Battery corrosive electrolyte leak is decreased by using an adjustable mechanical retention device to bias a plurality of batteries toward a first end of the series configuration wherein the adjustable mechanical retention device creates a snug mechanical fit at a bottom surface of the metal can of the terminal battery. When the device in which batteries are being used is a flashlight, a two-piece tail cap can be used, an inner member of which is driven by an outer member, and tail caps of existing flashlights can be replaced so that a strong tail cap spring no longer provides a biasing means against the terminal contact of the terminal battery.