The focus of this thesis involves the chemical, physical and engineering aspects related to the investigation of the primary cause(s) of the performance issues of lead acid batteries using qualitative and quantitative research methods. In this research, various real time products have been tested with the view to improving performance by resolving any issues before they launch into the consumer market.
The investigation primarily deals with high rate discharge lead acid battery subjected to long standing times in hot environmental conditions. It is important to note that these properties may be adjusted for different products and operating environments.
The technique presented in this thesis was tested and verified by using results obtained from experiments conducted at the YUASA Ltd battery laboratories and the Centre for Automotive & Power System Engineering (CAPSE) labs at the University of South Wales (USW) using 20°C as the ambient temperature operating condition.
The Yuasa valve regulated lead acid (VRLA) SWL2500 was the battery under investigation and it was selected as the representative model for characterisation in all subsequent work.
The devices inspected came from a range of different storage applications and environmental conditions. Initial research indicated that the positive active material (PAM) was the major cause for the low discharge performance with the electrochemical analysis of the battery showing the presence of significant levels of unconverted lead sulphate (5.52%). PAM microstructure investigation and porosity analysis show other key factors that limit the surface available for the electrolyte to diffuse into the plates and consequently limit the overall efficiency of the battery. These aspects include the build-up of large lead sulphate crystals, which are difficult to break down in the charge process and may have arisen during the standing time period at high temperature, where the batteries experience accelerated self-discharge.
To identify the causes of pore blocking effect that occurred in the low performance positive active material it was necessary to develop a mathematical model system to detect the real pore geometry. In the first instance, to ensure that the cells studied in this investigation were in a high charged state, a monitor system was developed. This system employs one simple equation to predict the equilibrated cell voltage after a small rest period. The proposed method for monitoring and predicting the open circuit voltage (OCV) for a VRLA battery system was applied to the low and standard performance product, providing estimation errors of just 0.2%. Secondly, an analytical equation system was used to simulate different pore geometries.
The analysis was based on the variation of the pore impedance Z with frequencies w to obtain complex plane plots at high frequencies. The shape of the plots changed in the way that could identify the shape of the pores of the electrode active material.
These features are related to the coupling of the double-layer capacitance and the solution resistance in the pore, and appear at high frequencies.
For the model validation purpose, electrochemical impedance spectroscopy (EIS) experimental data was used.
The correlation between pore structure geometries and the related battery efficiency is also addressed. This investigation may clarify the possible reasons for low performance batteries. Identifying the benchmark pore geometry parameters may be useful for the battery producers to improve the efficiency of their products. Various recovery methods are also included in this investigation to disperse the build-up of lead sulphate crystal that limits the electrolysis process in the low performance batteries. These techniques were capable of improving the low performance product capacity by 29%.
In light of the factors that have been identified as affecting the capacity ability of the low performance product, it was decided to intervene in the manufacturing process. A change in design, density composition and curing process of the positive active material to meet the new customer requirements was implemented.
The PAM density was increased to improve the mechanical strength and thus the suitability for extreme environmental conditions (35°C). The density increases reduce the volume of electrolyte inside the plate. This caused the reduction of high rate performance, to counteract performance reduction the positive active material volume was increased by changing the design of the positive electrode.
Furthermore, to prevent the formation of large sulphate crystal structure on the PAM the curing formation process was improved.
With these modifications the Yuasa SWL2500 achieved more constant capacity performance and long life expectation in extreme environmental conditions.
|Date of Award||12 Sep 2017|
|Sponsors||KESS & Yuasa Battery (UK) Ltd|
|Supervisor||Jonathan Williams (Supervisor), Kary Thanapalan (Supervisor) & Peter Stevenson (Supervisor)|
- Valve Regulated Lead Acid Battery
- Battery Manufacturing
- HIgh Discharge Rates