Additional insulation for the heater. It is required at the beginning and end of the battery stack (arrangement is shown on the second image on the left and right).
The PCB is made of FR4 material. FR-4, or FR4, refers to a class of flame-resistant and flame-retardant composite materials made from epoxy resin and fiberglass fabric. The abbreviation FR stands for flame retardant and complies with the requirements of UL94V-0.
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