8-cell charger handy and this is a backup so I don't expect to swap frequently.
2) I don't care if the camera's battery door doesn't close completely or if I have to drill some tiny holes to run the wires.
3) I don't need a power level indicator.
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Old Sep 25, 2012, 10:32 AM   #2
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The third terminal is a sensing terminal. This can be used for testing battery voltage under load, or possibly connected to a thermal sensor in the battery pack. If you hook up an external battery, be certain you use the proper negative terminal.
7.2V is the nominal voltage, and will read higher with a full charge and no load. Use 6 cells for your external battery.
If your camera manual has specs for current, use that number for your fuse, multiplied by 1.25. If not, I would start with a 2A regular blow (for AA battery)
When you run cells in series, only the voltage is additive, so you would have only the current capacity of one cell.
This could be a tricky installation if you intend to leave the camera battery installed while running on the external battery. Most cameras which have jacks for external power, automatically switch the internal battery out of the circuit when the plug is inserted. Without this, the two batteries will be wired directly in parallel, and the one with the higher voltage will discharge into the lower. If the voltage difference is high enough, this is very like a short circuit. If your camera battery is lithium ion, there is a possibility of fire. A rectifier as well as a fuse for your external battery will prevent reverse discharge of the camera battery, which would be my biggest worry with this setup.

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Old Sep 27, 2012, 11:19 AM   #3
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Why such odd approach (ie, thru the battery compartment)?

The normal procedure is just to make a big ass external power pack (one you wear) and plug it into the camera's external power source jack.

This site features external power pack projects (commercially available as well as build it yourself).
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Old Sep 27, 2012, 12:06 PM   #4
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The E-PL2 does not have a DC input jack, so it's likely that neither does the E-PL3.
Personally, I would probably just keep a spare battery with me.

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