Charging several phones in the field
Anyway the simple approach is to grab a few car power point (lighter socket to us old folks) USB chargers and use a 12V battery. Put a fuse on the thing before anything else. That will be the simple way to get full-bore charging (but you might need to segregate apples and androids since they do it differently, and possibly also some different brands of androids) - else any 12V-5V converter will do the job at the low-power range.

Some of those probably do use a DC-DC converter, some may not - if you could identify which do, they are likely to be more efficient than those that don't. 

You'll also need a low-voltage cutout to shut the system down when the battery is drained.







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