# An Examination of Thermal Features' Relevance in the Task of Battery-Fault Detection
## 2 Experiment Design  
12 V/63 Ah lead-acid batteries were used in this experiment. First of all, high resolution thermograms were captured to discover significant temperature differences during battery malfunction. As presumed, a thermal map inhomogeneity is detectable in the battery with internal defects (Figure 1).  
Having confirmed this, the experiment itself was conducted. Batteries were connected into the series string, containing from three to five batteries according to the experiment battery set. Before the experiment, all measured batteries were diagnosed for their current capacity (Table 1). Batteries with capacity below 50% were considered damaged.  
Figure 1: Thermogram of damaged and undamaged battery.  
An infrared camera was mounted on the ceiling of the laboratory for an automatic data acquisition with repetitive period of 1 min. The temperature range on the camera was set manually. The lower limit was set according to the ambient temperature and the upper limit according to the battery maximum allowed temperature. Value of the battery surface emissivity was estimated with the emissivity tape and set to 0.96. Voltage of each battery was measured with a data acquisition card with the same repetitive period as in thermogram acquisition (1 min). Batteries were mounted on a sheet of structural insulated panel which provided thermal insulation between the batteries and concrete floor in the laboratory (Figure 2).  
A voltage source was used as a charger. Its voltage was set according to the number of batteries in a battery set, multiplied by float charge voltage of the battery (13.6 V). This is the so-called constant voltage charging mode. Charging and discharging current was limited to 15 A to prevent the battery damage. Ideally, all the batteries in a series string have the same properties, which leads to a uniform distribution of voltage in each battery (Figure 3a). If one battery from the battery set is damaged, voltage distribution across the string is influenced by this malfunction. This failure leads to the overcharge or undercharge of the rest of batteries in the battery set (Figure 3b).  
This is the reason why our experiment did not include any battery set containing only new batteries (with 100% capacity). A battery set containing only new batteries with nominal capacity has a uniform voltage distribution without any overcharging possibility.  
Fault state of the battery was detected according to its voltage. If voltage of the battery overreached its maximum allowable value during the charging process (14.5 V), the battery state was labeled as fault. Similarly, when voltage of the battery dropped below the minimum allowable value (10.5 V) during discharging process, the battery state was also labeled as fault.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**(\%)**} & \multicolumn{6}{c}{**Battery Set**} \\ \cline{2-7}  & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline Battery 1 & 67 & 67 & 67 & 67 & 67 & 67 \\ Battery 2 & 76 & 76 & 76 & 76 & 76 & 22 \\ Battery 3 & 76 & 76 & 76 & 76 & 25 & 25 \\ Battery 4 & 53 & 22 & 24 & - & - & - \\ Battery 5 & 65 & 65 & 65 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: Percentage values of battery actual capacity (from nominal value).  
Figure 2: Thermogram of damaged and undamaged battery.