Calendar Aging of LiFePO4/Graphite Cells in a 250kW/500kWh Li-Ion Battery Stack Integrated in a Solar Farm
Authors: Kubiak Pierre , Cen Zhaohui , Belharouak Ilias 
Published: February 2020

The worldwide large scale integration of renewable energies will require a large deployment of energy storage solutions. Lithium-ion batteries will be an ideal choice as stationary energy storage systems to be integrated with solar photovoltaics as the PVs can generate energy and the battery can store and deliver this energy on demand with a minimal cost to end users [1,2]. Unlike portable electronics and automotive applications, the batteries for stationary applications are not constrained by volume and weight. Therefore more attention can be turn on cost, safety and battery lifespan rather than on energy and power density. In this context LiFePO4/graphite Li-ion technology is a valid option for stationary applications as LiFePO4 presents lower voltage and energy density than conventional cathodes materials such as LiMO2 (M = Ni, Co, Mn) or spinel LiMn2O4, but it offers other advantages especially in terms of cost, safety and cycle life.

This Li-ion technology has been chosen as energy storage system to equip the hybrid solar power generation/storage micro-grid system installed near Doha in Qatar. The purpose of this micro-grid is to investigate the feasibility of implementing renewable energies in an efficient way under the harsh climate conditions of Qatar. It also offers a unique opportunity for investigating the cycling conditions and aging behavior of Li-ion batteries, and providing statistics on a large number of cells. Generally, aging is studied in labs by screening various cycling and storage conditions on a limited number of cells. The effects of calendar aging LiFePO4/graphite cells strongly depend on the cell state of charge and on the storage temperature and are mostly affecting the negative graphite electrode [3-5].
In this study we analyze a complete battery stack after 3 years of storage at high state of charge. The battery stack is a BYD 250kW/500kWh Li-ion battery that contains 7 strings in parallel made of 168 LiFePO4/graphite cells in series. The whole system consists in power convert system (PCS), Li-ion battery, battery management system (BMS), fire protection system, lighting system, ventilation and air-conditioning system, and grounding system. Communication between PCS and SCADA system allows checking in real time and record data such as, voltage, current, temperature, SOC value, SOH value, and charge or discharge state at the battery, string and cell level. The first results show that the overall capacity of the battery has only been slightly affected by the aging. However, a very limited number of cells show significant damage due to self-discharge. Further results will be presented and discussed during the meeting.
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