Incredibly, my iPhone died while walking in NYC, but was still able to use the transit card at a gate. It goes into a low power mode that still has the payment/transit card powered.
I use a lithium booster battery pack for an old car I use, and have used the battery pack to jump-start my car multiple times on a freezing-cold day (sub 10f) while trying to locate a store with a new car-battery in stock.