Tesla Roadster BMB Undervoltage Shutdown (Lockout) Voltage Calculation
Intro
As you might know Tesla published some of «sources» 🤡of their electric car.
Actually it doesn't have original sources, just final Schematic, Gerber, Assembly, Pick-n-Place.
Board
Just several references photos of Battery Monitoring Board (BMB):
| TOP | BOT |
 |  |
Calculations
The system powered using non-isolated DC-DC LTC1976 converter with VOUT=5V.
As you can see it has special resistor divider, which defines system undervoltage (shutdown) value:
 |
|
I opened LT1986 datasheet and find diagram of the undervoltage lockout mechanism:
Reassign resistor values according to the formula and undervoltage lockout schematic:
|
|
Define shutdown threshold of voltage VSHDN and shutdown input current ISHDN:
 |
 |
Using formula from datasheet calculate exact low-voltage lockout value:
 |  |
As a result we have minimum powering voltage is VUVLO = 29.1117V
Analysis
Since it designed to use with 10S Li-Ion battery, the minimum voltage of one cell is:
 |
According to the SoC table 3V is 0%, so calculated value is corresponds to the real world.
Why they have VIN=22V-42V I don't know, since calculated value is higher 🤔
Calculation Report
Created using SMath Studio:
💾 Source: lt1976_shutdown_voltage_tesla_roaster_bmb.sm
📄 Document: lt1976_shutdown_voltage_tesla_roaster_bmb.pdf
Conclusions
| 🟢 | Lockout (shutdown) voltage of Tesla Board Monitoring Board (BMB): 29.1117V |
| 🟢 | Since BMB designed for 10S 9S, the minimum cell voltage is 2.91V 3.23V |
| 🔴 | Labeled minimum powering voltage 22V is incorrect, since due UVLO it should be 29.1117V |
| 🟡 | Using low-voltage lockout (shutdown) used in battery systems to automatically enter very low-power consumption when battery voltage is low (~0%) |
| 🟡 | Since voltage divider always connected use high-resistance values |