Lithium Ion Based Power Pack Solution for Military Equipment

Abstract

Sophie thermal sight is a Portable Military Electronic Device (PMED) being used by Pak Army. The sight is powered by a Nickel-Cadmium (NiCd) based pag belt. Numerous OEM provided pag belts for Sophie thermal sights are held out of action. Besides the high OEM cost (1.2 Mn Pkr), the cells and the charging modules being vintage are not available. This thesis work provides an alternate economical solution by conversion of the NiCd based pag belt into Lithium (Li) ion based peg belt having same performance with more than 50% reduction in weight. A detailed study was carried out to identify the challenges of military environment; the same have been translated into technical parameters for the battery of PMEDs, Sophie thermal imager in our case. A selection matrix is formed basing upon the key military challenges and a comparison of Li ion, NiCd and Nickel-Metal Hydride (NiMH) based batteries is carried out to reach a logical rationale for selection of battery technology. It is found that Li ion based batteries are better suited for PMEDs due to their longer battery life, low self-discharge, less weight and low maintenance requirements. The selection of battery shape and Li ion cell type is also carried out keeping in view the major requirements of battery for military applications. Consequently, a 4S4P 11.4Ah NMC based battery pack is designed. An algorithm is developed and implemented in hardware with battery pack protection and management IC BM3451 and arrangement for measurement of individual cell bank voltages via serial port through Arduino board. The hardware is tested for validation of performance on C8000 Cadex Battery Testing System along with a test circuit based upon BQ76920. The charge and discharge characteristics at 2A and 5A (different C-rates) for both circuits with two identical battery packs are carried out and voltage recovery monitored for upto 30 minutes after end of discharge cycle. The experimental results validate the performance of battery pack by cut-off at designated thresholds of voltages and performance of cell balancing. The results show that performance of different cells of the pack in series and parallel is different for same charge and discharge current. The voltage recovery for individual cell banks is more for high discharge C-rates and for cells at low SOC levels since polarization voltage changes more at higher C-rates and low SOC. A system level benefit of alternate economical solution with improved performance to the existing problems of NiCd based pag belts is provided. In addition, the charge and discharge data at 0.17C and 0.44C can be used for estimation of battery state and battery model parameter identification of a 4S4P Li Ion (NMC) battery pack.