iPhone 5SE battery drains to 10% when OFF

I’ve been backwards & forwards with Apple Support phone calls, but not getting any closer to solving this problem, only standard answers. This phone has a weak spot around 40-50% battery power. If I switch it OFF with these levels of battery, it will either need to be connected to charger in order to switch back on, or it will switch on with ONLY 10% battery (every time - it’s always 10). It is losing up to 40% battery when SWITCHED OFF. It also cut out at 44% battery last week and would not power back on until I did soft reset - eventually started up with 10% before dying again; jumped back to 44% when connected to charger. Low screen brightness, app refresh off for most except those necessary eg Strava, lots of apps removed eg Facebook, restored from back up etc etc etc. It’s still happening and is so unreliable - please someone help!