Hello The Postie,
It sounds like your Apple MagSafe Battery Pack isn't charging your phone correctly unless you try to charge the pack, right?
You may find more information here:
How to use your MagSafe Battery Pack - Apple Support
Charge your MagSafe Battery Pack and your iPhone
If both your iPhone and MagSafe Battery Pack need to be charged, you can charge them at the same time. Plug the MagSafe Battery Pack into a power adapter and then attach your MagSafe Battery Pack to your iPhone. Using a 20W or higher power adapter, the MagSafe Battery Pack can charge your iPhone with up to 15W of charging power.
You can also charge both if you attach your MagSafe Battery Pack to your iPhone, then plug your iPhone into a power source. You might want to charge this way if you need to connect your iPhone to another device while charging, like if you're using wired CarPlay or transferring photos to a Mac.
If you charge your iPhone and MagSafe Battery Pack at the same time, your iPhone might charge to 80% or higher before your MagSafe Battery Pack begins charging."
If you continue to have the same problem, you can contact Apple Support.
Regards.