IPhone 7 plus battery life

Here are a few things you can try.

Open the settings app, and click "battery." You might see a message that says "Service now," which is a sign that it's time to replace/service the battery (or replace the phone). Rechargeable batteries inevitably decline over time, so this is nothing unusual. Otherwise, follow the steps below:

1. Turn on "low power mode" whenever you can.
2. Do a "soft reset" (no data is lost) by holding down the volume down and power buttons for a few seconds. This will make your iphone restart.
3. Turn off notifications for as many apps as possible.

What is the battery health on that old device?

80%, or lower means to get it replaced.