Sodium hexa meta phosphate (SHMP) is an inorganic salt with a polymeric structure which produces massive branches of hexa meta phosphate anions (HMP−) in an acidic environment. The main goal of the presented work is to investigate the influence of SHMP as an electrolyte additive on the performance of the lead-acid batteries. To achieve the point, the examinations are done in two separate ways: i) Investigation electrochemical behavior of lead alloy electrode, by studying Hydrogen and Oxygen Evolution Reaction (HER and OER) with electrochemical techniques including; Cyclic Voltammetry and Linear Sweep Voltammetry. Then, Scan of Electron Microscopy (SEM performed to study the surface of the electrode after electrochemical treatments. By increasing SHMP concentration, overpotential of hydrogen and oxygen evolution is increased due to improving particle size of PbSO4 which consequently lessens water loss. ii) Real battery tests including; water consumption test, capacity measurements and High Rate Partial State of Charge (HRPSoC) as a cycle-life examination. Also, Scan of Electron Microscopy from the surface of the negative electrode and X-Ray Diffraction (XRD) analysis of negative electrode plate at the charge and discharge state after cycle-life test are prepared for study the PbSO4 particles size and quantity or the morphology of sulfating layer. Addition of SHMP to the electrolyte of the lead-acid battery increases cycle-life numbers and accordingly prolongs battery life. Therefore, SHMP is a promising electrolyte additive and it would be used with a combination of additives to increase the overall performance of a lead-acid battery.