Making a lithium polymer circuit
Protection circuitry is a highly critical element in a safely usable Lithium-polymer or Lithium-ion cell. Raw (unprotected) cells are less commonly sold, i.e. many times, the manufacturer of the battery includes protection circuitry as part of the battery; however, for safety, do not assume this is the case and check the datasheet of the battery.

Purpose of Li-ion/LiPo protection circuitry

The protection circuitry serves to protect against short-circuit, over-discharge, and over-charge conditions. Loosely speaking, the circuitry relies on an over-current cutoff, an under-voltage cutoff and an over-voltage cutoff respectively for ensuring against those three conditions. Sometimes, protection circuitry also includes thermistor-based overtemperature protection.

Unprotected versus Protected batteries

You MIGHT conceivably get by with an unprotected battery, for simplicity, if you have certain components on your PCB, e.g., a fuse (prevents overcurrent/short-circuit), an undervoltage-supervisor IC (prevents overdischarge), and a smart battery-charging IC (prevents overcharge).

But my personal assessment is: Unless you are able to constantly monitor the battery, do NOT use an unprotected battery; protection circuitries are fairly simple to implement (e.g. using pre-packaged protection ICs), or alternatively, a protection-added battery is easy to purchase.

How to implement Protection circuitry (for a Two-cell pack)

If you choose to implement it yourself (versus buying a protected pack), then here are two protection ICs you can consider for your scenario (i.e. two-cell Li-ion or LiPo):


BQ29312A from Texas Instruments
LV51135T from ON Semiconductor