Impact of Short Circuits on VRLA Battery Systems

A "shorted" lead acid battery has the capability of delivering an extremely high current, 100 to 1000 times the typical discharge current used in most applications. Electrical systems using batteries must be properly protected to avoid potentially dangerous fault conditions. In this paper, we compare the short circuit currents as predicted using generally accepted estimation methods versus actual measured values for individual batteries and battery systems. Practical considerations such as the effects of temperature, state of charge and type of circuit protection device are also presented. A battery's short circuit current is typically estimated by dividing its open circuit voltage by its internal resistance. While the true DC internal resistance can be determined using a series of discharge tests, it is often simpler to directly measure the battery's impedance or conductance using an AC test signal; several test units are commercially available. Due to differences in resistance versus impedance values, the calculated short circuit currents will naturally be different from those obtained using traditional DC test methods.