Some phones are better with battery life than others, if like me you've got a multitude of old smart phones lying around, use one of the good ones with viewranger as a dedicated GPS unit. Saves a fortune, has a bigger screen and are generally a lot lighter.
I used to use a HTC desire that had a spare battery but i'm now using an old S5 as a dedicated GPS. My S8 also has viewranger on it, but I switch it off when out, it can be used as a back up if there's a drama.
The S5 has the SIM card and all apps removed Wi-Fi etc off, so it doesnt use any power searching for a signal. It's basically a shell now. I was out around braemar yesterday, did about 8 miles over 4 hours, recorded the route and it used about 20% of the battery. If I'm out for days I have a battery pack to top it up every night. I've found over the last 3 or 4 years using viewranger that it's always been spot on, even when I've been convinced it was lying to me