iPhone battery health is low

If the Battery Health is less than 80%, the battery needs to be replaced.

iPhone Battery Replacement - Official Apple Support

If battery health drops, this is normal and not an error. With time, and charging cycles to be more precise, battery health will decline this is a normal process.

Sometimes it will look like that the battery health is declining fast, but after that it will keep to a standstill for a long time.

When it hits below 80%, your battery is eligible for replacement