Battery Cell Monitor with EIS BQ79826Z-Q1

Texas Instruments' BQ79826Z-Q1 is a stackable 26-series battery monitor for high-voltage battery packs. It performs the core cell supervising function — a dedicated ADC per channel measuring every cell voltage to ±1.7 mV across −40 °C to 125 °C, temperature sensing on 20 configurable GPIO, and passive balancing — and adds an integrated EIS engine that measures cell impedance in place.

Conventional monitoring reports voltage, current and temperature, which describes how much charge a cell holds. Impedance describes the condition of the cell. Measured across frequency, it separates into regions that map onto distinct physical processes: ohmic resistance exposes internal shorts from manufacturing defects or crash damage; the charge transfer region tracks the solid electrolyte interphase (SEI) layer, whose deterioration indicates cell age and predicts failure; the diffusion region gives state of charge, state of health and how hard the cell can currently be charged.

Conventional pack protection is reactive. Over-voltage and over-current protection, temperature sensors and pressure sensors all report a condition once it has occurred, and they measure from the surface of the cell or the pack. Impedance measurement is proactive: it observes the electrochemical behaviour inside the cell, which is where degradation and failure begin. That difference — detecting a developing problem rather than recording one that has already happened — is the argument for putting an EIS engine in the monitoring device.

The regulatory driver is explicit. China's GB 38031-2025, effective July 2026, requires at least two hours between a single cell entering thermal runaway and any fire or explosion outside the pack — up from five minutes under the previous standard. Temperature sensors typically react only after multiple cells have already entered runaway, and pressure or gas sensors detect venting minutes before a fire. Neither resolves individual cells. Per-cell impedance does, and it detects the dendrite growth that causes internal shorts as it develops.

• Cell coverage: 26 channels (26S/24S), 0 V to 5.5 V per cell, −2 V to 2 V busbar range, 143 V absolute maximum
• Voltage accuracy: ±1.7 mV from −40 °C to 125 °C, dedicated ADC per channel with synchronized redundant measurement
• EIS engine: 0.01 Hz to 3.5 kHz, 1 % impedance accuracy at 1 A excitation and 200 µΩ, global and local excitation, < 5 µs current/voltage synchronization between devices, and up to five frequencies excited simultaneously to shorten measurement time
• Balancing: passive, integrated FETs, up to 300 mA, odd/even with programmable PWM control
• Stacking: up to 128 devices; stack, ring, multidrop and split-ring topologies; 2 Mbps daisy chain, up to 4 Mbps with dual SPI
• Diagnostics: cell over/under-voltage and over/under-temperature, open wire detection, two die temperature monitors, redundancy paths for cell and GPIO voltage, limp home mode, fault detection time < 100 ms for 800 V packs or 250 cells
• Functional safety: developed under ISO 26262, system and hardware capability up to ASIL D; AEC-Q100 automotive qualified
• Power: integrated DC-DC converter; < 5 mA active, as low as 20 µA in sleep with monitoring, < 10 µA shutdown
• Package: 100-pin HTQFP, 12 mm × 12 mm

EIS does not replace DC voltage measurement — it extends it. The device is intended for engineers designing battery system electronics: cell supervising circuits, battery monitoring units and the surrounding BMS architecture. It is a component to be designed in, not a finished battery management system.