If you are going with a smartphone ..... Check you can open the back up and buy a 2nd battery. Iphones for that reason are a no no
Don't do this. Replacing a battery is a bit of a pfaff, getting both batteries back to charge means plugging phone into wall, waiting until it if fully charged, swapping batteries and charging the other battery. If you buy a new phone, the spare battery almost certainly won't suit it. Naked li-on batteries are a fire hazard and are banned from commercial flights.
Much better to get something like
this, which will charge your phone multiple times, any other small usb device you have (like a garmin!), can be used by others (I supplied enough charge to another rider on an audax so he could boot his phone, contact his riding buddy and arrange to meet at the end of the ride).
Maybe one day I'll buy a garmin and realise I was wrong, but I love using a smart phone for this. l load preplanned rides into
OSMAND, track my rides using endomondo or strava. Endomondo is great if you are doing an interesting ride and have data, as people can live track you on the road. If I turn on airplane mode, and mostly keep the screen off, I can get a lot of hours recorded. And just one device, chatting gently in my ear on long rides (podcasts), keeping me in contact with family and tracking my rides. I've just bought a
topeak bag for the phone and have
another one for the battery, to keep it handy. So far with my
new phone the longest ride I've done is 60km, so haven't come near to running out of power.