Acid content sensitive colorimetry of poly-N-phenyl-o-phenylenediamine for state of charge determination of lead-acid battery

Abstract

The ex-situ colorimetric method of battery acid estimation was developed using poly-N-phenyl-o-phenylenediamine (PPOPD) to accurately determine the lead acid battery's state of charge (SoC). PPOPD was synthesized by the in-situ oxidative chemical polymerization of N-phenyl-o-phenylenediamine monomer (POPD) using ferric chloride as an oxidizing agent in ethanol solvent. The UV–visible absorption peak of PPOPD in acidic solutions at 518 nm showed a sharp positive linear slope with increased acid content. The charging or discharging mechanism of the battery produces changes in battery acid content. A fully charged lead-acid battery contains approximately 39.7 % sulphuric acid, whereas its acid content decreases by approximately 10 % on discharge. The PPOPD polymer showed a change in absorbance with a change in the lead-acid battery's acid percentage, which can be correlated with the state of charge. The absorbance of PPOPD solution was calibrated linearly with the sulphuric acid percentage (6–40 %), a similar acid range produced inside the battery during charging or discharging. The absorbance of PPOPD solution obtained in 10 % and 40 % sulphuric acid at 518 nm was set as the lower and upper limits (Amin and Amax) respectively for SoC determination. Therefore, the state of charge in any lead-acid battery under investigation can be determined by measuring the absorbance of the PPOPD solution. The state of charge obtained from the colorimetric method was compared with an open circuit voltage method using a 12 V lead-acid battery.