Battery Calibration and Cell Balancing in Tesla Packs
Understanding how Tesla’s Battery Management System (BMS) and Battery Monitoring Boards (BMBs) operate can be complex, but it’s crucial when diagnosing range loss, imbalance errors, or capacity issues.
A Tesla battery pack is designed as a series of modules, each containing bricks of individual cells. These cells are charged and discharged together, but not all cells age identically, which can lead to imbalances.
How Imbalances Occur
During charging and discharging, some bricks in the battery pack may store and release energy at slightly different rates. Over time, this variation can grow, resulting in some bricks reaching higher or lower voltages than the others.
If left unchecked, this imbalance could cause:
✅ Overcharged bricks near 100% SOC (State of Charge)
✅ Undercharged bricks near 0% SOC
To prevent this, Tesla’s BMS actively monitors voltage levels across the battery and makes adjustments through cell balancing.
How Tesla Handles Cell Balancing
Unlike some battery management systems that can actively balance packs by shifting energy from high-voltage bricks to low-voltage ones, Tesla’s BMS only has passive balancing.
🔧 How it works:
- The BMBs contain resistors that dissipate excess energy from higher-voltage bricks by converting it into heat.
- This process gradually brings all the cells into alignment by bleeding charge from overperforming sections.
Tesla has refined how and when the BMS triggers this process over the years through software updates, though details remain largely undisclosed.
The Myth of “Clipping Out” Bad Cells
Some sources claim that removing a faulty cell can extend battery life, but this isn’t a reliable fix.
📉 If a single cell in a brick fails and is physically removed:
- The total capacity of that brick is permanently reduced
- The brick will charge and discharge at a different rate, causing long-term imbalances
- The BMS will attempt to compensate, triggering balancing resistors to overwork
Ultimately, if a module has a dead or degraded cell, replacing the entire module or the entire battery is a better solution.
Replacing a Tesla Battery Module – Does It Work?
In theory, you can swap out a single module from a donor battery pack, but this is rarely successful long-term.
🔍 Why?
- Every Tesla pack ages as a whole—all cells degrade together.
- A replacement module may have a different degradation profile, leading to uneven charging/discharging.
- The BMS quickly detects mismatches and may trigger errors like BMS_u029 (capacity mismatch).
🛠️ What works better?
If a pack is failing, a complete battery swap (with a matched replacement pack) is the most effective solution.
Tesla Battery Management Calibration vs. Cell Balancing
Tesla’s BMS has two key functions related to keeping the battery healthy:
1. Calibration (Ensuring the BMS has accurate data)
🔹 The BMS continuously estimates battery capacity and health based on previous charging cycles.
🔹 If the car is always charged the same way (e.g., never discharged below 50%), the BMS may misestimate range.
How to Improve BMS Calibration:
- 🔋 Let the car rest overnight without charging (Sentry Mode OFF)
- ⚡ Charge to different levels occasionally (avoid always charging at the same level)
- 📱 Turn off third-party apps that keep the car awake
Letting the car sleep across a range of States of Charge (SOC) gives the BMS a clearer picture of total capacity.
2. Cell Balancing (Ensuring all cells remain in sync)
🔹 Over time, slight voltage differences between bricks can cause imbalance.
🔹 The BMS compensates by using passive balancing resistors to drain energy from higher-voltage bricks.
How to Trigger Cell Balancing:
✅ Charge to 100% and keep the car plugged in even after charging finishes
✅ Leave the car connected to AC power, allowing the BMS time to balance cells
✅ Monitor energy flow—charging may continue at very low amperage for an hour or more
🔍 If 100% charging is not possible:
This could indicate a BMS calibration issue that needs to be addressed first.
How to Identify a Highly Degraded Battery
If your car is showing significant range loss, consider these steps before assuming the pack is failing:
1. Run BMS Calibration:
- Let the battery drop below 20% SOC
- Leave the car off and unplugged for several hours
- Fully charge to 100% and let it sit until charging completely stops
✅ Expected Result: The range display may improve as the BMS recalibrates.
Advanced: Checking Battery Health in Service Mode
Tesla vehicles have a hidden service menu that can run a full battery health check.
⚠️ Note:
- This fully discharges and recharges the pack
- It can take several hours
- Misuse of the service menu can cause unexpected issues
📌 Recommendation:
Instead of running service tests, following the manual calibration steps over several weeks can achieve similar results safely.
Final Thoughts
Battery balancing and calibration aren’t the same thing, but both play a role in maintaining optimal range and longevity.
Key Takeaways:
🔧 BMS Calibration = Ensuring the car has accurate data about charge levels
⚡ Cell Balancing = Aligning individual cell voltages across the battery
By following best practices, you can recover lost range and extend battery life without unnecessary interventions.
