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However, now that energy will not be dropping, I think keeping weight as low as we relatively can is actually an important factor to consider. MS15’s battery cells already had the most minimal gravimetric and volumetric energy density that we possibly could have had with commercially widely available options (5Ah, 21700 form factor). So weight-wise, our greatest room for improvement lies in the overhead weight added from the rest of the battery system instead of the battery cells.
Comparing P45B and 50S
In our entire electrical system, power inputted into the system is equal to the power used by the system, this is true by the law of conservation of energy.
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The electrical system gets power from the batteries, solar array, and regen braking. Those powers inputted into the system are consumed by the motors, PCBs, losses, and probably some other things. This is probably a big simplification but it probably covers most of the power consumed (with the rest being negligible here).
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Solar power is the only power we are able to get externally. Battery power is limited, we start with full SOC but the only way to get new energy in is from solar. So the value of battery power tells us the net power of the car (the battery makes up for whatever power the solar can’t). If the battery is providing power we know we are running energy negative, if the battery is gaining power we know we are running energy positive, and if the battery is neither providing nor gaining energy we know we are running energy neutral.
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Power consumed by the motor can be represented using all the power our car needs to overcome to move (acceleration, drag, rolling resistance, gravity when on a gradient). I will also assume a 90% efficiency as a constant.
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For power loss from internal resistance, the power loss can written using P=I^2R.
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Using KCL, we can write
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And then we can further write
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Each component of motor power are
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Plugging all of them in, we get
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These are the main equations I will use to calculate battery power consumption, battery heat production, battery energy consumption, and total heat generation.
How much Heat Generation is Acceptable?
A lot of teams ran a passive pack with no cooling during WSC. Ideally we would also do the same for MS16, but it’s not an option due to reg change forcing some form of forced air cooling on all battery packs (and it must be turned on whenever the pack is connected). However, this doesn’t stop us from having a pack that is functionally passive for all intents and purposes, with “fans” playing an unimportant role for cooling.
Air cooling is pretty complicated in that it is not easy to make cooling uniform for all cells. Based on those two factors, I think we should (in-principle) aim for a p-count that allows us some weight savings (since we’ll have to have fans and ducts anyway, we don’t get to reduce design complexity by running no cooling) while also aiming for a p-count that generates very little heating such that we don’t have to worry about serious module hotspots complicating our design or bottlenecking our driving later on.
So first I’m going to try to figure out what the point of diminishing returns is for increasing p-count (at what point are we adding p-count without actually increasing thermal safety?
Ideally we would have power draw data for the whole 7 day race, but we don’t. And we didn’t have telemetry to log our FSGP current profile. So I’ll try to do some sort of simple model with some big assumptions.
The equation for heat production in Watts from IR loss is
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A typical tour day is 9 hours, from 9am to 6pm. I will take Psolar from a calculator for solar insolation that I found online from UNM’s website. I’ll use data for the coordinates of Nashville as a constant for solar insolation. Based on this website, the more South-West we move in the states, we should be getting more sunlight throughout the race week. So my Psolar value should be more conservative than what we actually get.
Here is the matlab script I will run for heat generation and power consumption when considering the whole race (scenario 1):
| View file | ||
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Here is the matlab script I will run for considering short term high power scenarios (scenario 2):
| View file | ||
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50S vs P45B Scenario 1
The first scenario I will consider will be a 36S8P configuration based on the specs of the Samsung INR21700-50S cell (max charging temp of 50 deg C). I will also assume this configuration will give a total car mass of 296kg. I am assuming that the P45B configuration will have no problem helping us reach our target 300kg total car mass, and that the difference in weight between the two battery packs will be 3.5kg (2kg for battery difference, 2kg for overhead weight, I think this should be reasonable…).
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How much Heat Generation is Acceptable?
A lot of teams ran a passive pack with no cooling during WSC. Ideally we would also do the same for MS16, but it’s not an option due to reg change forcing some form of forced air cooling on all battery packs (and it must be turned on whenever the pack is connected). However, this doesn’t stop us from having a pack that is functionally passive for all intents and purposes, with “fans” playing an unimportant role for cooling.
Air cooling is pretty complicated in that it is not easy to make cooling uniform for all cells. Based on those two factors, I think we should (in-principle) aim for a p-count that allows us some weight savings (since we’ll have to have fans and ducts anyway, we don’t get to reduce design complexity by running no cooling) while also aiming for a p-count that generates very little heating such that we don’t have to worry about serious module hotspots complicating our design or bottlenecking our driving later on.
So first I’m going to try to figure out what the point of diminishing returns is for increasing p-count (at what point are we adding p-count without actually increasing thermal safety?
Ideally we would have power draw data for the whole 7 day race, but we don’t. And we didn’t have telemetry to log our FSGP current profile. So I’ll try to do some sort of simple model with some big assumptions.
The equation for heat production in Watts from IR loss is P = I^2R, but if I make it a function of bunch of things like car speed, mass, gradient, etc, then it comes out like this. I took the Voltage part out cause it looked neater like this and also cancelled out with some things.
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A typical tour day is 9 hours, from 9am to 6pm. I will take Psolar from a calculator for solar insolation that I found online from UNM’s website. I’ll use data for the coordinates of Nashville as a constant for solar insolation. Based on this website, the more South-West we move in the states, we should be getting more sunlight throughout the race week. So my Psolar value should be more conservative than what we actually get.
Here is the matlab script I will run for heat generation and power consumption when considering the whole race (scenario 1):
| View file | ||
|---|---|---|
|
Here is the matlab script I will run for considering short term high power scenarios (scenario 2):
| View file | ||
|---|---|---|
|
50S vs P45B Scenario 1
The first scenario I will consider will be a 36S8P configuration based on the specs of the Samsung INR21700-50S cell (max charging temp of 50 deg C). I will also assume this configuration will give a total car mass of 296kg. I am assuming that the P45B configuration will have no problem helping us reach our target 300kg total car mass, and that the difference in weight between the two battery packs will be 3.5kg (2kg for battery difference, 2kg for overhead weight, I think this should be reasonable…).
The datasheet for the 50S is a little rough because it gives a lot of different numbers for max charge temp and talks about “re-charge release” and “discharge release” (I’m not able to confirm what they actually mean, Samsung never responded to my emails), I am going with 50 deg C because if you scroll down to “Pack Design Guidelines” Samsung says to not exceed 50 deg C charge temperature range and 80 deg C discharge temperature range. I think it’s better to err on the safer side and go with 50C.
This pack should run the hottest since it has the lowest parallel count. I will assume the car runs at a constant speed of 80km/h and with constant solar.
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