SLUAB20A May 2025 – July 2025 BQ41Z50 , BQ41Z90
In a battery gauge based on the resistor and capacitor equivalent circuit, the gauge relies on a resistance model measured for a new battery or updates the resistance model when the load current is stable for a long enough time that the battery transients settle. When the battery load is highly dynamic, intervals of stable current long enough to verify transient settling are not frequent enough to enable accurate tracking of the resistance increase as the battery ages. In these applications, gauges based on the resistor and capacitor equivalent circuit underestimate the resistance of the battery. In these situations, the IR drop is underestimated, so the gauge overestimates the remaining capacity of the battery. In this situation, the overall system can need to shut down prematurely, and the terminal voltage can approach the minimum value faster than the gauge predicts.
However, in applications where there are frequent intervals of stable load current sufficient to allow the battery transient response to settle, traditional gauging algorithms are able to track the increasing resistance of the battery and provide accurate predictions of battery remaining capacity.
An alternative approach to using a resistor or capacitor equivalent circuit with selective updating is to use a more accurate broadband model of the battery. As shown in Figure 4-1, a broadband battery model can provide accurate predictions of the battery terminal voltage during the transient response relaxation interval. A properly designed broadband battery model can generate the transient response accurately for arbitrary load current conditions, not just step responses.
The Dynamic Z-Track algorithm relies on a broadband model of the battery response that can generate accurate battery terminal voltage estimates for dynamic load currents over long time periods. The Dynamic Z-Track is parameterized by the same values as Impedance Track based gauges: the battery resistance normalized to room temperature versus DoD and maximum battery charge storage (Qmax). The Dynamic Z-Track algorithm uses a correction factor to the IR drop in the simple resistor and capacitor battery equivalent circuit to generate accurate battery terminal voltage predictions and battery resistance estimates. The Qmax parameter is estimated using the same techniques as Impedance Track based gauges.