Introduction: GoPro Hero2 Parallel Battery Expansion Pack

DISCLAIMER: I'M NOT RESPONSIBLE IF YOU DAMAGE YOUR CAMERA DURING THIS INSTRUCTABLE
LiPo battery safety information: Lithium Polymer Etiquette by Radioactive_Legos
Skip to Step 2 if you simply want to see where to solder your wire internally.

THIS IS NOT: a portable USB charger. This IS a means to connect multiple high capacity LIPos to your Hero2.
WHY: 1) Extended battery life for extended time-lapse photography or multiple videos where rechargeable power is unavailable.
           2) Charging through portable USB chargers can be cumbersome and wasteful (portable USB chargers must step down the voltage to 5V then the camera must step down the voltage once again to charge the internal battery. Not to mention charging the camera through the USB port doesn't give an accurate prediction of remaining battery life when using the screens indicator. 
HOW: Disassemble camera, find Batt (+) and Ground (-), solder external wires for external batteries.
CRITERIA:  Don't block any rear BackPac accessories (e.g. wifi, screen)  + capable of running without original Hero2 battery.
BENEFITS: The camera is capable of charging the external single cell LiPo batteries attached to it. Also the camera has a low voltage cut off so you don't have to worry about damaging your external batteries. 

Yes, if you don't want to use an external battery pack there is enough room within the case to hide the loose wires.

BATTERY DISCLAIMER: You can connect any battery configuration you wish as long as it's capable of a 0.550A discharge and you don't charge it past 4.2V (a single cell LiPos' top off voltage) before connecting it to the camera.

FUTURE IMPROVEMENTS: add rubber grommets around the case holes to make it more weatherproof.
An alternative if you're up for the task is to melt aluminum/steel contacts in the protective case to act as a waterproof power bypass. Steel contacts would allow for magnetic inner/outer wire terminals. 
*power through the bus interface is not recommended as the gauge is very fine.

My Parts list:
4.0mm Low Profile Gold Plated Bullect Connector (USA)
0.8mm Connectors (only available from Hong Kong)
4 x 5000 mAh Single Cell LiPo batteries (USA)
2m of 16 AWG speaker cable (thin insulation)
Coloured heat shrink
Drill bits + torch or drill (can simply heat bits to force through plastic)
Solder + soldering iron
Optional: female + male connectors + banana plugs (for easy, but not pocket friendly, removal)

Step 1: Disassemble the Camera

We need to open the camera and find suitable solder points. I used the instructions provided by ifixit to disassemble the camera and start probing. Follow their instructions until you've completed Step 8 (speaker removal).

After you disassemble your camera you'll notice that GoPro uses large side rails to supply power to the the lower boards. Although not all rails are either batt / gnd I've labeled the two that I've determined are in the photos. 

To find the boards battery (+) and ground (-) I probed with my multimeter until I found two easily accessible solder points where there wasn't any voltage drop between the motherboard battery terminals and motherboard circuit. The resistance between the motherboards battery pin terminals and the rails were <1 ohm (as measured by my Canadian Tire multimeter) with a 0.000V drop in standby and under use.

Note: you don't have to remove the front cover as I did unless you want to visually see 'BATT' printed on the front motherboard. Just removing the back case exposes enough room to work with.

Step 2: Solder Your Wire

The Hero2 consumes its 1100mAh battery in approximately 2 hours: this is a current draw of 550 mA. When choosing your wire size you have two options:

1) 24 AWG (0.577 A continous rated @ 0.084 ohm / meter) or thicker for only supplying power to the camera
2) 21 AWG (1.2 A, @ 0.041 ohm / meter) or thicker for also charging external batteries off the camera

I decided to go with option #2 because I had extra 16 AWG speaker cable and I presume there will come a time when I won't have my high current LiPo charger and will need to charge my external batteries (which are in parallel with the cameras internal battery) directly off the camera which is capable of at least 1C (1100 mA).

Pre-tin your wire of choice and melt it into the rails solder globs.
*you may want to angle up the negative wire to give yourself more clearance when trying to close the back case during reassembly (see next step for clarification). If you place the wires directly to the side you won't have an issue interfering with the cameras bus interface in the back panel. 

*the lens fits perfectly in a roll of solder. This let me protect the lens while preventing the camera from moving around.

Note: 1st image courtesy of www.ifitit.com

Step 3: Drill Holes in Case

Before soldering your wires you may want to determine where you'd like your holes in the back case to be. I simply used a caliper/ruler and the external mic input as reference between the case and motherboard to drill my holes. After drilling into the camera case you can also drill into the larger camera cases.

Step 4: Reassembly + Adding Wire Terminals

Performing the reverse of the original ifixit instructions: reassemble the camera and pull the wires through the back case as you slide everything into position.

From here strip the ends of the wires and add your terminals of choice. I went with tiny bullet connectors so I wouldn't have any issues closing the case with the wires inside for when I use the camera under normal use.

As a safety precaution I colour coded the wires from the camera and battery pack and left some unheated heat shrink overhang so the terminals can slide into one another while preventing accidental shorts. 

Step 5: Sizing and Assembling Your Battery Pack

Approximate calculations:

Given from experience: 1100 mAh = 2h of use

Battery capacity (mAh) = (time required in hours / 2) * 1100 mAh

For e.g., the batteries I have pictured below are 5.0 Ah each thus giving me approximately 9h of camera use (excluding the internal battery).

CAUTION: before connecting your camera wires to your battery pack I recommend that the voltage of the cameras internal battery be the same if not slightly higher then that of the external pack and that you make the connection when both batteries are floated (fully charged, peak voltage drops more readily under load to self-equalize). Unless connected through a resistor first to equalize voltages, the external pack is capable of a very high discharge and may dump too much current into the Hero2 battery. Fortunately these larger capacity batteries are capable of high charge rates and as such the internal battery can harmlessly dump power into them during equalization. From history I've connected the external pack (4.08-4.15V) to the Hero2 (4.18-4.20V) directly without any problems.

Step 6: Find Something Interesting to Shoot

Here is a time-lapse of some cedar work being done on the neighbours roof. The perspective is from the camera mounted on another neighbours gutter/roof. It's 8.5 hours of footage (6 pictures taken per minute = ~3000 stills). The screen was still showing 2/3 battery life (5000mAh external + 1100 mAh internal) but since I'm yet to run the battery to empty I can't comment on how much actual remaining capacity there was. 

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