Aviation battery maintenance is continuing to evolve. Much recent effort has been devoted to battery redesign to totally maintenance free or non-maintainable batteries. These batteries are placed into service and replaced at predetermined intervals. Still, some batteries are failing before their scheduled replacement period. For this reason attention is being focused on methods to transition batteries to an on-condition maintenance status.Nickel-Cadmium (NiCd) and Valve Regulated Lead-Acid (VRLA) are used to start engines, provide emergency back-up power, and assure ground power capability for maintenance and pre-flight checkout. Various Lithium-based battery chemistries are also now being developed and considered for use in these applications.As these functions are mission essential, State of Health (SoH) recognition is critical. SoH includes information regarding battery energy, power and residual cycle life. This document describes an SoH recognition technique for on-board aviation batteries that utilizes a totally passive diagnostic device (PDD). The PDD monitors on-board electrical system for battery current, voltage and ambient temperature. Being totally passive, there are no active signals introduced to the battery or aircraft electrical system, thereby eliminating the possibility of any interference with the vehicle electrical and operating system.A procedure for data sampling and analysis of transient and stationary battery voltage and current under normal aircraft operating conditions resulting in a matrix of battery parameters will be discussed. A matrix of parameters (MoP) including but not limited to, ohm and chemical resistance, instantaneous and dynamic open circuit voltage forms the basis for SoH recognition. Additionally, data from a demonstration of the PDD battery SoH determinations on actual F/A-18 equipment will be presented.This presentation will further describe on-going efforts to develop an SoH recognition technique for on-board Li-Ion aviation batteries. Li-ion battery operation is accomplished under precise controlled conditions. In a Li-Ion battery system, PDD data are sampled and processed both in charging and discharging modes. Advantages of the capability of the PDD to provide early signs of impending battery failure or simply early identification of the inability of the battery to carry out a necessary function, or the need for off-line battery maintenance making the battery an on-condition maintenance item, will be discussed.