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Bq40370 ✦ Updated & Plus

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Bq40370 ✦ Updated & Plus

Report: Texas Instruments bq40370 Lithium-Ion Battery Gauge

Security and Encryption

One final note for reverse engineers: The bq40370 supports SHA-1 Authentication. Many OEMs enable the "Authenticate on Seal" bit. This means that even if you perfectly emulate the SMBus protocol, the host laptop/tool will send a 160-bit challenge to the bq40370. If the chip does not return the correct hash (computed by a secret key burned into the IC's ROM), the host refuses to turn on the system. You cannot extract this key via side-channel attacks easily.

Steps:

  1. Connect SDA, SCL, VSS, BAT+ to the pack.
  2. Launch bqStudio → Select bq40370 from device list.
  3. Set chemistry (load a .chem file for your cells).
  4. Configure protection:
    • Set OV threshold (e.g., 4.25V/cell)
    • Set UV threshold (e.g., 2.8V/cell)
    • Set current limits (OCC/OCD/SCD)
  5. Configure CEDV parameters:
    • EDV0, EDV1, EDV2 voltages (at C/10 rate)
    • CEDV coefficients (a, b, c, d)
  6. Set cell balancing (internal balancing current ~5–10 mA).
  7. Calibrate:
    • Offset calibration (CC offset, board offset)
    • Current gain calibration (using known load)
    • Temperature calibration (TS1, TS2)
  8. Write to data flash and seal the device (lock configuration).

⚠️ Warning: Incorrect CEDV parameters will cause SOC jump errors. Always validate with a full discharge test.

Core Architecture: Impedance Track vs. CEDV

To understand the bq40370, you must first understand TI’s proprietary Impedance Track™ technology. bq40370

Traditional fuel gauges use CEDV (Compensated End of Discharge Voltage) algorithms, which measure voltage and current but struggle with accuracy under dynamic loads. The bq40370 utilizes Generation 2 Impedance Track. This means it does not just measure voltage; it measures the internal impedance (resistance) of the battery cells in real-time.

Why this matters: As a lithium-ion battery ages, its internal resistance increases. A standard gauge might show 40% remaining, but under a heavy load (e.g., a power drill or a laptop CPU spike), the voltage might sag below the cutoff threshold, causing a system shutdown. The bq40370 predicts this sag by tracking impedance, providing a Remaining Capacity (RM) reading that remains accurate even under pulsed loads. Connect SDA, SCL, VSS, BAT+ to the pack

bq40370 — Technical Overview and Reference

BQ40370 vs. BQ40Z50: Which is Better?

A common question among design engineers: Why use the bq40370 when the BQ40Z50 is newer?

| Feature | bq40370 | BQ40Z50 | | :--- | :--- | :--- | | Algorithm | Impedance Track (Gen 2) | Impedance Track (Gen 5) | | Series Cells | 2 to 4 cells | 1 to 4 cells (with taps) | | Protection | Basic OV, UV, OT, Short | Enhanced (pre-charge, CTO, FET temp) | | Current Drive | Standard PFETs | High-side NFET drivers (more efficient) | | Legacy Support | Linux kernel support (old) | Requires new SMBus drivers | | Availability | NRND (Not Recommended for New Designs) | Active | Set OV threshold (e

Verdict: For new product development in 2024-2025, do not use the bq40370. TI lists it as "NRND." Use the BQ40Z50-R2 or BQ40Z80. However, if you are maintaining legacy medical or industrial equipment, you must stick with the bq40370 for drop-in compatibility.