Internal resistance

The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of circuits.

In practice, the internal resistance of a battery is dependent on its size, state of charge, chemical properties, age, temperature, and the discharge current.

[1] A large part of this drop is due to the increase in the magnitude of the electrolyte diffusion coefficient.

Many equivalent series resistance (ESR) meters, essentially AC milliohm-meters normally used to measure the ESR of capacitors, can be used to estimate battery internal resistance, particularly to check the state of discharge of a battery rather than obtain an accurate DC value.

In use, 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;[3] thus, it is a good indicator of expected life.

Internal resistance model of a source of voltage , where ε is the electromotive force of the source, R is the load resistance, V is the voltage drop across the load, I is the current delivered by the source, and r is the internal resistance.