Aviation battery maintenance is trending toward on-condition maintenance. Nickel-Cadmium (NiCd), Valve Regulated Lead-Acid (VRLA), or Lithium-Ion (Li-ion) batteries are used to start engines, provide emergency back-up power, and assure ground power capability for maintenance and pre-flight checkout. As these functions are mission essential, recognition of battery state of health (SOH) is critical. SOH includes information regarding battery energy, power and residual cycle life along with monitoring overall battery safety. This paper describes an SOH recognition technique for on-board Li-Ion aviation batteries and discusses a passive diagnostic device (PDD), that analyzes input data derived from normal system parameters such as battery current, voltage and ambient temperature. These parameters are monitored in a totally passive mode eliminating the need for active signals to the battery. Active signals are restricted or even prohibited in order to avoid any interference with the vehicle electrical system. A procedure for sampling and analyzing transient and stationary battery voltage and current and establishing a matrix of battery parameters (MoP) is discussed. The basis for SOH recognition is based on a matrix of parameters containing values for ohm and chemical resistance, instantaneous and dynamic open circuit voltage, and Tafel coefficients of electrode reactions. Because Li-ion battery charging is provided under precise controlled conditions, data are sampled and processed both in charging and discharging modes. Advantages discussed are the capability of the PDD to provide early signs of impending battery failure or simply the inability of the battery to carry out a necessary function, or the need for off-line battery maintenance.