Anyone got experience on how much less runtime you get out of batteries when the temperature drops to between -5 and -15 Celcius? The batteries in question are IDX Endua 10S and I got two and suspect I need to more to work an entire day(about 6 hours shooting) for a upcomming project. Camera I'm using is the Hpx500.

I've done some shooting in those temps and I suspect that I lost about 25% of shooting time because of the cold. I was shooting with a Canon XH A1.

I have heard that if you have a camera cover you can use hand warmer packs which might extend your battery life.

My experience years ago before I began using a heating cover for my Canon XM2, was that I lost around 50% of the battery capacity in the temperature range you mention, and maybe up to 75% loss in -15 to -20C. (Using standard Canon batteries)

Now when I am using a heating cover for the camera, the temperature inside the cover stays well above 0C, even when the outside temperature is down to -25C.