The flat line
Notably there are no flat lines in the NiMH battery curve. During the daylight interval, where no images are being taken, it would be desirable to use as little current as possible. There are a few issues here;
The UNO board is powering serial - USB hardware and some LEDs.
SD card is possibly taking current when not datalogging.
Sleep modes have not been effective, they seem to break the datalogging and don't turn off the extra hardware.
Action here requires a stripped down processor board. (like this one http://www.gammon.com.au/forum/?id=11109 ). It would seem sensible to put the rest of the hardware on that board too and do away with plugs and shields. Lowering the clock speed of the Atmel328 may also be prudent.
Sleep modes may require closing the file and reopening/reinitializing the sd card instead of flushing the file. Further work is needed here. Other tricks to save power involve changing he data pins to inputs and using a mosfet to lift the ground of the SD card.
The yellow lineAt some point around 20hour the GoPro timing shifts out of the lighting window. Other tests have also exhibited half frames, but the results are using black frames of around 260kb. Extending the lighting window to 3 seconds will rectify this, but this is seriously detrimental o battery life.
Presently the system is momentarily pulling pin 12 of the GoPro connector low and the light comes on 2500ms later. Theoretically the timing of the first frame should be a known quantity but while experimental results appear to show this occurs at 2800ms this has not produced reliable results in the camera. (characteristic black and half black frames as the 'shutter' timing slips out of the lighting window.
Fixing this is going to require a hardware hack, two wires to close the shutter button remotely through another relay or an optoisolator.
Charging the GoPro
A two battery system has to present some loss of efficiency. At present anecdotal evidence suggests the camera will forget the date and time if the battery is removed for any length of time. It is imperative that the images retain correct date and time of creation.
Replace the NiMH with a high capacity lithium battery!
Power saving and the RTC
good thread here
Dump the power on the RTC when it isn't it use.