Keep a portable charge brick in your tank bag...with a tiny lead...should the battery run low (or a magnetic one if you're posh)
Or
Run a charge lead....from a power outlet on the bike.....into your tank bag.
It really isn't an issue.
Also.... iPhone's have small batteries.......get yourself a big ass android phone.
(no clue what phone anyone uses.....just throwing it out there)
One thing that I’ve noticed with MagSafe (wireless) PowerBanks, is that they cause phone to run quite warm, due to the nature of the induction charging. Perfectly fine when you are asleep or having closed all of the apps in the phone and left it alone to do its thing (that being charging).
Put said phone and power bank inside a confined space such like your tank-bag, whilst your using it as a navigator, along with say streaming music/radio (all wirelessly via BT or local WiFi) and suddenly the phone is running quite a bit warm. At same time, internal battery management systems, stops it from charging from a wireless PowerBank whilst also throttling back peak performance in order to cool down. Thats when things start to get laggy, Nav throwing a hissy fit. Your phones screen not responding and/or screen does go really dark, with faint message being displayed, telling you that your phone needs to cool down before you can use it again.
You are better off having wired charging solution. Better still, try to run Nav in the offline map mode. Download music playlist or two to your phone internal memory. This in turn will let the battery have an easier life. Remember, battery life depends on how often it is charged to a 100%, how hot do you let it run, etc.
Optimum battery charging (be it Electric Car or you phone) is between 20% and 80%*. On modern iPhones (iPhone 15 onwards), you can set the maximum charging to stop a 80% or above. This in turn does allow me to use my phone all day, with most of the time enough charge left for me to get in a van and start wirelessly charging whilst I drive.
Going back to tank-bags.
I have been using my Givi ST603 Tank-bag, along with their power hub accessory for the past 8-9 years. It is a hardwired power lead, that is permanently attached to the bikes battery, which then secured to a panel in front of the fuel tank (within the steering column aperture) from there I plug in a detachable fly lead that does plug into watertight transfer block on my tank-bag. Inside the tank bag, there is a 3 x USB-A** power port hub, which is plugged into the bung. Simple solution, that is neat, which does allow me to keep my phone topped up (via a foot long cable) when necessary, as well as I get to charge my overnight power bank for those times when I am camping for example.
*For example I run mine at 80%, this in turn resulting with phone being used relatively heavily (work) for the past 17 months, yet battery capacity still being at 94%.
** I am surprised that Givi haven’t released a USB-C version yet, seeing as USB-A has technically been knocked on the head by Eurocrats in Brussels.