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Importance of Isolation Monitoring in Electric Vehicles

With the recent news of Audi E-tron recalls due to battery pack leakages, the following information on isolation is important to understand and consider.

The article talks about moisture leak into the battery pack, so the first question to ask is why does increased moisture in the pack lead to issues?

Here are a few reasons:

  • Since pack voltages are high voltage DC, in the order of 400V and above, electrical isolation must be handled carefully to avoid potential leakage / shorts.
  • Increased moisture can potentially create a path of low resistance for electrons to flow, thus affecting isolation of the pack which can be dangerous to the user.

The next thing to understand here is what isolation is and how is it measured

  • Isolation faults can be between the positive and negative terminals, as well as from each of the terminals to chassis.
  • An isolation fault between the positive and negative terminals of the battery is essentially a low resistance path between the terminals of the battery which can be extremely dangerous. I have seen tools (like wrenches or screwdrivers) being evaporated when accidentally dropped in a pack with high voltages.
  • An isolation fault between a terminal and chassis could lead to leakage of current through the chassis. If a low resistance path from the chassis is found to earth or the other terminal, this could again lead to an extremely dangerous scenario.

Now that we understand the effect of an isolation fault, lets understand how we would measure it.

  • One common way of measuring isolation on board the vehicle is to use high (known values in the order of Mega ohms) resistance to chassis. In the diagram, you can see R1 and R2 are such resistances. R1 and R2 are switched on one at a time to create a swing in voltage of a specific battery terminal with respect to chassis. This swing in voltage is measured, and compared to factory default values (+/- a threshold) to know if isolation is normal. Incase such a test returns an isolation warning, the battery management system should put out an alert. This is the yellow warning light on the dashboard that the article talks about.

Hope this gave you an insight into isolation in battery packs with the help of a real world scenario on why it is so important.


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