ChrisTenone:
Hi Phil_Cox,How long is your flight? Typically our launch-to-land is about an hour, as the balloons are designed to rise til they burst. A 9 volt alkaline battery is sufficient for that time span. We no longer use the Sparkfun heating pads. Nichrome wire on a kapton taped enclosure works well. If you have access to a freezer that goes really cold, you can get a handle on the current needed to keep your particular package at the proper temperature (it's about -40 at apogee.) An insulating styrofoam enclosure is also good for protecting the stuff when it
crasheslands.
Hey Chris, thanks for sharing your flight experiance.
In short, I'm not sure how things will behave during the flight as the freezer only takes me down to -18. The payload is in a polystyrene box, the elctronics and batteries are seated in foam pads that are velcro'd to the inside of the box. This has essentially reduced the internal volume and therefore air to be kept warm. If all goes to plan, I'm hoping there will be useful flight data on the SD card (assuming we can find the thing of course!).
I suspect I am over thinking/engineering this but I am keen to keep all elements of the payload working. The camera's and Arduino seem happy working at 4 degrees (about 90 minutes in the freezer). I have a TMP36 monitoring internal temp, this controls the relay that closes the heat pad circuit at a given temp (i'm thinking of 5 degrees as the trigger temp?). I think I am going to try a 3.7v 18650 large mAh battery on a cheap 3.7v heat pad and see what happens in the freezer. This would be a weight saving on my current set up as no voltage regulator and heavy 9v batt!
Quick question, is your nichrome set up on all the time? This will be my nexxt experiment :O)
Best wishes
Phil