For anyone curious, these were the identified problems with our accumulator from this video (anything marked "unclear" or "unacceptable" in our submission). HV Warning stickers: Unacceptable. We expected this because they weren't on the accumulator at the time. One interesting thing to note is that we thought a sticker on the top would suffice for "all viewing angles", but since the car is in the way there needs to be stickers on the sides as well. Had extras, luckily. Separation on self-developed PCBs: Unclear. We thought we explained it decently in this video, but didn't seem to do it well enough. To be on the safe side at competition, we conformal coated the PCBs shown just so we could be sure we would pass on the first attempt. HV Path: Unacceptable. Here, they were unhappy with our positive locking mechanisms. We had not bent our cotter pins to prevent removal, and the tab of our tab washer wasn't precisely on the flat of the hex. At comp, we fixed a few problematic areas and they sent us on our way. Internals: Unclear. They wanted to double check that we had no always energized TS wires leaving acc. Given that this video is all they had to understand our system, this is understandable. We were able to convince them that this rule was followed quickly at acc tech. Internals - Cell Stacks: Unclear. We have fire-resistant materials surrounding the accumulator. We didn’t show this in the video, only verbalized it. Indicator Light: Unacceptable: Our light was not labeled at the time. Accumulator Container Connectors: Unclear: They were unsure of our TSMP Voltage rating. This was a simple verification of our connectors parameters at comp. Charging Leads Fused Appropriately: Not Shown. This was the toughest thing to fix at comp. Since our wire gauge steps down when leaving the accumulator, we must fuse those wires beyond using the accumulator fuse. To get a fuse with the required amperage, we used a number of smaller fuses each rated for about 10A. Connecting them in parallel yielded a much larger equivalent fuse. Each fuse was wrapped in kapton tape to ensure isolation between fuses. Our accumulator tech experience was a single failure on the first pass, due to the charging leads and HV Path requirements, and a pass on the second attempt.
We don't have good information about whether the BMS heats up in static air or not. Ours was located outside of the accumulator for packaging reasons, we found it difficult to fit it inside of the accumulator housing as it takes up a lot of space.
Answer from an alum that worked on the first iteration of that accumulator: My initial (somewhat of a cop out) answer is “all of it”. Every team will have different constraints they’re working around as well as different skills and proficiencies that will work to their advantage. What was challenging one year may be trivial the next due to any number of circumstances. Additionally, the accumulator is an integral part of the architecture of EV FSAE cars and presents (at least in the opinion of an EE) one of the more interesting systems integration problems on EV FSAE cars and EVs in general. If you want a simple answer, for us at that time, it was energy and cooling. We had no test data to go off of so we either had to make models and simulations with a variety of assumptions and/or conduct a number of tests to collect this data, all of which takes time. Young engineers will often make the assumption that money is the enemy in engineering projects but, significantly more often than not, it’s time. The first big challenge is answering the question “how much energy does it need to have?”. Being able to accurately model how much energy the car as a system expends to quickly drive through a sea of cones is important. It’s also important to understand some of the intricacies of how the battery is going to react to the different load cases when driving the car. Depleting a battery at 1C for 10 seconds and depleting a battery at 10C for 1 second are going to have different net energies required to provide that power for the given time/load. Thermal modeling also becomes somewhat of a challenge. In a perfect world, each of these cells is sitting in open free stream air and can have air cooling it on all surfaces. Obviously, this isn’t the case. At worst, some of these cells are nestled deep within a stack of other cells that are also producing significant heat. Routing air ducts quickly becomes a packaging challenge given the typical location of the battery and making sure that air is actually flows properly through these ducts can be more challenging than you would think depending on the implementation. You can water cool the pack, however this will add not insignificant size, weight, and complexity into the pack. You’re also introducing water into the battery which requires careful consideration and testing. Then you need to create a thermal model or do significant testing to validate the cooling solution will work. All of the factors mentioned above will make that more difficult/complex. Packaging is another significant consideration. Again, in a perfect world, this pack is just sitting on a table with all of the space in the world, but it’s not. Typically, it’s nestled inside the body of an enclosed car with a plethora of other systems of the car that would very much like to also occupy that space. It's here especially where you begin to understand that you're not just building a battery pack, you're building a battery pack that needs to live in a car in harmony with all of the other components around it. Adding more energy means adding more cells which means adding more size and mass. What’s the sweet spot for energy/mass/size of the accumulator? Is it worth adding complexity/scope for better cooling? What shape should the accumulator be? Where should the accumulator be in the car? None of these questions have correct answers and every single FSAE team in the world will have a different answer to each of those questions. That is far from an exhaustive list of all the questions you need to be answering when you’re designing a battery pack but gives an idea of some of the things you need to consider.
@@Junkman1217 thank you so much for the insight with which you have answered my (perhaps overly) vague question, I really appreciate it! What you have written is really interesting and things that I will be sure to take into account when trying this out next school year at my team! Thank you
For anyone curious, these were the identified problems with our accumulator from this video (anything marked "unclear" or "unacceptable" in our submission).
HV Warning stickers: Unacceptable. We expected this because they weren't on the accumulator at the time. One interesting thing to note is that we thought a sticker on the top would suffice for "all viewing angles", but since the car is in the way there needs to be stickers on the sides as well. Had extras, luckily.
Separation on self-developed PCBs: Unclear. We thought we explained it decently in this video, but didn't seem to do it well enough. To be on the safe side at competition, we conformal coated the PCBs shown just so we could be sure we would pass on the first attempt.
HV Path: Unacceptable. Here, they were unhappy with our positive locking mechanisms. We had not bent our cotter pins to prevent removal, and the tab of our tab washer wasn't precisely on the flat of the hex. At comp, we fixed a few problematic areas and they sent us on our way.
Internals: Unclear. They wanted to double check that we had no always energized TS wires leaving acc. Given that this video is all they had to understand our system, this is understandable. We were able to convince them that this rule was followed quickly at acc tech.
Internals - Cell Stacks: Unclear. We have fire-resistant materials surrounding the accumulator. We didn’t show this in the video, only verbalized it.
Indicator Light: Unacceptable: Our light was not labeled at the time.
Accumulator Container Connectors: Unclear: They were unsure of our TSMP Voltage rating. This was a simple verification of our connectors parameters at comp.
Charging Leads Fused Appropriately: Not Shown. This was the toughest thing to fix at comp. Since our wire gauge steps down when leaving the accumulator, we must fuse those wires beyond using the accumulator fuse. To get a fuse with the required amperage, we used a number of smaller fuses each rated for about 10A. Connecting them in parallel yielded a much larger equivalent fuse. Each fuse was wrapped in kapton tape to ensure isolation between fuses. Our accumulator tech experience was a single failure on the first pass, due to the charging leads and HV Path requirements, and a pass on the second attempt.
Hello!!
Why is this Bms out of the accumulator? You think that this model from Orion heats up too much? My team is thinking about buying it.
We don't have good information about whether the BMS heats up in static air or not. Ours was located outside of the accumulator for packaging reasons, we found it difficult to fit it inside of the accumulator housing as it takes up a lot of space.
What was the toughest part in designing your accumulator
Answer from an alum that worked on the first iteration of that accumulator:
My initial (somewhat of a cop out) answer is “all of it”. Every team will have different constraints they’re working around as well as different skills and proficiencies that will work to their advantage. What was challenging one year may be trivial the next due to any number of circumstances. Additionally, the accumulator is an integral part of the architecture of EV FSAE cars and presents (at least in the opinion of an EE) one of the more interesting systems integration problems on EV FSAE cars and EVs in general. If you want a simple answer, for us at that time, it was energy and cooling. We had no test data to go off of so we either had to make models and simulations with a variety of assumptions and/or conduct a number of tests to collect this data, all of which takes time. Young engineers will often make the assumption that money is the enemy in engineering projects but, significantly more often than not, it’s time.
The first big challenge is answering the question “how much energy does it need to have?”. Being able to accurately model how much energy the car as a system expends to quickly drive through a sea of cones is important. It’s also important to understand some of the intricacies of how the battery is going to react to the different load cases when driving the car. Depleting a battery at 1C for 10 seconds and depleting a battery at 10C for 1 second are going to have different net energies required to provide that power for the given time/load.
Thermal modeling also becomes somewhat of a challenge. In a perfect world, each of these cells is sitting in open free stream air and can have air cooling it on all surfaces. Obviously, this isn’t the case. At worst, some of these cells are nestled deep within a stack of other cells that are also producing significant heat. Routing air ducts quickly becomes a packaging challenge given the typical location of the battery and making sure that air is actually flows properly through these ducts can be more challenging than you would think depending on the implementation. You can water cool the pack, however this will add not insignificant size, weight, and complexity into the pack. You’re also introducing water into the battery which requires careful consideration and testing. Then you need to create a thermal model or do significant testing to validate the cooling solution will work. All of the factors mentioned above will make that more difficult/complex.
Packaging is another significant consideration. Again, in a perfect world, this pack is just sitting on a table with all of the space in the world, but it’s not. Typically, it’s nestled inside the body of an enclosed car with a plethora of other systems of the car that would very much like to also occupy that space. It's here especially where you begin to understand that you're not just building a battery pack, you're building a battery pack that needs to live in a car in harmony with all of the other components around it.
Adding more energy means adding more cells which means adding more size and mass. What’s the sweet spot for energy/mass/size of the accumulator? Is it worth adding complexity/scope for better cooling? What shape should the accumulator be? Where should the accumulator be in the car? None of these questions have correct answers and every single FSAE team in the world will have a different answer to each of those questions. That is far from an exhaustive list of all the questions you need to be answering when you’re designing a battery pack but gives an idea of some of the things you need to consider.
@@Junkman1217 thank you so much for the insight with which you have answered my (perhaps overly) vague question, I really appreciate it!
What you have written is really interesting and things that I will be sure to take into account when trying this out next school year at my team!
Thank you