A six year old phone needs replacement, not just a battery replacement due to security updates you are not getting. If you cannot afford an S series at the moment, you would do well with an A series until your situation improves. Until you can afford a replacement, I would suggest setting a routine to remind you to charge your battery when it's at 35%. Make sure you have a current backup in case your phone decides to die completely. You can make a full backup to a flash drive using Smart Switch. You will need a flash drive equal in size to the current amount of space utilized on your device, but depending on what you choose to backup, SS will tell you what size you need.
Anyway, go to modes & routines and set up the following routine:
