As the commonts say, check seatguru to see if you're likely to have at-seat power. I don't recall every being on a Delta or KLM flight that had at seat power in economy, they most likely will in business. If it's got a newer version of the in-seat entertainment it might have a USB socket and that might be powered, although it depends (again, check seatguru) and it also depends how power hungry your mobile device is.
I carry a portable battery pack that will recharge my phone fully a few times or recharge my tablet about half to three-quarters. Having that fully charged and my tablet fully charged gets about eight to ten hours of use which is normally enough for me.
Also, get an app that monitors battery usage (you'll already be in flight mode which helps with the battery drain), close out apps you're not using, turn down the brightness. There are a lot of things you can do to minimize the battery usage of a device.