the voltage across the terminals of a disposable battery driving a load decreases until it drops too low to be useful; this is largely due to an increase in internal resistance rather than a drop in the voltage of the equivalent source. In rechargeable lithium polymer batteries, the internal resistance is largely independent of the state of charge but increases as the battery ages due to the build up of a passivation layer on the electrodes called the solid electrolyte interphase; thus, it is a good indicator of expected life. See also Norton's theorem Output impedance References Student Reference Manual for Electronic Instrumentation Laboratories (2nd Edition) - Stanley Wolf & Richard F.M. Smith Fundamentals of Electric Circuits (4th Edition) - Charles Alexander & Matthew Sadiku External links Interconnection of two audio units - Output impedance and input impedance Electrical resistance and conductance es:Impedancia de salida ja:内部抵抗 tr:İç direnç