An obvious concern with using any electronic device for satellite navigation is the lifetime of the battery, particularly given the high amplification levels needed to use weak GPS signals from space.
For mobile phones, there is a cheap solution: USB "power bank" (external batteries) can be purchased for as little as £5 and are small enough to slip in a pocket.
Even the cheapest ones should last a walk but for around £20 you can get something that has many times the capacity of the phone's internal battery (over ten times that of the iPhone 6 battery, for example) so with those you really can walk all day long and probably all night (some include a torch!). The flat ones are convenient as a phone can sit on the top and be secured by a couple of elastic bands! Amazon category: portable power banks
If that happens, the app also offers you some options to conserve battery power if you are "caught short" and need to eek-out your battery.
The first step is to switch off WiFi and Bluetooth as these use a fair bit of battery power and you don't need them to do a walk. On iPhones you can do this by swiping 1 finger upwards from the bottom of the screen to show the toolbar.
You can also dim your screen as much as you can get away with as this is another large drain on the battery, although for viewing outside you'll need it brighter than for inside, particularly in the sunshine. During the walk, you can also switch the screen off entirely by pressing the power button during long stretches between directions. The iWalk app will continue to run in the background, even when your screen is turned off, so will still beep/vibrate when there's a new direction to follow. On Android you can use a free app to disable the screen lock during the walk so you don't have to unlock your screen each time you press the power button.
If all of that is still not enough, the app will warn you again:
The first emergency measure you can use is to disable GPS tracking when the app is in the background (i.e. when the screen is turned off). This means you'll have to switch on the screen and allow the app to get a GPS fix to check your position, but for potentially long stretches in between your phone can be using minimal battery. If even that is not enough then the app offers you a last resort - to suspend GPS tracking entirely:
This means you can still use the directions text and map, just like a walk book, and you can switch on the GPS just for a moment if you're not sure where you are on the map.
For mobile phones, there is a cheap solution: USB "power bank" (external batteries) can be purchased for as little as £5 and are small enough to slip in a pocket. Even the cheapest ones should last a walk but for around £20 you can get something that has many times the capacity of the phone's internal battery (over ten times that of the iPhone 6 battery, for example) so with those you really can walk all day long and probably all night (some include a torch!). The flat ones are convenient as a phone can sit on the top and be secured by a couple of elastic bands! Amazon category: portable power banks
Battery monitoring in the iWalk app
During the walk, the iWalk app will monitor how fast your battery is draining and estimate how quickly you are likely to finish the walk based on your current walking speed.
Saving battery during planned stops
If you decide to have a long stop during the walk (e.g. for a swim or meal), you can use the "Pause GPS" button which can be found by pressing the extended menu button "...". This will temporarily stop receiving location updates which allows the phone to shut down the satellite receiver to save battery power. Don't forget to un-pause it afterwards to re-enable location tracking before you resume the walk!
Battery safeguarding measures
If it looks like the battery is definitely going to run out before you finish the walk, the app will warn you so you can connect an external battery.
If that happens, the app also offers you some options to conserve battery power if you are "caught short" and need to eek-out your battery.
The first step is to switch off WiFi and Bluetooth as these use a fair bit of battery power and you don't need them to do a walk. On iPhones you can do this by swiping 1 finger upwards from the bottom of the screen to show the toolbar.
You can also dim your screen as much as you can get away with as this is another large drain on the battery, although for viewing outside you'll need it brighter than for inside, particularly in the sunshine. During the walk, you can also switch the screen off entirely by pressing the power button during long stretches between directions. The iWalk app will continue to run in the background, even when your screen is turned off, so will still beep/vibrate when there's a new direction to follow. On Android you can use a free app to disable the screen lock during the walk so you don't have to unlock your screen each time you press the power button.
If all of that is still not enough, the app will warn you again:
The first emergency measure you can use is to disable GPS tracking when the app is in the background (i.e. when the screen is turned off). This means you'll have to switch on the screen and allow the app to get a GPS fix to check your position, but for potentially long stretches in between your phone can be using minimal battery. If even that is not enough then the app offers you a last resort - to suspend GPS tracking entirely:
This means you can still use the directions text and map, just like a walk book, and you can switch on the GPS just for a moment if you're not sure where you are on the map.