TomBlunty Is this a new phone or an old phone? Have you or the previous owner brought the phone for any repair or had any physical or water damage?
After that, depending on your scenario, please check these.
If this is a Pixel 8 series phone or older (Pixel 8a exclusive), do not use Device Diagnostics (not functional and cannot fix (issuetracker.google.com)). Instead:
Go to Settings > About phone > Battery information and check all fields: "Manufacturing date," "Date of first use" and "Cycle count."
If your Pixel is a Pixel 8a or newer (any Google Pixel released after May 2024, where Google switched to a 1000-cycle Li-ion battery):
You can go to Settings > System > Device diagnostics > Component health > Battery status and check the remaining full charge capacity of your battery. If you want to read other stats, Google does explain it all (support.google.com).
Remember batteries can fail due to time, heat, being kept on low charge (<10%) or full charge (>90%) for an extended period of time, water damage, physical damage and multiple other factors. Consider a battery change if needed.