I wouldn't be afraid to show up at a track with one of those chargers that Ext2Rob suggested. They IMO have great specs for a dual AC/DC charger. That's what I like. I don't like having to worry about separate power supplies.
I've been keeping up with EXT2Rob and his use of the charger. Those types of buttons are my only concern; not on any one particular charger...just in general. Many electronics use them. I've had $500 guitar effects processors' buttons fail me in a short amount of time, and my lowest of the lowest priced budget chargers, with those types of buttons, have been holding up to 6 years of use so far (knock on wood
).