FSAE EV Rules 2014 Questions

[uG]

Dear FSAEe colleagues,
Dear Tobias,

I have two questions so far on this year rules improvements. :-)

EV3.4.8: "[...] The contained energy of a stack is calculated by multiplying the maximum stack voltage with the nominal capacity of the used cell(s)."

- Why? Usually, energy is measured with nominal voltage. Maybe it refers to the way the 6 and 12 MJ limits have been decided?

EV8.3.3: "The charger must incorporate an interlock such that the connectors only become live if is correctly connected to the accumulator."

- We are using Brusa NLG5x3 chargers. They automatically detects when the output is disconnected, and it quickly (<0.1 sec) disables the HV output. The detection is made, I think, when it realizes that the current equals zero. Would this software interlock comply with the rule?

Kind regards,

P.S. As some of you might have notice, Tobias Michael decided to start a blog.
P.S. #2: Is there a difference between a molten salt and a thermal battery?

[Adam Farabaugh]

3.4.8. makes sense, I think. They want to set a maximum energy within an un-separated segment. If nominal voltages were used, then at max charge the energy limit could be exceeded.

Where did you hear that energy is measured at nominal voltage? At 3.7V a li-po cell has less energy than at 4.2V...

[Francis Gagné]

Great to see that blog, it's a very good idea!

3.4.8. makes sense, I think. They want to set a maximum energy within an un-separated segment. If nominal voltages were used, then at max charge the energy limit could be exceeded.

Where did you hear that energy is measured at nominal voltage? At 3.7V a li-po cell has less energy than at 4.2V...

If you look at the following graph :


The total available energy is the area under the curve (depends on your discharge rate). To estimate it, you use the Nominal voltage, which is approximately the mean voltage of the flat part of the curve times the capacity. If you use max voltage, you will overshoot the real energy capacity of the battery. In regard to the 3.4.8 rule, I think the use of the maximum voltage is playing it safe, which is alright with me.

[TMichaels]

Hey All,
if the contained energy is calculated with the nominal voltage then the result is the nominal energy, right? But the rules talk about maximum energy contained between firewalls for example, thus we have to calculate with the maximum energy. Yes, it is that easy

Regarding the interlock: Only hardware interlocks count, no software solutions, like more or less for everything safety-related in the rules.

Well, molten salt and thermal batteries is kind of redundant. We just wanted to make sure that nobody shows up with thermal batteries which are not made of molten salt.

[uG]

Haha. Thanks, as usual, Tobias.

[uG]

Dear FSAEe colleagues,
Dear Tobias,

Rule EV4.8.1 claims that: "The HVD must be above 350mm from the ground and easily visible when standing behind the vehicle."

Would it be allowed to have the HVD being made in two parts:
1) One part being hidden
2) One part (remote handle) being above 350mm off the ground and easily visible when standing behind the vehicle.

By the way: could "behind the vehicle" be "behind the vehicle, but slightly on the side"?

Also, EV8.3.5 stipulates that "When charging, the AMS must be live and must be able to turn off the charger in the event that a fault is detected." Is it enough to stop the charge (send a stop charge command to the charger) or it has to electrically disconnect the charger from its AC supply.

On a more general topic now, I'm wondering how suited do you think of the design points are split for electric cars. I think that the electrical design points should be worth more than what they are (around 25/150 @ FSAEe in Nebraska). Do you think we should even create an electrical design event (or regulate the amount of points the electric part of the design should be worth)? I also feel that the cost event is not well suited for electric cars, but I can't talk with experience on this :-).

Thanks, and regards.

[rz-jacks]

Dear FSAEE colleges, I'm currently working on the design of the battery pack and I have a question regarding the rules:

In EV3.3.3 is states that "The separation must affect both poles of the segment. This separation method must be used whenever the accumulator containers are opened for maintenance and whenever accumulator segments are removed from the container. Maintenance plugs requiring tools to separate the segments will not be accepted. Maintenance plugs must include a positive locking feature which prevents the plug from unintentionally becoming loose."

Q1: what do they mean by a maintenance plug? is this a specific type of connector they are referring to? and does the "tool-less" rule only apply to these?
Q2: Does this mean that whatever method we use to interconnect the segments needs to be tool-less? for example, couldn't we use a simple busbar arrangement in which the segment poles are bolted together? I must be missing something here because it wouldn't make sense to use 6 high current "tool-less" connectors (assuming you have 6 segments) that are going to be extremely expensive. I'm trying to understand what this rule is trying to safeguard against.

Thank you!
Rene.

[uG]

Hello Rene,

Q1: As far as I know, a maintenance plug is simply a connector that you disconnect when open the battery pack (when doing maintenance). No interlock is needed since the maintenance plug opens the series path in your battery pack (no electrical difference potential will come out of the battery pack).

Q2: You can use bolts as long as each separation has "a maximum static voltage of less than 120VDC and a maximum energy of 12MJ" (EV3.3.3). Note that it can be 6MJ if you use other chemistries than LiFePO4.

If you wonder why you need not to require tools to open the maintenance plugs, Tobias explained it well previously, on the FSG's forum:

"The intention is, that the maintenance plugs shall always be removed whenever the accumulator container is opened. If you need tools for this, it may prevent you from following this approach, just because of lazyness or unavailability of the correct tool for example. The logic could be: "I am only working for 5mins at the accumulator container, I will not start searching for tool xyz now." This should not happen, if you could just unplug the connectors by hand."

P.S. You can also refer to what was discussed on the FSG's forum last year. Please note that there was no positive locking mechanism requirement at that time.

Regards,

[tobias.rheinlaender]

Hello everybody, hello Tobias,

I have a questions concerning the following rule:

EV2.1.2 Motors must be contained within a structural casing where the thickness is at least 3.0 mm (0.120 inch). The casing must use an Aluminum Alloy of at least 6061-T6 grade or better if a casing thickness of 3.0mm is used. If lower grade alloys are used then the material must be thicker to provide an equivalent strength.

Note: Use of a higher grade alloy does not enable a reduced thickness to be used.


At this point, I am not sure whether the rule refers to an additional housing or the motor case itself. (Although I must admit, I have yet to come up with a clear meaning for ‚structural casing‘ - perhaps this is crystal clear to native speakers…)

In Tobias Michaels blog, he states:
EV2.1.2:
Change: A minimum strength/thickness requirement for motor housings has been added.
Impact: Many teams may have to re-design their motor housings or at least check whether it fulfils the new requirements. This rule has been added due to lots of discussions this year at scrutineering about the necessary thickness of motor housings.

So, this sound promising, but it is not official, so...

We employ a rather sturdy motor out of a series-production vehicle that has a casing that might fulfill the material and thickness requirements. Would this be enough or is an additional casing mandatory?

In case the motor case is enough, what kind of documentation would be required and where would we put is? (SES, I assume)

Concerning the geometry of the case, if necessary: Since the new rule is akin to the scatter shield rule T8.4.4, we have been wondering if some kind of cylindrical case with open ends would be enough to comply with the rule (and contain exploding motors...) If not, how wide can openings for ventilation and such be and where should or could they be placed?

Thanks alot!

Kind regards,
Tobias R
Combined University Racing Berlin (CURB) - Germany

[Puneet Arora]

Hello Everyone,

I am not clear about this rule:

EV1.2.8 All components in the tractive system must be rated for the maximum tractive system voltage.

What rating is the rule talking about? The insulation rating? Or something else? Most components come with only insulation ratings and not supply ratings. They rather have current ratings. (e.g. fuse, wires, etc).

In particular, does this rule prohibit use of motors with maximum supply voltage greater than max accumulator voltage? (The motor has sufficient insulation rating).

Thanks for helping.




Puneet Arora
IIT Bombay Racing
Racing up the ladder!!