What kind of battery are you using yourself? My external battery is just an iPhone specific morphie juice pack with a 1500 mAh capacity and I thought that was pretty typical for an auxilary iPhone battery. Ocassionally I use specialized larger capacity batteries for offroad lighting, but a couple 5,600 Ah batteries will run 100W of lighting for a couple hours and thats considered lots of battery for a bicycle! You're using even more than that! 18,000 Ahs ??
Don't see why you were even concerned about battery drain, but recharge times must be interesting. Good luck on the trip Pete. When do you leave?