Cooling and Preheating of Batteries in Hybrid Electric Vehicles

The performance of a hybrid electric vehicle (HEV) depends strongly on the performance of its high-voltage battery pack, which is influenced by temperature. We have been working on thermal management of batteries in HEVs, including cooling and heating issues. In cold temperatures, batteries perform poorly because of high internal resistance; the vehicle may start slowly. The battery may need to be preheated by heating the internal core, heating the external of a module with electric heaters or a hot fluid, or heating around each cell in a module with electric heaters or a hot fluid. We used finite element thermal analysis to analyze the transient thermal behavior of a typical battery for each preheating method and compared the energy required to heat the battery. Heating the internal core with alternating current (AC) through battery terminals was the most effective and energy-efficient method. Although direct current (DC) can heat the battery, it may damage the battery. We found that 100 Amp, 60 Hz AC heating was effective for warming up a non-operating 16 Amp-h lead acid battery at -40°C to deliver an acceptable performance. However, 60 Hz AC heating is good for electric vehicle applications. For HEV applications, higher frequency currents must be used for smaller and lighter power electronics and for an on-board generator. We have tested the feasibility of a high frequency heater circuit for on-board vehicle use. Preliminary results have shown that applying a 60 A, 10 kHz current to a nickel metal hydride pack initially at -20°C restored the battery performance close to its +25°C performance in less than 3 minutes. This paper provides an overview of battery thermal management progress for HEVs, the results of finite element thermal analysis, and experimental results of AC heating of batteries. Copyright © 2003 JSME NOMENCLATURE m = module mass (kg) p C = specific heat of the module (J/kg °C) Ibat = current through the battery (A) T = module temperature (°C) 0 T = initial module temperature and ambient temperature (°C) t = time (S) = ∆t time difference (S) Rbat = Battery Internal resistance (mΩ) q = heat rate added to in the battery (W) Q = amount of energy/heat added to the battery for preheating in t ∆ seconds (J or Wh) h = heat transfer coefficient (W/m/°C) HEV = hybrid electric vehicle EV = electric vehicle NiMH = nickel metal hydride Li-Ion = lithium ion W = width of a module (m) L = length of the module (m)