Fig. 1. FBG array layout and spectrum. (a) A schematic diagram; (b) a layout inside and outside the battery; (c) glass encapsulated; and (d) spectrum of the FBG array.

Fig. 2. FBG temperature sensitivity in the temperature range of 20°C–60°C.

Fig. 3. Battery simulation. (a) A geometry model used in the simulation. (b) Temperature distribution of the battery—a cross-section view.

Fig. 4. Changes in the temperature and temperature rate of the inner and outer surfaces of batteries under three different charging and discharging rates: (a)–(d) Voltage current, internal and external temperature and temperature rate change and difference between internal and external temperature changes at charging and discharging rate of 0.5C; (e)–(h) voltage current, internal and external temperature and temperature rate change and difference between internal and external temperature changes at charging and discharging rate of 1C; (i)–(l) voltage current, internal and external temperature and temperature rate change and difference between temperature and external temperature changes at charging and discharging rate of 1.25C.

Fig. 5. Comparison of the measured and simulated internal temperatures when the battery is charged and discharged in the second cycle at 0.5C.

Fig. 6. Capacity test with the FBG array inserted and no FBG array inserted in the battery. (a) Capacity and voltage changes. (b) Localized amplification of voltage changes during a single charging and discharging cycle.

Fig. 7. Changes of internal and external temperature during battery charging and discharging: (a), (b) Voltage current and difference between internal and external temperature changes at charging and discharging rate of 0.5C; (c), (d) voltage current and difference between internal and external temperature changes at charging and discharging rate of 1C; (e), (f) Voltage current and difference between internal and external temperature changes at charging and discharging rate of 1.25C.

Fig. 8. Battery state change at 2C charging and discharging rate. (a) Voltage and current change; (b) internal and external temperature change; (c) difference between internal and external temperature changes.

Table 1. Center Wavelengths and Sensitivities of FBGs

Table 2. Battery Charging and Discharging Test Procedure

Table 3. Material Properties

Table 4. Entropy Values Based on Different SOCs