Juice lost in 30 minutes of outdoor use

Screen at full brightness for 30 minutes for a iPhone 5 is not that far out. If in doubt take it to an Apple store and they will diagnose. Might be time for a new battery

 

Max brightness is a battery killer, and if you were surfing the Internet on 3G as well, 20% is not out of the realm of possibility.

 

The screen uses more power than any other piece of hardware in your phone and if you have it a MAX it will take it toll on the battery.

 

On top of it all it depends on how good the signal was, what you were doing on the phone, what else might have been running in the background and/or had notifications come in, etc., etc., etc.

 

Not normal. Head to apple Store ASAP. Ask for a manager

 

if this occurred recently can we assume the outside ambient temperature was above 20 degrees C?

but even if it was warm outside, going from 100% to 85% takes about 45 minutes on my iPhone 5.

but then when it hits about 50% the pace of the drop occurs much faster.

iPhone 5's are on their last legs.