My mom's iPhone battery is suddenly draining very fast (its losing 1% in a 10 second time period when the screen is on), so I checked the battery health in the Settings app, and it says it has 51% maximum capacity. We are going to replace he battery regardless! But even if the battery is very bad i don’t think it’s supposed to drain this fast. My question: what can I do right now to stop it from draining so quick while I look for replacement options?
Once they start to go, they go fast.
Mine went from 'perfectly fine all day' to 'need to carry a charger with you' in about a month.
Switch it into Low Power mode for now, ring Apple & arrange to take it to an Apple Store or for them to collect it.
Battery change takes 2 hours if you take it in or 3 days by post. [They say it can take longer but that's direct experience.]
Do it soon because the low price battery replacement offer ends in September, so I've heard.
You must release it from Find my iPhone before they will accept it.
Safest is to fully back it up & erase it before handing it over.
If you have a spare iPhone, restore it from that backup & drop the SIM into it for the duration.