Closing apps does not help much
Modern phones freeze apps you are not using, so swiping them away saves very little. Reopening an app from scratch can even use more power than resuming it.
The big drains are the screen, the cellular radio, and GPS. Focus on those.
Where the charge really goes
The screen is usually the biggest user, so turn brightness down. Weak signal is next: a phone searching for service works hard, so in a basement or a thick-walled building, switching to Wi-Fi or airplane mode for a while helps. GPS and navigation keep the radio and screen busy, so close the map once you know the way.
Video, games, and hotspot sharing are heavy too. Save them for when you are near a charger.
The percentage is an estimate
The battery number is the phone's best guess. It can drop quickly under heavy use or in cold weather, then recover a little once the phone rests or warms up. An older battery drops faster still.
So 15% is not a fixed amount of time. Plan with some margin.
What low power mode trades away
Low power mode stretches your battery by pausing background refresh, delaying mail, and trimming visual effects. It is worth turning on early, around 20%, not at 2%.
The trade-off is that some notifications arrive late. If you are waiting on an important message or a ride, keep that app open.
What to do right now
Dim the screen, turn on low power mode, and close navigation if you do not need it. Then decide how you will get charge back before you hit zero.
A Juiceboxx station rents a portable power bank for $2 per 30 minutes, capped at $12 per 24-hour day, and your phone charges while you keep moving. Start the rental while you still have enough battery to open the app. The app guide shows how, and charging on the go has more tips for a long day.