Yeah, if you don't have easy access to a charger or Wifi, I would keep it in airplane mode. When it's fighting for signal, your battery life will be in a free fall.
I assume low signal means you didn't have a good signal. That in and of itself will kill your battery life quickly. Second your top used app is the browser, so therefore a lot of screen on time.
He posted this after the fact and probably not from the location of the low signal.
Also, I would think low signal only registers when the battery usage for each unique app is done while the majority of the percentage used is during that one dot/bar period and not for just say 1% out of 10% total usage?
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You probably have background app refresh running? With that bad of a signal at work, it'll kill ya.
I used to go to work in a really bad reception area on my 5s and after 9 hours I would be at 65% after a full charge and very, very light usage.
With my 6 and typically the same usage I was at 94% yesterday. Here at least, maybe the marketed new LTE bands is helping, and not just the larger battery?
I also have a 6+ and that phone in the same conditions leaves work at 96%.