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Thread: EV4.5.8 - Plastic used in HV "stackup"

  1. #1

    EV4.5.8 - Plastic used in HV "stackup"

    Our team has recently been picked up (luckily) on facebook for a rule that we missed in our accumulator segment design.

    EV4.5.8:
    “All tractive system connections must be designed so that they use intentional current paths through conductors such as copper or aluminum and should not rely on steel bolts to be the primary conductor. The connections must not include compressible material such as plastic in the stack-up.”

    Our cells are in layers, the tabs are insulated by a plastic which also provides the mechanical clamping force for the series connection.

    Initially we queried the rule for clarity, asking:
    1. What exactly is the term “stack-up” referring to?
    2. How is the term “compressible” defined, and what material is this with reference to?
    3. Following from 2), in the last sentence where an example of a compressible material is given as plastic, is this an error and meant to refer to a material like rubber? If this is not an error, does EV4.5.8 therefore strictly prohibit the use of all plastics within the “stack-up”, or just the use of plastics exhibiting elastomer type behaviours? I.e. could a less compressible plastic like Polypropylene be used?

    We also asked for a material recommendation.

    The reply:
    Dear ####

    Rule EV4.5.8 relates to materials which may creep or deform when compressed causing loosening of battery terminals or high current connections. The stackup is the assembly of the electrical connection components, retention device and insulation components. High current connections between battery terminals or cable lugs, rely on maintaining a consistently high compression force to guarantee a reliable conductive path. Plastic materials such as polypropylene will relax after being compressed, causing the connection to come loose, which risks malfunction and fires or short circuits.

    The FSAE rules committee cannot provide specific design advice in regards to material selection for your vehicle.

    FSAE-A Rules Committee
    With this information we then consulted with an academic at our university who is a specialist in polymer failure modes, fatigue, creep ect.. (http://researchers.uq.edu.au/researcher/111)

    He made a material recommendation (Glass filled Acetal). We then performed calculations using the material manufacturers data to get an estimate of the expected creep we may experience in our application. It was well below 1%, with a corresponding <4% reduction in clamping force if held at at 40 degrees C for 410 days. The short term effects were negligible, however we additionally specified a monthly re torquing of the clamping bolts.

    We also asked for further clarity on "Compressible"
    - at what weight % polymer does a material becomes "compressible"?
    - is FR4 therefore compressible?
    - what materials are people expected to use? Are natural fibres, MDF acceptable?

    The follow up:
    FSAE rules prohibit plastics materials in the stackup as the mechanics of bolted electrical connections are difficult to analyze and cannot be guaranteed to meet the level of safety required. In particular, it is difficult to predict what the actual contact area of the bolted connection is on a macro scale, and so high clamping forces must be applied and maintained to ensure joint integrity. Materials used in bolted electrical connections must be dimensionally stable and not soften or creep across the expected operating temperature of the connection. Hydrocarbon based plastics do not meet this requirement.
    FSAE is an engineering competition, however the rules are intended as a bounding box in which students may compete fairly and safely and as such some possible solutions to problems are not acceptable. In order to maintain consistent application of the rules, your suggestion of retightening the retaining bolts following a maintenance schedule is not acceptable.
    The materials you have suggested below (wood, acetal, GFRP) are not acceptable substitutes as they all suffer from creep and operating temperature problems to some degree. The FSAE rules committee cannot provide specific materials design/selection advice to your team, however, we would encourage you to keep looking, as there are suitable materials available, which will not creep and will maintain performance to high temperatures. Engineers with experience from the power industry may be able to assist you.
    FSAE-A Rules Committee
    Now we have the problem that:
    - a leading industry professionals recommendation is unacceptable
    - creep for the material we want to use is negligible in this application, but still unnaceptable
    - any segment using PCB material in any clamping format may be illegal (how many teams is this?!?)
    - brittle materials such as ceramics are ok, but our confidence due to cracking is low
    - we would really like to avoid mechanical modifications to our cell tabs (welding, drilling) as it renders them potentially useless for future designs, plus the risks involved.

    What have other teams in this situation done?

    Edit:
    We have solved our issue with a design change. This is still an important note for all the teams out there using PCBs or any other epoxy matrix composites for clamping the cell tabs.
    Last edited by Mitchell; 08-11-2016 at 06:06 PM.
    UQ Racing

  2. #2
    Use metal.

    You did creep calculations, but did you consider vibration environments as well? Most fasteners will relax more than 4% of their preload much faster than that.
    Also, do a test if you can. This is the easiest way to find out the truth.

    I don't think any teams are using PCBs in the high current path clamp-up, because it's not within the rules. If they have gotten through tech with that, it's a mistake.
    Penn Electric Racing

  3. #3
    Quote Originally Posted by Adam Farabaugh View Post
    Use metal.

    You did creep calculations, but did you consider vibration environments as well? Most fasteners will relax more than 4% of their preload much faster than that.
    Also, do a test if you can. This is the easiest way to find out the truth.

    I don't think any teams are using PCBs in the high current path clamp-up, because it's not within the rules. If they have gotten through tech with that, it's a mistake.
    Metal is not an option as it needs to insulate. The less metallic components inside an accumulator the better.

    I don't really understand your (correct) point about bolt relaxation. Are you saying is creep is irrelevant because the effect of bolt relaxation will be worse? Bolts are allowed.

    The rules use a broad brush to rule out all plastics, including GFRP. The point is there are GFRPs will not creep under the strictly monitored operating conditions that exist in all fsae accumulators. They should be allowed. The teams using PCBs for clamping are out there. Our 2014 car passed through elec scruitineering with great positive feedback and yes we were using PCBs to clamp the tabs.
    UQ Racing

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