# An Examination of Thermal Features' Relevance in the Task of Battery-Fault Detection
## 1 Introduction  
A typical large scale uninterruptible power supply usually consists of a higher number of elementary batteries connected in series strings or series-parallel groups. Nowadays, most advanced uninterruptible power supplies (UPS) are equipped with a complex battery management system (BMS) in order to maintain their sustainable operability and prevent their degradation. The voltage of each battery is measured and the charging and discharging current of every battery in a string is controlled by balancers. This approach is necessary, especially for Li-ion batteries which are sensitive to overcharge and deep discharge [1].  
In industrial purposed UPS, the lead-acid batteries are most frequently used due to their lower price and higher resistance to overcharge and deep discharge processes [2]. This implies a long payback period for any BMS application and most of the lead-acid based UPS are operated without battery management. Their charging process is controlled only by the measured battery voltage.  
Modern BMS evaluates the condition and charge level of the batteries according to their voltage level, charging or discharging current and the cell temperature [3]. When the cell is damaged, overloaded or overcharged, it generates additional heat [4], which underlines the importance of temperature monitoring included as a part of the modern BMS. Temperature sensors must be placed directly on the cell surface, or inside the cell itself for proper functioning. In most applications, a simple thermistor is used for temperature monitoring. However, there are other types of sensors,developed especially for BMS purposes. Semiconductor CMOS sensors can provide high accuracy and linearity [4]. Flexible micro temperature sensors can be fabricated into the cells to provide high accuracy measurement with a short response time [5]. Temperature sensors may be also produced by a printed electronics technology [6], which can also positively affect their price. In special applications, an optical fiber can be used for the external and internal cell temperature monitoring [7]. This method is especially suitable in noisy environments.  
In general, only contact sensors are being used for temperature monitoring of batteries. An infrared thermography, such as in studies of the thermal characteristics acquisitions [4] and factory diagnostics [5], was applied mostly for onsite inspections. Such an estimation of a battery condition according to its temperature leads to the overall difficulty and cost decrease while the performance remains the same or at least comparable.  
Our article describes an experiment of the following parts:  
* To gather an original dataset of charging and discharging lead-acid battery phases containing their voltage levels and temperatures that will be taken by an infrared thermography device placed above them.
* To extract valid features from the thermal images and to evaluate their clarity and relevance for a possible comparison.
* To demonstrate the relevance of the features by developing a fault detection model.  
Such a study, according to the available literature, has been carried out for the first time and in case of success, it can lead to a design of an innovative and low cost BMS based on contact-less infrared thermography measurements.  
This is usually done by a maintenance personnel. Goal of this paper is to describe the possibility of infrared thermography for long term monitoring and estimation of the condition of batteries in a string of UPS. Whole experiment was driven by the idea of creation of infrared thermography supplement to maintenance.