Safety Concept

When designing a battery pack / system it is important to think about and describe the safety concept. This needs to be considered in a systematic way and needs to cover:

  • electrical
  • mechanical
  • thermal
  • chemical

In each of these we need to look at the lifetime of the battery and the safety of it when being handled and used:

  • manufacture
  • logistics
  • installation
  • use
  • repair
  • end of life

The safety concept should include all aspects. A good example of this is the HV electrical safety concept for the VW ID4 [1].

  • Color coding of the high-voltage wiring and connectors
  • Safety markings on all high-voltage components
  • Accidental contact protection
  • Emergency cut-out connections: Maintenance connector for high-voltage system Fuse on the A-pillar with a small “flag”
  • Pilot line
  • Insulation resistance monitoring
  • Electrical isolation between high-voltage system and body (terminal 31)
  • Active discharging
  • Passive discharging
  • Crash shutdown
  • Monitoring of the high-voltage relays
  • Short-circuit test
  • Short-circuit shutdown
  • Detection of open circuits in high-voltage wires

This HV safety concept clearly shows the steps used to minimise risk to users in normal use and in case of an accident. Below we show the steps as described and also some of the design elements we see in the battery pack of the VW ID4.

As you can see these are a mix of physical and software based features. The first measures are labels and colour coding.

VW ID4 safety with warning labels
Warning labels are a first line of HV safety
VW ID4 safety with colour coding
Colour coded HV cables

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The battery pack is a sealed box with finger proof connectors. These finger proof connectors remove the need for an HVIL system. However, A “Pilot Line” or HVIL is used between the battery and the maintenance connector.

VW ID4 battery and description of the parts

The battery pack internal HV connections are all made down the central spine of the battery pack.

This means that in the event of an accident and the battery enclosure being damaged it is unlikely that a finger could touch the HV system. Also, this means the HV system is well protected from shorting to the pack enclosure.

The battery management system monitors the isolation resistance of the HV system. This sits on top of the design that will meet creepage and clearance requirements.

The VW training on this is quoted in the right hand text panel.

The insulation monitoring checks the electrical isolation of the high-voltage potentials to the chassis. When the value falls below a threshold of 510 kOhm, a yellow warning lamp illuminates on the instrument cluster.

A red lamp appears when the value falls below 90 kOhm. DC charging is either deactivated or prevented.

The insulation resistance monitoring is initiated by the Engine Control Module (J623) and performed by the Battery Regulation Control Module (J840). Among other things, it is part of highvoltage system activation, which is monitored by the high-voltage coordinator. Its function and circuitry is similar to the insulation resistance monitoring in the e-Golf.

In the event of a crash a signal will fire the HV pyrotechnic fuse and isolate the negative terminal of the battery pack. All of the HV components will then actively or passively discharge to a safe working voltage.

The battery management system monitors the voltage across the terminals of each of the contactors, if it determines that they are not functioning correctly, it inhibits the system.

Conclusion

We have used the VW ID4 as a reference to describe the HV safety concept approach. In this you can see that the design has layers of physical and control. This means that you need more than one point of failure for the HV system to be unsafe.

This approach is applied to the mechanical, thermal and chemical aspects of the Battery Safety Concept to give a total safety concept.

References

  1. The High-Voltage System in the ID.4, Self Study Program 811213, VW, 2021

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