Battery modelling and usage guidelines in Solar Home Systems

Abstract

Nowadays, there are still some regions in developing countries lacking electrification. A solution to electrify thehouseholds in these areas with off-grid Solar Home Systems (SHS) has been proposed for some time. However,battery storage in SHS is always the limitation in both cost and lifetime point of view. Thus, a battery withstable behaviour, longer lifetime and less maintenance as well as a lower price is highly desired in SHSs. Withthe narrow choices on economical commercial battery technologies, there is another approach to improve thebattery behaviour in SHSs.This thesis is aiming at exploring a solution from usage perspective to maintain the battery behaviour in long-term, in SHSs. In order to achieve the goal, a tool and a method were proposed to provide a practical solutionfor battery performance preservation in SHSs.In this study, two battery technologies which are commonly applied in SHSs were explored: the LiFePO 4battery and the Valve Regulated Lead-acid batteryFirstly, an accurate battery dynamic model based on the electrical equivalent circuit was constructed for bothbattery technologies separately. Series of experiments were performed to obtain the relevant parameters. This model was built for low current applications, which is lower than 1 C, typically suitable for SHS applications.This model was on battery cell level and with a great accuracy with a < 2% errorSecondly, a performance based battery lifetime prediction model was built, and the battery capacity wasselected as the index of the ageing process. The modelling was achieved by applying a new concept, which isthe rate of normalised capacity fading, with respect to capacity throughput. Another series of experiments wereoperated for the exploration of the relationship between stress factors and the rate of battery ageing. Theexperimental data is the foundation of the battery lifetime model.Thirdly, a usage guideline for each battery technology was proposed by analysing the lifetime test data. Thenone practical usage guideline application method was simulated. The application of the usage guideline has anoticeable improvement in the battery behaviour in long term scale.In conclusion, the modelling of the battery including both dynamic behaviour as well as lifetime predictionprovides a tool for future exploration. With this tool, the design and sizing of the battery storage system, as wellas the management of appropriate battery usage in SHS would be easier. The method proposed for batterycapacity preservation is the usage guideline. The usage guidelines offered wide choices on user sideimplementations regarding battery capacity preservation.