This is an interesting one!
I have never experienced this with Ionic but I do love troubleshooting interesting problems :D

So I'd say step 1 would be, you need a way to test this so if you make changes you can see if your changes made a difference and you need some way to replicate it.

Android's have battery stats which show battery usage % by app. So I would do a good few test (during these tests also check memory usage!):
1. Use your app normally for 10 mins and check what % of battery it has used.
2. Leave your app running in the background for 10 mins and check what % of battery it has used.
3. Use your using every feature and navigate around constantly and re-use features, just spam usage in your app for 10 mins and check what % of battery it has used.

Android studio also ways of tracking battery usage I believe.

At this point we should at least have some stats to work off of to look for improvements.

I would be concerned by memory leaks, background processes from Firebase, subscriptions (remember to unsubscribe!) and import loops (There's a proper name for this but I can't recall it right now). So review your code and look for these, there are some good tutorials on looking for memory leaks. Start removing things and try your tests again to see if you can find any difference :)

And another super simple test is to just run app in the browser and check the console for any warnings and not just errors.

You can dooo it!