An Engineer's Guide Balancing EV Battery Insulation & Thermal Management

Engineers designing high-voltage (HV) battery systems face an age-old challenge: the tradeoff between electrical safety and thermal management. This problem becomes especially acute for applications with limited space available such as an FSAE race car or small energy storage units. Engineers usually pose two questions about Nomex insulation paper thickness:

1) how thick should it be, since I can't find a specific thickness requirement in the rule book?

2) can I place my cooling fan directly against it for maximum efficiency of cooling?

These are excellent questions, the answers to which are key in high-voltage system design. While the answers do not involve rules but instead "physics" and system design principles. As engineers who specialize in automating electric vehicle battery production lines, we shall attempt to outline some key design principles involved.

Part I: Understanding Insulation - Thickness Is Wrong

Measure Engineers often make the mistake of mistaking thickness for insulation specs (i.e. "must use 0.5mm Nomex paper"). Unfortunately, such rules won't exist within FSAE or IEC specifications - instead "Dielectric Strength" should be measured instead - this material property measures how much voltage a material can withstand per unit thickness before becoming conductor (in other words "breaking down") This measurement can either be expressed either in kV/mm or V/mil.

Select Insulation Carefully

As an engineer, your function is to select insulation based on physics calculations rather than rigid thickness rules created arbitrarily:

1.Determine Max Voltage: Analyse all conditions where surge charging might take place in order to ascertain its maximum system voltage (including surge charging).

2.Assess Surge Charge Capabilities (charging surges).Referring to a Specification Sheet: Determine which Nomex type paper (i.e. Nomex 410 or 411 etc) you wish to use and locate its "Dielectric Strength".

3.Employ a Safety Factor: Verify that the total dielectric strength of your selected paper thickness exceeds your maximum voltage limit safely. Engineers usually follow an industry practice by employing a safety factor of at least 2x to 3x. ​​​​​​​

Example:

Maximum System Voltage = 600V ​​​​​​​

Dielectric Strength of Nomex Paper = 15 kV/mm (15V/mm) ​​​​​​​

Minimum Thickness Requirement = 600V/15V/mm = 40mm ​​​​​​​

Apply Safety Factor of 3x (40u*3 = 120mm or 0.12mm).

In this scenario, using either standard 0.13mm or 0.17mm Nomex paper would be optimal in terms of safety.

Your second consideration should be mechanical strength ​​​​​​​

when selecting the thickness for your battery pack. Paper must withstand tear-ripping, puncturing and abrasion during construction of the battery as well as processes involving high vibration (like in racing cars). Therefore teams often opt for intermediate thickness (e.g 0.25mm) even though it might "over-specced electrically", to ensure mechanical performance throughout its lifespan of use in their vehicles.

Part II: Cooling Fans and Electrical Isolation​​​​​​​

We now turn our attention to cooling fans and electrical isolation, specifically fan placement near Nomex paper. Your intuition to be cautious with this decision is absolutely right; Nomex paper serves to separate cells from airflow. But now the fan itself (including its casing, motor and wiring) becomes part of your HV environment. Tech inspectors will assess your design against two key electrical safety criteria when inspecting it:

Creepage: the minimum distance that current can travel along an insulator surface and

Clearance: when placed directly against Nomex material, clearance becomes nonexistent due to puncturing of Nomex fabric - in this instance there would no clearance between fan casing and HV cell and fan.

Solutions and Decision Paths

You have two safe design paths open to you: ​​​​​​​

Path A (Recommended) Non-Conductive Fan (All Plastic)

It provides an easy, safe, and cost-effective design solution using all plastic frames, housings and blades as the non-conductive material.

Additionally there exists a fan with this setup.

By doing this, even if a fan touches Nomex, its body cannot become electrified and create a conductor path. ​​​​​​​

Care must be taken to insulate and direct low voltage power leads away from HV terminals for best results. ​​​​​​​

Path B (Complex and Not Recommended) Metal Finish Fan

If you wish to use a fan with a metal enclosure, its use can be exceedingly complex; ​​​​​​​

Option 1 (Earthing): requires connecting its metal case directly with chassis grounding. ​​​​​​​

Option 2 (Insulation): requires spacing the fan away far enough from any of the HV components (cells and busbars) so as to strictly abide by all clearance regulations as though Nomex paper weren't present - invariably undermining its purpose of cooling directly against the pack. This path (Path B) often results in more heat being generated.

Conclusion: Adopt an Engineering-First Philosophy

When designing high-voltage battery systems, simply following rules isn't enough, designers need an in-depth knowledge of physics that governs them to create safe and reliable designs. Thickness alone doesn't provide adequate insulation, dielectric strength and mechanical strength are more crucial. ​​​​​​​

For component placement purposes "space" alone doesn't suffice - creepage clearance are key factors to take into consideration instead. At our engineering and automation facilities we employ this physics-first thinking to help engineer and automate high-voltage battery systems from manufacture through automation processes that we employ for high voltage EV/ESS battery manufacturing systems.