When should I replace my Dell latitude 7212 battery?

In most case when the laptop is not functioning properly, you can not tell the problem is battery or adapter. You can read this article before taking further step. This is from www.dell.com.

Verify AC adapter functionality

To ensure that the Dell battery is getting charged correctly, the AC adapter should be functioning normally. Verify if the AC adapter is recognized correctly by the PC.

To verify if the PC is recognizing the AC adapter correctly:

Restart the PC.
When the Dell logo appears, tap F2 key until the message Entering Setup appears.
Verify the AC Adapter Type in the BIOS options.
If AC Adapter Type is None - Ensure to connect the AC adapter to the PC.
If AC Adapter Type is Unknown - Follow troubleshooting steps to resolve AC adapter issues. If AC Adapter Type is recognized correctly, proceed to the next step.

dell latitude 7212 

Brand new dell latitude 7212 7.6V 4342mAh laptop batteries

Voltage: 7.6V
Capacity: 4342mAh (34Wh)
Battery cells: 4-cell
Chemical: Li-ion
Battery Color: black
Condition: new original battery
Warranty: 9 months