To charge say 50 average smart phones how many 12 volt car batteries will I need?

[Warren_19]

To charge 50 average smartphones how many 12 volt car batteries will I need?

 

[Someone Somewhere]

Car batteries aren't really a good choice; they're not meant for long-term discharge like that. You want deep cycle SLAs.

10W.h is a reasonably common/large cellphone battery, so we want maybe 1.5kWh, given that we don't want to fully discharge the battery, and the converters aren't 100% efficient. That's about 125A.h at 12V.