| The theoretical specific capacity of lithium iron phosphateis 170 m Ah/g.Due to its high safety,lithium iron phosphate is widely used in the field of electric vehicles.Until now,charging of lithium iron phosphate based lithium ion batteries in electric vehicles still cannot be achieved at low temperatures(T ≤ 0 ℃),because the conductivity of lithium-ion in phosphate lithium-ion battery is very low.The temperature has played an important role on the charging and discharging processes of the large capacity lithium-ion power batteries,therefore,the researches on liquid-cooled thermal management system of the large capacity lithium iron phosphate batteries have important scientific significance and practical application value.needs to be developed.This thesis mainly includes the following research contents:(1)The electrochemistry and thermal properties of 15 Ah cylindrical lithium iron phosphate batteries were studied.The experimental results showed that in the temperature range of-5 ～ 10 and 0 ～ 20℃,the charging and discharging capacity significantly increased.And we compared the specific heat by using calorimetric method and mass average method respectively,the calculated specific heat is 7.5% less than the calorific value.We used calorimetric method to study the thermal behavior of the battery charging and discharging process under different C rate,and the results showed that the heat generated by charging process under 1/3C rate is 0.9 k J,which was less than that generated by discharging process under the same C rate.In addition,under the 1/3C ～ 2C and 25 ～ 45℃,energy efficiency of the tested battery is more than 95%.Besides,the experimental results showed that under certain temperature range,with the increase of temperature,charging and discharging capacity are increased significantly.(2)We studied the effectiveness of several cooling material in maintaining a lower surface temperature and temperature gradient of the battery in passive thermal management system.The results showed that when we used PCM,water or water with foam aluminum as cooling material,even under the current rate of 3C,the average surface temperature rise is less than 8,the highest surface temperature difference is less than 16.In the mean time,the internal temperature of the battery will be within the scope of security.In addition,through comparative analysis of initiative and passive temperature control methods,combined with the analysis of battery internal resistance and heat generation,we selected the liquid cooling system,which can give consideration to the both functions of cooling in summer and heating in winter and can be applied to the thermal management of high-capacity power lithium batteries for pure electric commercial vehicles.(3)Based on the Guoxuan Power 15 Ah cell cylindrical lithium iron phosphate batteries,we designed,developed and verified the liquid-cooling system of large-capacity power batteries.The cooling experimental results of high temperature in40℃ showed that the maximum temperature difference of charging in 0.75 C was 4,maximum temperature difference of discharging in 1C was 6,maximum temperature difference of discharging in 0.5C was 4 and less than 8,which was complied with the design standards.In the heating process in low temperature of-20℃,the temperature rise rate of batteries was 0.59/min,maximum temperature difference was 9.2 and less than 10,which was complied with the design standards.When the liquid cooled battery pack was filled with 250 k Pa compressed air and kept at 150 s,we measured the leaks.The results of the three experiments were all less than 1.5m L/min,which met the requirements.The sub pressure drop of external line in the inlet fluid temperature of25℃ and flow rate of 12L/min was 10 k Pa,and the pressure drop of battery pack in flow rate of 12L/min and cooling fluid of 25℃ was 37 k Pa and less than 45 k Pa,which satisfied the requirements.To sum up,the design of the liquid cooling system achieved the expected effect.In the fifth chapter,conclusion and innovation of this paper was summarized and deficiency was analyzed,and the prospect was put forward. |
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