Before looking at buying an additional battery it is worth looking at ways to save your battery usage.
We know that batteries on phones can run down quite quickly, especially when the device is not new. Whether using the iFootpath App or not it's a good idea to have a backup battery with you. They are not too expensive these days and can give you the peace of mind when you are out and about and you are not able to plug in your charger.
Here are a few suggestions:
Anker PowerCore 5000 Portable Charger, Ultra-Compact Power Bank with Fast-Charging Technology, External Battery for iPhone X / 8 / 7 / 6s / 6 / SE, iPad, Samsung Galaxy and more
Anker Power Bank Astro E1 5200mAh Ultra Compact Portable Charger, External Battery with PowerIQ Technology for iPhone, iPad, Samsung, Nexus, HTC, Huawei and More (Black): Anker
Ultra ® Edition 2500 mah Charger Power Bank Cases for iPhone 5 5s and SE models rechargeable close fitting shockproof battery back up case iOS9 slim (Grey and Orange): UltraByEasyPeasyStore