Battery Management System (BMS) Isolation Sensing

A battery management system (BMS) provides multiple functions that can be broken down into the following main categories.

  • Sensing and high-voltage control:

    • Measure voltage, current, and temperature.

    • Control contactor, pre-charge.

    • Ground-fault detection / isolation sensing.

    • Thermal management.

  • Protection against:

    • Over-charge, over-discharge, over-current, short circuit, extreme temperatures.

  • Interface:

    • Range estimation, communications, data recording, reporting.

  • Performance management:

    • State-of-charge (SOC) estimation, power-limit computation, balance/equalize cells.

  • Diagnostics:

    • Abuse detection, state-of-health (SOH) estimation, state-of-life (SOL) estimation

The isolation sensing feature on a BMS should detect the presence of a ground fault and is a core safety feature and answers the question if it is safe to touch a battery terminal and chassis ground at the same time . The battery “should” be completely isolated from chassis ground and the isolation sensing feature verifies it. In the US Federal Motor Vehicle Safety Standards (FMVSS) states that isolation is sufficient if less than 2mA of current will flow when connecting chassis ground to either the positive or negative terminal of the battery pack via a direct short circuit. Other standards specify slightly different values, warning equals 2mA, whereas fault equals 10mA. In the circuit diagram, see picture below, the paths between the battery and chassis ground are drawn as red resistors; ideally these have infinite values.

BMS Circuit

The “isolation resistance” Ri is the lesser of R1 and R2, this results in that Ri must be greater than Vb / 0.002 = 500 Vb. For the BMS to sense / measure that the battery pack is sufficiently isolated from the chassis it must measure Ri. The BMS cannot measure Ri directly, instead it measures V1 and V2 through a high-impedance measurement circuit, and both V1 and V2 are positive. The resistors R1 and R2 form a voltage divider and the BMS wants to find the smaller of the two resistances, and this results in the following logic: - If V2 > V1  find R1, means there is a fault on the low side. - If V1 > V2  find R2, means there is a fault on the high side. - I1 = I2, V1/R1 = V2/R2

Fault on low side – find R1.

When there is a fault in the low side, we want to find R1 and insert a large known resistor R0 between the battery and the chassis ground through a transistor switch, see circuit diagram below.

BMS Fault Low Side

Using Kirchoff’s Current Law (KCL) it is possible to calculate R1. Isolation is deemed sufficient if Ri > Vb/0.002 or R1> 500Vb.

Fault on high side – find R2.

Like when we had a fault in the low side, difference here is that V1>V2 and that we want to find R2 and to achieve this we insert a large known resistor R0 between the battery and the chassis ground through a transistor switch, see circuit diagram below.

BMS Fault High Side

Using Kirchoff’s Current Law (KCL) it is possible to calculate R2. Isolation is deemed sufficient if Ri > Vb/0.002 or R2 > 500Vb.