1 Basic Information, 1. Basic Information

Common HV Battery Failure Sources

1. v1 – C26/C27 BMB Failure (Voltage Sensing Capacitors)

What Happens?

  • The plastic shield inside the v1 packs rubs against two capacitors (C26 & C27) on the voltage sensing circuit, causing them to corrode over time.
  • When the Battery Management Board (BMB) can no longer accurately read voltages, the BMS starts throwing multiple errors.

Common Error Codes:

  • BMS_u018 – Short-term voltage imbalance. (Clears itself after repair, may need to sit overnight.)
  • BMS_u029 – Capacity imbalance. (Latches and requires a manual reset with Toolbox.)
  • BMS_f107 – High voltage sensor failure. (Also requires a reset with Toolbox.)

Fix:

  • Replace the damaged BMB capacitors and verify proper voltage readings.
  • Errors must be cleared manually with Tesla Toolbox after repairs.

2. v2 – Sense Wire Failure

What Happens?

  • The tiny sense wires that measure brick voltages break off their copper pads on the ribbon cables.
  • Brick #3 is especially prone to failure because it’s mounted upside down and has the longest flexible ribbon cable, which hangs in the air instead of lying flat.
  • This is especially common in accident-damaged packs, where impact forces can break the delicate sense wire welds.
  • Some believe thermal expansion plays a role—the module and flex cable expand and contract at different rates, causing fatigue fractures.

Common Error Codes:

  • BMS_f107 – High voltage sensor failure. (Must be manually cleared with Toolbox after repair.)

Fix:

  • Open the pack and repair or replace the broken sense wires.
  • Use better reinforcement techniques to prevent reoccurrence.
  • Manually reset BMS_f107 using Tesla Toolbox.

3. Rusty Fuse Cover – v1 Water Intrusion

What Happens?

  • Early v1 packs have a metal fuse cover that faces upwards.
  • In rust belt regions, thousands of packs have failed because water seeps through the rusted fuse cover and into the battery pack.
  • This triggers an internal isolation fault (BMS_f123), which shuts down the HV system.
  • If not addressed quickly, rust spreads inside the pack, corroding collector plates and making it unrepairable.

Common Error Codes:

  • BMS_f123 – Internal isolation fault (water intrusion). (Clears if dried, but corrosion is irreversible.)

Fix:

  • Time is critical! If water is inside the pack:
    • The only way to dry it properly is to fully disassemble and remove each module.
    • Blowing air, using rice, or draining water through the umbrella valves will NOT work.
  • Preventative maintenance:
    • Inspect the fuse cover regularly—you can see it through the frunk on RWD cars, but AWD (dual-motor) cars require pack removal for inspection.
    • Perform a Pressure Test and reset the umbrella valves when the pack is out.
    • Catch rust early! This is one of the most avoidable pack failures.

Other Contributing Factors to Battery Pack Failures

Environmental Moisture Ingress

  • C26/C27 capacitor corrosion (from issue #1) is worsened by moisture in the pack.
  • Most failures happen near the front of the pack, where moisture enters as the pack “breathes.”
  • Condensation accelerates corrosion, especially where the plastic shield touches C26/C27.
  • A good discussion on this topic: Tesla Motors Club post.

Resistive Cells (Rarely the True Issue)

  • Some popular YouTube videos suggest that removing a single bad cell can revive a failing pack.
  • In reality, true cell failures are rare—most “bad modules” in v1 packs actually have failed BMBs, which cause incorrect voltage readings.
  • The BMS compensates for faulty readings, which can create a real but correctable imbalance.
  • Tesla does NOT diagnose pack issues at the Service Center.
    • If your car is under warranty, Tesla will simply swap in a refurbished pack.
    • The old pack gets sent to a 3rd-party repair facility, refurbished, and installed in another Tesla.
  • Until 2020, no Model S had gone out of warranty, meaning independent repair knowledge was limited until recently.
  • Today, 3rd-party repair shops are uncovering these issues, leading to better repair methods and a deeper understanding of pack failures.

Final Thoughts

The Big-3 failures (C26/C27 BMB, Sense Wire Breaks, and Water Intrusion) account for the majority of v1 and v2 battery pack issues.

  • Pack failures are increasingly repairable, but quick action is crucial—especially for water-related faults.
  • Preventative maintenance, such as regular fuse cover inspections and umbrella valve resets, can prevent expensive repairs.
  • As Tesla repair knowledge grows, we will continue to refine the best repair strategies.

What will we learn next? 🚗⚡

SBecker

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