I'll be heading back to Japan this winter and have started preparing my gear.
Always indispensable, my iPhone is a constant navigator, translator, and camera on such trips. Battery life quickly suffers with the extra use so external batteries are a must.
External battery packs have significantly dropped in price in the last few years. I previously had recommended the Mophie Juicepack Powerstation 4000 mAh for $80, which is enough to re-charge the iPhone more than 2.5 times.
You can now get the top-rated IntoCircuit Power Castle for $30 with the $10 off discount code
YBURK9FK for 11200 mAh. I picked one up immediately.
These are also great for conferences.