1. INTRODUCTION Hybrid drivetrain hardware combines an electric motor and a transmission, gear box, or hydraulic unit. With many hybrid electric vehicle (HEV) hardware designs the transmission fluid is in contact with the electric motor. Representatives of some OEMs and tier suppliers have expressed concerns about the electrical properties of automatic transmission fluids (ATFs) - specifically, that the ATF may be sufficiently conductive to short circuit the electric motor. The implication was that the conductivity of the oil should be as low as possible in order to ensure transmission reliability. This concern was echoed in a recent paper on lubricants for hybrid electric vehicles [ 1], whose authors recommended an arbitrary upper limit for new oil conductivity. However, the strategy to pursue the lowest possible conductivity does not consider the potential for oils with insufficient conductivity to allow static buildup and discharge, which can lead to equipment damage and increase the rate of oil degradation [ 2, 3, 4, 5]. In principle, there are two concerns regarding electrical conductivity of ATFs and other lubricants. The conductivity should be low enough so that the lubricant is a good electrical insulator, but also high enough so that it can dissipate static charge. In practice, the difficult task is to determine the range of acceptable conductivities for a particular application and hardware configuration. How high is too high? How low is too low?We have also heard concerns about to what extent the accumulation of metal increases ATF conductivity and whether this could also to lead to a short circuit of the motor. Though concern over ATF conductivity may seem new in light of the introduction of HEVs and EVs into the marketplace, these characteristics have been considered before when electronics were introduced into automatic transmissions [ 6]. Though the authors stated that the electrical conductivity is important in light of increasing use of electrical components, they did not offer guidelines. The absence of electrical characteristics as part of ATF specifications for the following 25 plus years suggests that while understanding the value is important, in practice this characteristic is not problematic. These conversations have prompted us to conduct a fundamental research project to better understand the electrical conductivity of ATFs. In this paper, we will present data that address the following questions. • What is the electrical conductivity of commercial ATFs, and how much variation is there? • Which ATF components have the largest effect on conductivity? • How does conductivity change in use? • What are the effects of oil aging, metallic wear debris and dissolved metals? Electrical conductivity is proportional to the number density and mobility of charge carriers. The mobility of charge carriers is determined primarily by their charge, size, and the viscosity of the fluid.Electrical Conductivity of New and Used Automatic Transmission Fluids Chris McFadden, Kevin Hughes, Lydia Raser, and Timothy Newcomb Lubrizol Corporation (The) ABSTRACT Hybrid drivetrain hardware combines an electric motor and a transmission, gear box, or hydraulic unit. With many hybrid electric vehicle (HEV) hardware designs the transmission fluid is in contact with the electric motor . Some OEMs and tier suppliers have concerns about the electrical properties of automatic transmission fluids (ATFs). Lubrizol has conducted a fundamental research project to better understand the electrical conductivity of ATFs. In this paper, we will present conductivity data as a function of temperature for a range of commercially available ATFs. All fluids had conductivities ranging from 0.9 to 8x10-9 S/cm at 100 °C and can be considered insulators with the ability to dissipate static charge. Next we will deconstruct one ATF to show the relative impact of the various classes of lubricant additives. We find