BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI

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106 Vlad Marţian et al there is also the advantage of efficiency which for the electric engine is around 80% -90%. The main obstacle in producing on a mass scale this type of vehicles is represented by the storing capacity of the electrical energy i.e. the batteries. Actually the storing capacity is not enough, so one of the main directions of research is to improve the storing capacity of the batteries. This increase in energy density and also the need for drawing high powers form the batteries has another side effect such as increasing the temperature of the battery. The working temperature of the battery is a very important parameter, for example for a Li-ion cell an increase in temperature of 15°C will reduce the life of the cell by about 50% (Asakura, Shimomura & Shodai, 2003). The temperature has also another effect on the charge/discharge of the battery and also on the storage capacity of the battery. These parameters i.e. charge/discharge and storage capacity is quantified by using the term SOC (State of charge). In the work of Zheng Popov and others (Zheng, Popov & White, 1997) an optimum temperature for a battery is around 25°C, even if now there are batteries that can have a maximum temperature of 85°C (Winston, 2011). The current discharge/charge rate grows as the temperature approaches the optimum due to increased ion mobility and also due to modifications of internal resistance of the battery, but after the optimum the current charge/discharge rate stats to decrease due to oxidations that happen inside battery. Increasing the temperature over the functioning domain make the batteries to have a catastrophic failure, and not only the performance of the battery will be diminished but also irreversible oxidations occur and the battery becomes useless (Jiangang, et. al., 2006). For these reasons, toghether with RAAL S.A., we began to investigate the necesity of a cooling system for batteries equiped in EV and HEV. This paper presents the first step from many that includes battery modelling, the modelling of cooling modules, the modelling of an automatic driver, experimental tests of the cooling modules, and thermal test on the battery pack, etc. 2. Battery Models The literature has many models which vary in complexity. There are complex models that use quantum mechanics for describing the battery at chemical reaction level (Parthasarathy et. al., 2002), (Aron, Girban & Pop, 2010), finite element models that describe the spatial dynamics in the battery (Sievers, Sievers & Mao, 2010),electrochemical models, electrical equivalent circuit (Matthias et al., 2005), Dynamic Lumped parameters models, tabulated battery data models. To model the battery as close to the reality as possible every model has to take into account the parameters on which the battery depends on, and these parameters are a few. One of the most important parameter that the battery has is the so called state of charge, SOC, or the electrical energy stored in the battery. This parameter depends on other parameters of the battery as the
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 3, 2012 107 current drawn from the battery, the time that the current has been drawn, and the capacity of battery, and can be express in mathematical form as It SOC = 1 − , . (1) C ∫ t I ()d t t 0 (2) SOC( t) = 1 − , C where I – the current drawn (A), t – time (s), C – the battery capacity (A.s) Other parameters of the battery include the temperature of the battery, the internal resistance and the open circuit voltage. In the remaining paper we will only describe the electrical circuit models which are the base for the model in this article, for other model types you can see (Gomadam, Weidner, Dougal, & White, 2002). 2.1. Simple Model The simplest model used consists of a constant resistance R b in series with an ideal voltage source E 0 , sketched in Fig. 1. Fig. 1 – Simplest model. Even this is very simple form electrical point of view; this model does not take into account the true internal resistance of the battery, which is highly related to the state of charge (SOC). In this case the draw of energy is unlimited. Another drawback of this model is that it does not take into account the thermal energy generated during discharge. There are other, improved, electrical models, some of which modify the internal resistance according to the SOC, and also include other parameters that take into account the dynamics of the electrical current during discharge. One of this improved a model that is worth mentioning it is the Thervein model. 2.2. Thervein Model This is another basic battery model which describes a battery with an ideal voltage source (E 0 ), internal resistance R and a capacitance C 0 which represents the actual capacitance of the battery, and also an over-voltage resistance R 0 (Ziyad & Salameh, 1992). The main disatvantage of this model is
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