Tesla HV Battery Cell Balancing Guide

1. Introduction

High Voltage Battery consist with number cells (bricks). Cell has its own Stage of Charge (SOC). All cell should have similar SOC. If SOC will be not be similar. BMS will trigger imbalance alerts (BMS_u029, BMS_a062, BMS_064, BMS_a066, BMS_w117 etc.).

BMS_a066_SOC_Imbalance_Warning

Cell (Brick) Voltage are very critical for car proper operation and for safety.

2. Potential causes

There could be many possible causes for imbalance. Some of most reasons could be followings

Malfunction (Faulty Readings)

In certain circumstance/scenarios, system may not read correct readings and system will generate imbalance alerts. Please contact us to get know more about it.  

VHS

Voltage sense harness issue. Please contact us for more details and how to clear.

Alerts codes: BMS_a129, BMS_f107, BMS_w177

Battery Age

How old is you car? Is it older than ... years?

Over car age, cell impedance is natural process. There are few factors in that it might not good to repair HV Battery. If car has more than .. years old and has already drove .... kilometer etc.

Outside Temperature

One of reason could be that surrounding temperatures is very low. (Temperature is less than -XX °C (degree Celsius).

Recent Repair

Have you performed recently any repair and got this alert. Please contact us. We can help you to clear it.

Battery Monitoring Board (BMB)

Corrosion on BMB. 

3. Do I need to replace HV Battery?

Imbalance alert does not mean that you need to replace High voltage Battery. Please do not replace high voltage battery without professional diagnosis to save your 10-20 thousands euro/dollars.

There are lot cases/scenarios, car will show imbalance alert. It does not mean that high voltage battery need to replace.

Imbalance alerts may clear after cell balancing.

Brick Voltage Difference 

If the brick voltage difference is less than .. mv. There are best practices to clear imbalance alerts. Please contact us for more details.

Lot of workshop does not have knowledge, experience. So they just recommend to replace. OEM does not repair. OEM will replace battery, which could cost 10-20 thousands. 

Please feel to contact us, we will be happy to help you for affordable solutions. 

4. How to diagnose imbalance alerts  

There are different ways to diagnose alerts. It may also vary depend on alert code. Please contact us with your particular alert code  

Mobile Application

We have developed our mobile application which reads different high voltage battery readings (Brick Voltage, Brick Temperature, Voltage difference etc). and help to diagnose BMS Alerts. Please contact us for access. It will be available soon on Apple Store.  

  • Brick Voltage Max
  • Brick Voltage Min
  • Brick Voltage Diff
  • PT_BMS_SOCCELLMAXDBG
  • PT_BMS_SOCCELLMINDBG
  • MinPackTemp

Erratic brick voltage measurement

Brick voltage difference is less than .. mv. Please contact us for more details. 

5. Steps to clear

5.1 Software clear

Some of alerts may be related to software bugs or glitches.

Software Bug and Glitches

One possible reason could be software bug or glitches. 

  • Did you receive this alert after updating recent software version? 
    • If yes, it could be related to a software bug and may be resolved in a future software update.
    • Please send us screenshot of the Service Alert, and we will inform you if it could be related to software bug. Link

Firmware not Updated

One of the main reasons could be that firmware is not updated. 

  • Have you checked for the latest software update and installed the most recent version?
    • If not, it is recommended to install latest software version.
    • Please ensure that your car is running the latest software version.
How to install latest software version

Controls > Software > Update Available

For more details, please refer to another guide how to install the latest software version. Link

5.2 Reset via Mobile Application

Run UDS commands

In some cases, running UDS will clear. Please contact us for more details. We have developed mobile application for it.

5.3 Reset via Service Mode, Toolbox

Reset SOC

Service Mode - High Voltage - Reset SOC

Slightly unbalanced

If difference between the State of Charge (SOC) minimum (brickVoltageMin) and SOC maximum (brickVoltageMax) is greater than 5%.

BMS_w164

BMS_f164

BMS_w166

BMS_f166

6. Best practices to balance cell, clear imbalance alerts 

There are couple of best practices to balance cells, clear imbalance alerts. Please contact us to consult best practice for your car/case alerts code

Some of most common best practices

Following steps will allow for cell balancing

  • Set the charge limit to the daily recommended value via Touchsreen, Mobile App.
  • Keep car plugged in on AC charger as often as possible, if possible overnight, it will allow system to balance bricks voltage. 
  • Best practice is to have high SOC for cell balancing.

How to set charge limit

Touchscreen - Charging - Charge Limit 

How long it could take to balance cells.

It may vary depend on which alerts do you have and how much is brick voltage difference. It may take couple of weeks/months. There are couple of alerts (BMS_u029), it may take around 9 months. 

Please contact us for more details.

7. Alert codes

BMS_w064_SW_BrickV_Imbalance

Model S and X (2012-2020)

BMS_a177_OpenVshDetected

BMS_a129_SW_BMB_VSH_Connector

BMS_a107_SW_Cell_Voltage_Sensor

BMS_a062

  • BMS_a062_SW_BrickV_Imbalance
  • The battery management system (BMS) detects that there is a brick voltage imbalance.
  • The BMS has detected that the difference between the maximum brick voltage and the minimum brick voltage in the high voltage (HV) pack is greater than expected.

BMS_a062_SW_BrickV_Imbalance

BMS_a070_BrickV_Imbalance_Limiting

FAQ

Do I need to replace High Voltage Battery?

In most of cases no. It is recommend to try with cell balancing. 

What happens, when car get imbalance alerts?

Imbalance will reduce car range. 

Does it have impact on performance?

No, it will just reduced range. 

When BMS triggers Imbalance alerts?

There are different kind of Imbalance alerts, which triggers at different conditions. For some models, its 10 and for some models its 20%. When there will brick voltage difference more than ... mv.

My car has slight imbalance, can it recover over time?

Yes, it my recover naturally over time or with best practices. Please contact us to get know how it may recover.

Best practices to reduce SOC Imbalance?

  • Keep car plugged in on AC charger as often as possible. 
  • Set charge limit to recommended daily charge limit.

How long it may take to balance?

It may take 8 weeks.

What happens if there is imbalance?

High Voltage BMS will not allow system to charge or discharge if brick voltage difference is high or low.

What happens when car/HVC asleep?

When car/HVC asleep, it allows BMSs to perform brick balancing.

What is HV Energy Capacity?

What is Bleed resistors?

 

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Disclaimer: Tesla, Inc. was not involved in the creation of this document. Tesla did not provide us any info. Tesla did not edit, review or authorize us. We gather this information from Tesla official website and other resources.

Note: It is your responsibility to double check and confirm that the procedures/info mentioned in this blog are applicable to your specific car before performing them. Please contact us or our workshop partner to make sure/confirm. We will be happy to help you.