# Battery Cell Recommendation for SAE Formula Hybrid



## arbartz (Sep 27, 2013)

Okay, so those of you that are familiar with the SAE Formula Hybrid competition will probably know that the rules are a tad excessive and don't really reflect those in the real world. 

Specifically when we have to build our battery pack there are a bunch of rules that make it very difficult for a college team to build. I could go into all the specific rules, but in general they give us these limitations:


Can't parallel a bunch of cells without having each one individually fused and having the BMS able to detect if any single cell fuse blows.
Can't use pouch cell Lithium packs unless we meet very stringent regulations on limiting pouch cell expansion and make sure every material used in the pouch cell container is able to withstand 150C before melting, all while being UL94V0 listed.
Can't really use cells that require welding tabs since we simply don't have the equipment to do so. 
Leaving us with prismatic cells and cylindrical cells that have threaded contacts.

So, given those regulations I've found it extremely difficult to find suitable Lithium cells. The specific chemistry really isn't a concern, just as long as we can meet these pack spec requirements.

We are planning on using an EMRAX 207 motor. If we go with the Medium voltage version, we'll plan around having the following specs:


Fully charged voltage no greater than 300VDC
Continuous discharge current capability no less than 160A
Total pack capacity no greater than 5,561.25Wh when measured at the 2C rate (30min to full discharge)

If we go with the Low Voltage version we'll need these specs:


Fully charged voltage no greater than 135VDC
Continuous discharge current capability no less than 400A
Total pack capacity no greater than 5,561.25Wh when measured at the 2C rate (30min to full discharge)


Right now this leaves me looking at the Headway 38120HP cells, in an 84S2P configuration for the MV motor. They honestly seem to be the only decent option I could find, but I can't be sure that they will hold up to their ratings. So, I had the team buy 2 of them from Headway Headquarters and have some test time scheduled at Rockwell Automation to do load testing on them. We plan on fully discharging them at a bunch of different rates all the way up to their burst rating, only stopping the test if the cell temperature exceeds the rated 60C. (I'll post the results here if you guys are interested) 

So, if you guys have any other suggestions for cells to try, let me know, we'll take all the help we can get!

Thank you all for any help!


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## bigmotherwhale (Apr 15, 2011)

I have a 2013 toyota prius plug in hybrid battery pack
4kwh, 56 series cells 20ah each, 207volts, 150amp fused.

it uses Panasonic prismatic cells. 
its near perfect for your application.
i got mine cheap at a breakers, if you find one of these it will work for you as is its also designed for hybrid applications so i imagine the charge rate is high.


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## arbartz (Sep 27, 2013)

bigmotherwhale said:


> I have a 2013 toyota prius plug in hybrid battery pack
> 4kwh, 56 series cells 20ah each, 207volts, 150amp fused.
> 
> it uses Panasonic prismatic cells.
> ...


I'll have to look into that. The only concern though is that it was 150A fused. I'd really be worried about how much the cells are meant to handle and wouldn't want to push it too far past that. Thanks!


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## bigmotherwhale (Apr 15, 2011)

yeh, i will be pushing that limit when i use the pack, i think it could be conservatively rated considering the size of the batteries and their internal resistance, i would bet that they could easily handle 160A 
30kw is a pretty decent amount of power for a hybrid or small ev but for a race car if speed is the name of the game i dont think so, whats the conditions of the race? cant you use ultra capacitors to better effect? 

why cant you use nissan leaf cells? surely they meet all the criteria in their stock format?


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## arbartz (Sep 27, 2013)

bigmotherwhale said:


> yeh, i will be pushing that limit when i use the pack, i think it could be conservatively rated considering the size of the batteries and their internal resistance, i would bet that they could easily handle 160A
> 30kw is a pretty decent amount of power for a hybrid or small ev but for a race car if speed is the name of the game i dont think so, whats the conditions of the race? cant you use ultra capacitors to better effect?
> 
> why cant you use nissan leaf cells? surely they meet all the criteria in their stock format?


Well the Leaf cells are 60Ah from what I can tell, which would be too high for us since we need close to 300V, that only allows us for up to about 20Ah. We could drop our voltage by quite a bit, but then we'd have to get a different motor.


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## bigmotherwhale (Apr 15, 2011)

arbartz said:


> Well the Leaf cells are 60Ah from what I can tell, which would be too high for us since we need close to 300V, that only allows us for up to about 20Ah. We could drop our voltage by quite a bit, but then we'd have to get a different motor.


I see, i realise now you said 160A constant not peak so that pack is no where near what you need. 

Have you looked into lithium ion capacitors? they would certainly give you the burst ratings you desire.


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## dmwahl (Aug 25, 2014)

I'd expect the high power headways to work well for you, they should do 160A continuous from 2P until depleted. Don't expect long life out of them that way, but should last at least a couple years.

The requirement for individual fusing may seem ridiculous, but at my school several years ago (after I was gone) the clean snowmobile team had a short circuit in the main battery pack that caused some major damage. You might try designing some fusible links to use as interconnects, they're relatively cheap and easy to use. As far as detecting when one blows, watching for any sudden changes in cell terminal voltage can detect a blown fuse, similar to broken wire detection.


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## dougingraham (Jul 26, 2011)

arbartz said:


> OSo, given those regulations I've found it extremely difficult to find suitable Lithium cells. The specific chemistry really isn't a concern, just as long as we can meet these pack spec requirements.
> 
> We are planning on using an EMRAX 207 motor. If we go with the Medium voltage version, we'll plan around having the following specs:
> 
> ...


I can't help but think there is something here causing confusion. A 5.56kwh pack at 300 volts requires an 18.5 AH cell. In order to reach 160 amps continuous it would require the ability to do 8.6C discharge. If you were to discharge the pack at 160 amps continuous your run time is only 7 minutes. It doesn't matter what kind of cells you use, that would be all you get.

I assume the 5.56 kwh pack size is mandated by the rules. Where does the 160 or 400 amps continuous come from?

You are almost always better off going to a high voltage system and letting the controller do a current amplification.

Lets assume something like a Soliton Jr with a 340 volt 500 amp limits. Make a pack out of 40 AH CALB cells. 5561.25/40 = 139 volts. This would be 43 cells in series assuming the nominal voltage of 3.2 volts is acceptable. This would give a peak output power of approximately 139 volts times 400 amps or 55.6 kw (74.5 HP). Continuous output would be 16.7 kw (22.36 HP) and it could do this for about 20 minutes.

Maybe the races do only last 7 minutes in which case you would need that 8.6 C or greater continuous rating. That with the pack capacity figure of 5561.25 wh gives you everything else.

Where exactly does the hybrid part come in?


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## arbartz (Sep 27, 2013)

bigmotherwhale said:


> I see, i realise now you said 160A constant not peak so that pack is no where near what you need.
> 
> Have you looked into lithium ion capacitors? they would certainly give you the burst ratings you desire.


I haven't looked into them, but I'll give them a look now. Thanks!



dmwahl said:


> I'd expect the high power headways to work well for you, they should do 160A continuous from 2P until depleted. Don't expect long life out of them that way, but should last at least a couple years.
> 
> The requirement for individual fusing may seem ridiculous, but at my school several years ago (after I was gone) the clean snowmobile team had a short circuit in the main battery pack that caused some major damage. You might try designing some fusible links to use as interconnects, they're relatively cheap and easy to use. As far as detecting when one blows, watching for any sudden changes in cell terminal voltage can detect a blown fuse, similar to broken wire detection.


Well, like I said, I have a few of the headway cells on the way for testing, so we shall see if they will work or not. As for the fusing, I understand that it is important, but the detecting when one blows is what was annoying. The BMS I'm designing should be able to detect it just fine anyways, so I guess it's not a huge deal.

Thanks!



dougingraham said:


> I can't help but think there is something here causing confusion. A 5.56kwh pack at 300 volts requires an 18.5 AH cell. In order to reach 160 amps continuous it would require the ability to do 8.6C discharge. If you were to discharge the pack at 160 amps continuous your run time is only 7 minutes. It doesn't matter what kind of cells you use, that would be all you get.
> 
> I assume the 5.56 kwh pack size is mandated by the rules. Where does the 160 or 400 amps continuous come from?
> 
> ...


I realize that we would get a very short run time, but we don't actually plan on fully draining the pack at 160A, we just want to make sure that it can safely handle what the motor will be able to draw if we push it to the limit.

That 160A comes from that we are going to be using an EMRAX 207 MV motor as our traction motor. So we want to get the most out of it.

We would go with a higher voltage, but the rules set a hard limit at 300V. We could go the other way and use the LV version of the EMRAX 207, which means we would have a lower voltage, but would have to source more current, which just makes everything else physically larger.

We are doing a Series hybrid since no other team has done it yet. Teams are limited to a 250cc Gas 4 Stroke engine, so since the CBR250R engine we have produces about 25HP (18.64kW), we are going to re-purpose our old Agni 95R motor as a generator since it's rated for about 20kW. We'll have the engine running at it's most efficient RPM and have that drive the Agni motor as a generator to charge the battery pack when it starts getting low.

Hopefully that cleared everything up.

Thanks!


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## dougingraham (Jul 26, 2011)

Ok, I read through the Formula Hybrid rules.

Apart from the engineering challenge LiPo pouch cells make the most sense.

300 volts is the max allowed and this is the charging voltage. For your typical cells this is 4.2 volts per cell. There are some "UHV" cells that you charge to 4.35 volts. No idea if these are worth the additional cost but it would be worth investigating. This means that with UHV the cell count max is 68 cells in series. With regular LiPo cells it would be 71 cells. With LiFe types it would be 83 although you could sneak this up if you change the charging algorithm to terminate the CC phase at a lower voltage than 3.6 per cell. Using the 71 cell limit your peak pack voltage will be 298.2 volts and the nominal voltage will be 262.7 volts.

I was not able to see where the 5.56kwh max pack size comes from. The number I found is 5400 wh. With a 5.4 kwh pack at 262.7 volts you can have a maximum of 20.55 ah for your cells.

Even if you use a 12 volt motor there is no good reason not to use the highest possible battery voltage. The motor controller (inverter) would need to be selected to handle the battery voltage and then you adjust it to match its output to the motor characteristics. The watts out will equal the watts in minus losses. The motor controller when combined with the motor will form a buck regulator which will act as a current amplifier so that the motor will see only the voltage the controller allows it to see and the motor currents can be much higher than the battery current. For example the battery at 263 volts could see 400 amps (105200 watts) and the motor could see 1000 amps at around 105 volts. The cables between the pack and the motor can be 1/0 in this example and 2/0 between the controller and motor.

The most straightforward pack would be 20 AH LiFe prismatic cells. At 5400 wh this means a nominal voltage of no more than 270 volts. 270/3.2 = 84 cells. If you limit the CC peak to voltage to 3.57 volts per cell you do not exceed the 300 volt limit. Your typical prismatic can do 3C continuous which means you could exhaust the pack if you used it at that rate in no more than 20 minutes. In the 20 AH format the cells typically can do 10C without any problems for 10 to 20 seconds. Continuous power would be 16.2 kw and peak power would be 54 kw.

Building the more complicated LiPo pack using 50C continuous and 100C burst cells will give you a pack that can do 262.7 kw continuous and 525.4 kw for around 10 seconds. The energy density of these cells is better than that of the LiFe prismatics and with careful engineering there should not be a weight penalty to meet all the additional requirements. The larger institutions will obviously do it this way unless there is simply no reason why you would ever need more power than the LiFe arrangement will put out.


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## arbartz (Sep 27, 2013)

> With LiFe types it would be 83 although you could sneak this up if you change the charging algorithm to terminate the CC phase at a lower voltage than 3.6 per cell.


That's kinda what I was planning on doing so that if it made sense to add a cell or two to make an even number of series cells, then I could do it and just limit the max charge voltage.



> I was not able to see where the 5.56kwh max pack size comes from. The number I found is 5400 wh. With a 5.4 kwh pack at 262.7 volts you can have a maximum of 20.55 ah for your cells.


Okay, so the rules have two limits. 4449Wh for Hybrid and 5400Wh for Electric Only. We are going to be making a Series Hybrid, so we have to abide by the Hybrid limit. Now, they calculate that capacity by the following formula:

```
Vnom * Ah * 0.8 = Wh
```
 Another caveat though is that the Ah must be measured at the 2C rate (taking exactly 30min for a full discharge). For us, this means we have a 5,561.25Wh limit.

```
4449/0.8 = 5561.25
```



> The motor controller (inverter) would need to be selected to handle the battery voltage and then you adjust it to match its output to the motor characteristics. The watts out will equal the watts in minus losses. The motor controller when combined with the motor will form a buck regulator which will act as a current amplifier so that the motor will see only the voltage the controller allows it to see and the motor currents can be much higher than the battery current.


Okay, now I'm officially confused. I understand how buck and boost and a bunch of other topologies work, but I'm not sure how the typical inverter would do that without a separate internal buck-boost converter, which would make it huge unless they were running at insane switching frequencies (like 1MHz+). Anything less would result in an extremely large inductor being needed for the conversion, not to mention the power switching devices. Now I guess they could maybe somehow use the inductance of the coils in the motor itself, but I still don't see how it would work. Especially in an 80kW system.

I created a table of some different options that show all the specs I need to know. There are a bunch of options that "work" in theory, it just comes down to balancing which is the best for our purpose and what we can actually make with the limited resources we have available to us. Let me know what you think.

Thanks!


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## dougingraham (Jul 26, 2011)

arbartz said:


> Okay, now I'm officially confused. I understand how buck and boost and a bunch of other topologies work, but I'm not sure how the typical inverter would do that without a separate internal buck-boost converter, which would make it huge unless they were running at insane switching frequencies (like 1MHz+). Anything less would result in an extremely large inductor being needed for the conversion, not to mention the power switching devices. Now I guess they could maybe somehow use the inductance of the coils in the motor itself, but I still don't see how it would work. Especially in an 80kW system


It might help to think of the motor as the inductor in the buck topology. The energy leaves the inductor as mechanical rotation of the output shaft. This is pretty easy to see in a DC motor controller. In a three phase AC arrangement it is less clear because of the three half H bridges but it is still a buck regulator where the windings of the motor are the inductor.


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## arbartz (Sep 27, 2013)

Okay, I can imagine it as a buck converter, since it basically is one, just minus the output capacitor. I still have a problem with it though since the duty cycle is constantly changing in a SVPWM control scheme. Which means that the effective voltage of the buck converter is changing. Now I understand that it's basically what we want. The motor wants to see a sine wave, so the closest way to do that is to vary the duty cycle in a sinusoidal fashion.










If I understand what you were referring to correctly, we could have a higher voltage battery pack than our motor officially supports, but because it's acting as a buck converter the motor controller will just never allow the duty cycle to reach 100%. That way the effective voltage to the motor is lower. These seems to make sense, but it seems to be extremely inefficient. Wouldn't it be better to match the pack voltage to the motor so it can run all the way up to 100%?


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## coleasterling (Feb 29, 2012)

arbartz said:


> We are doing a Series hybrid since no other team has done it yet.
> 
> Thanks!


Besides that just not being correct and that MANY teams have done series cars, is that sound engineering? What analysis do you have saying that a series car will be faster than parallel? What is the point of the competition?


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## arbartz (Sep 27, 2013)

My mistake, I was under the impression that no Formula Hybrid team has done a true series hybrid vehicle yet. Our reasoning for doing series though is that we believe we can get a longer range by doing so. Essentially we're mimicking how the Chevy Volt works. We'll have a powerful traction motor that draws from a battery pack. That battery pack will then be charged by the engine powering a generator and then regeneration from braking. We will be repurposing our Agni 95R motor as the generator. So it'll be hooked up to the engine and the engine will be held at it's most efficient RPM. The output of the Agni motor should be around 84V. That will then be pumped into a team designed DC-DC converter / charger which will boost it to 300V and regulate the current for charging the batteries. 

The vehicle will likely not be faster in a straight line since we won't have the extra 25HP of the engine, but the motor is good for around 100HP burst anyways, so with 100HP in a 600Lbs car we'll still be traction limited on acceleration. Aside from all that, the point of this competition isn't really speed as much as it is efficiency. Now, I'll completely admit that as a team we are more concerned with speed as we're all big gear heads and just want to see this thing smoke the tires and be insanely fast. With that being said, we still have a concern for the efficiency, just not as much as we probably should.


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## coleasterling (Feb 29, 2012)

Na, it is actually pretty common. My now personally-owned car was a series car back in 2006 and 2007, even. There have been many. 

The point of the competition is not efficiency. The point of the competition, assuming you're doing it with the intent to win and not just as a science experiment you're using as a learning tool (completely valid approach by the way...but it won't help you win), is to amass as many points as possible. Does the point breakdown even favor having high drive efficiency?

When I did FH, we wrote a lapsim that helped us compare various concepts based on energy usage and raw time, that we then correlated directly to points. Before we ever did any detail design, we had actual data helping us decide which direction to go. We spent a couple weeks researching difference concepts and came up with our best guesses as to weight, tractive force,drive efficiency, etc..for each concept. The sim gave us back energy used and lap times based on those factors. We picked the concept that, when plugged into the scoring formulas, gave us the highest points.

You can't just look at your drive efficiency and say that a series configuration is better. You HAVE to look at the whole picture. Lets make some pretty general assumptions here...

You said your car will be 600lbs. The year I did Hybrid, our car weighed 440lbs. Assuming a similar chassis a series car will ALWAYS be heavier than a parallel car. Even considering final drive efficiencies, gasoline has higher energy density than the best lithium cells and you're carrying more of them...Especially at 300V. Then, you're carrying another 25lbs in motor, too. F=ma, right? For the same acceleration, a higher mass needs a higher force. Is the higher final drive efficiency of the series configuration enough to make up for your extra mass?



Edit: To not get too far off-topic here...There is a company on eBay selling off 12 cell SAFT modules from hybrid busses. They listed them as 144Ah packs with the same specs as the 12ah Headways. They were lying through their teeth and the cells are actually SAFT VL6P's with 6.8Ah. The module is around 300Wh and weighs 11lbs, so the energy density is extremely poor, but they will do around 12kW peak per module. Individual cells have screw terminals and weigh 0.9lbs. Assuming you could buy enough of them to sort the best cells and throw away the bad ones, they are may be better suited to the project than the Headways. Definitely cheaper. The company was asking an average of around $120 shipped per module. If you bought a bunch, they might lower the price. I have to warn you, they were EXTREMELY difficult to deal with, though.

http://www.ebay.com/itm/161838193194?_trksid=p2057872.m2748.l2649&ssPageName=STRK:MEBIDX:IT


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## bigmotherwhale (Apr 15, 2011)

arbartz said:


> My mistake, I was under the impression that no Formula Hybrid team has done a true series hybrid vehicle yet. Our reasoning for doing series though is that we believe we can get a longer range by doing so. Essentially we're mimicking how the Chevy Volt works. We'll have a powerful traction motor that draws from a battery pack. That battery pack will then be charged by the engine powering a generator and then regeneration from braking. We will be repurposing our Agni 95R motor as the generator. So it'll be hooked up to the engine and the engine will be held at it's most efficient RPM. The output of the Agni motor should be around 84V. That will then be pumped into a team designed DC-DC converter / charger which will boost it to 300V and regulate the current for charging the batteries.
> 
> The vehicle will likely not be faster in a straight line since we won't have the extra 25HP of the engine, but the motor is good for around 100HP burst anyways, so with 100HP in a 600Lbs car we'll still be traction limited on acceleration. Aside from all that, the point of this competition isn't really speed as much as it is efficiency. Now, I'll completely admit that as a team we are more concerned with speed as we're all big gear heads and just want to see this thing smoke the tires and be insanely fast. With that being said, we still have a concern for the efficiency, just not as much as we probably should.


You mention a separate dc dc converter? wouldn't it be much better to use the motor winding inductance just as its done with regenerative braking? you will save yourself alot of weight and it will be much simpler and more efficient.


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## arbartz (Sep 27, 2013)

coleasterling said:


> Na, it is actually pretty common. My now personally-owned car was a series car back in 2006 and 2007, even. There have been many.
> 
> The point of the competition is not efficiency. The point of the competition, assuming you're doing it with the intent to win and not just as a science experiment you're using as a learning tool (completely valid approach by the way...but it won't help you win), is to amass as many points as possible. Does the point breakdown even favor having high drive efficiency?
> 
> ...


Well as far as the point breakdown favoring efficiency, I'd say it kinda does in that the most amount of points for the dynamic events comes from the endurance run, in which the number of laps you complete has the largest effect on the score. 


































That being said, time is a factor in all 3 events, so we certainly don't want a slow car. Which is why we are going with an EMRAX 207. We'll have 100HP, that is more than enough to do very well in all of these events, having the extra 25HP of the engine isn't going to be that helpful since we'll already be traction limited (or so I'm told by the MechEng's on the team). 

I'll be honest though, I really don't have any hard numbers to back it up that Series hybrid is the way to go for us. Being an EE, I have absolutely no idea where to start for making a lapsim that could give us the information you talked about. I've brought it up before in meetings, but none of the MechEng's want anything to do with it. The best we've had so far is using OptimumLap to figure out the best gear reduction and the best e-motor for our purposes. Which is how I ended up at the 207. I put a bunch of different motors into the program and the 207 ended up being the best option given our limitations. That program doesn't have anyway to combine engine and emotor though, or show us energy used.

600Lbs is a goal weight by the Senior Design team this year. I'm not sure if they'll even hit that though since they are using all steel tubes again. Apparently we don't have the equipment or experience to do anything with carbon fiber. Anyways, you talk about mass of the car being higher because we are in a series configuration because we're carrying more batteries, but I think it would almost be the same since we would have the same amount of batteries regardless of series or parallel. We just want to max out our capacity. The only way the car would be heavier is by the extra motor being used as a generator, but we won't have the need for a coupling transmission like we would in a parallel hybrid. So while it may be a bit heavier, it'll only be a few pounds, and we're not that concerned about a few pounds. 

As for those SAFT modules, I'll certainly have to take a look. Thanks for the tip!



bigmotherwhale said:


> You mention a separate dc dc converter? wouldn't it be much better to use the motor winding inductance just as its done with regenerative braking? you will save yourself alot of weight and it will be much simpler and more efficient.


Well the generator motor we're using is a brushed DC motor, so we'll just have an 84V output from it that'll have to be boosted up. We were only planning on using it since we already had it. 

If I'm understanding correctly though, you're saying that with a 3-phase brushless motor we could use the windings as the coil for the boost converter?


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## bigmotherwhale (Apr 15, 2011)

Yes you can use the winding as a boost converter entirely, you can also add extra inductance to each leg of the three phases if the inductance in the winding isn't high enough. 

Im not sure if this can be done with a brushed motor.


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## coleasterling (Feb 29, 2012)

arbartz said:


> Well as far as the point breakdown favoring efficiency, I'd say it kinda does in that the most amount of points for the dynamic events comes from the endurance run, in which the number of laps you complete has the largest effect on the score.


My comments on points breakdown specifically said drive efficiency, not the overall system efficiency. The car that goes the most laps the fastest (assuming no breakage) is clearly the most efficient as a system (including driver!) That car may or may not have the highest combined drive efficiency. Remember again that gasoline has a FAR higher energy density than any battery. Even with a poorly tuned engine that's say...10% efficient, you STILL have more energy available (edit: add per kg). Until batteries get to a couple/few thousand Wh/kg, gas will go further in this competition. 

Edit: I need to qualify this. Yes, the energy is set at a cap and gas is given based on how much electrical energy you have. When you just look at the conversion efficiencies, electric wins out. Makes sense since it has so much higher drive system efficiency. Buuutttt, look at the entire car. You're trading drive efficiency for nearly 200lbs of battery if you're using something like the Headway 8ah or SAFT cells to max out the battery energy. The equivalent in gas weighs well under 10lbs. 



arbartz said:


> I'll be honest though, I really don't have any hard numbers to back it up that Series hybrid is the way to go for us. Being an EE, I have absolutely no idea where to start for making a lapsim that could give us the information you talked about. I've brought it up before in meetings, but none of the MechEng's want anything to do with it. The best we've had so far is using OptimumLap to figure out the best gear reduction and the best e-motor for our purposes. Which is how I ended up at the 207. I put a bunch of different motors into the program and the 207 ended up being the best option given our limitations. That program doesn't have anyway to combine engine and emotor though, or show us energy used.


OptimumLap isn't a bad resource to use. It is fairly easy to estimate with a series car, maybe not as much with a parallel, depending on how you use the motor. You've said you'll be holding your engine at the max efficiency point. You know what the fuel consumption is and the conversion efficiency through the generator, so you know how much energy you'll have available from gas. I don't remember if OL gives you energy used, but from there, you can estimate how much further you can go with that energy. Rough, but at least you have an idea.

I'll throw this out there...Do you think using gas to convert to electricity and feed your accumulator is more efficient than using it to directly drive the wheels? 



arbartz said:


> 600Lbs is a goal weight by the Senior Design team this year. I'm not sure if they'll even hit that though since they are using all steel tubes again. Apparently we don't have the equipment or experience to do anything with carbon fiber.


I don't know what your ME's are telling you, but you don't need carbon to be a lightweight car in FSAE or especially FH. I believe, although I could certainly be wrong, that our FH car was and is the lightest car to run in the hybrid class at 441lbs. We used a steel tube chassis and suspension that wasn't particularly optimized just because we knew we had an extreme time constraint (our team doesn't even meet each other until September every year). 

You've run the lapsim and my guess it that it has showed you that power matters more than weight for times...slightly. No? What happens if you lower your power, but lower the weight quite a bit, too? Our car was heavier than it should have been by at least 35lbs. We ran a Perm 132 at comp, but had a gorgeous little custom motor from Astro we originally planned on running. 6.8lbs and we put 48kW into it. It didn't like that for long 

Point is, 500lbs should be pretty doable if you don't have a hundred pounds of batteries. Have you looked into running another motor or having a custom wind of the EMRAX done to run on say, 100V instead of 300V? Maybe use OL to estimate how much power you need to keep similar lap times when you lower the weight of the car to mid 4-500 lbs. 



arbartz said:


> Anyways, you talk about mass of the car being higher because we are in a series configuration because we're carrying more batteries, but I think it would almost be the same since we would have the same amount of batteries regardless of series or parallel. We just want to max out our capacity. The only way the car would be heavier is by the extra motor being used as a generator, but we won't have the need for a coupling transmission like we would in a parallel hybrid. So while it may be a bit heavier, it'll only be a few pounds, and we're not that concerned about a few pounds.


This is mostly true. You're carrying the extra 25-30lb generator weight and you don't HAVE to run a giant accumulator with a series car, but then you've got to put more energy more quickly into it from the generator, which means upsizing the (or cooling) the generator and running the engine harder. I don't know where the balance point is. There's probably a nice local min somewhere that looks pretty nice. I really should have framed the argument around carrying more battery vs. less, not series or parallel. My apologies. You're very correct in that you could use a lot of battery in either configuration. It has been my experience in FH that the series cars all use more battery, however. 

Why do you have to have a transmission for the motor in a parallel configuration? We just coupled it straight to our differential on the opposite side from the motor. Are you directly driving the generator from the engine? No chains or belts? 



arbartz said:


> As for those SAFT modules, I'll certainly have to take a look. Thanks for the tip!


I've got to reiterate...That company is very difficult to deal with. They lied to me numerous times in our dealings and sent me modules with several defective cells that they claimed were good. Know that modules may have bad cells going into it and that you'll have to get extra modules and test every cell if you end up going that direction. I can send you a cell to test if you'd like before dealing with them at all. 

I'll post some pictures of our 2011 car here in a couple hours.


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## coleasterling (Feb 29, 2012)

Here are some pictures of our car: 

Note that it isn't even close to passing tech now. Rules were quite a bit different in 2011.

The second picture shows our entire accumulator and high voltage system minus the motor. We used 90C hobby lipo cells and only had around 500Wh of electrical energy. That was good for around 6 laps on the old (half as long as now) Endurance course at 80% discharge. If we wanted to really kill the cells, it could technically do another 1-1.5 laps. Our philosophy was that we wanted to be as mechanically efficient as possible and use the motor only for Accel, extra power during AX, and a safety in Endurance if we ran out of gas. We were somewhat close to the edge of finishing with just gas in good weather. In the end, of course it rained in New Hampshire, and we ended up having enough gas to halfway fill a gatorade bottle. We could have probably finished another whole Endurance stint if we used the electrical power, too. We were probably a little too conservative an should have gone faster. We never used the electrical power in Endurance. Our engine was a Yamaha wr250 with around 32-34hp and we didn't run regen. For our particular setup, we calculated that our regen efficiency would have been piss poor and that we saved more energy by not constantly back-spinning the motor, so we used a one-way bearing. Our particular philosophy was somewhat frowned upon in FH, but our results speak for themselves. 

The 4th picture shows the brushless motor we originally designed for the car. We intended the motor to see max duty on electric-only acceleration and that's about it. We would have had to have babied it quite a bit to get it to last during any issues in Endurance. It burned one extremely hot Texas day when we tried multiple accel runs in a row without letting it cool. Quite sad. We replaced it with a boat anchor of a Perm 132 and gave it around 50kW. It did quite well and we never had any issues. Funny, we had heard people complaining about us not using our motor during AX, claiming we weren't a hybrid (the rules at that time defined being a hybrid by finishing an e-only run in 10s or less...We did it in 5.7s), so we asked to do an exhibition run on electric-only. Unfortunately that was denied, but in practice, we were showing e-only being about 2-2.5s off the engine-only times. That would have put us in 3rd place for AX that year. 

Overall, we went for a super simple car that we could design and build in 5 months. The car worked on either system independently or both at the same time, adding redundancy and the low weight helped make us the fastest car out there as well as one of the most efficient as a system.


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## arbartz (Sep 27, 2013)

coleasterling said:


> My comments on points breakdown specifically said drive efficiency, not the overall system efficiency. The car that goes the most laps the fastest (assuming no breakage) is clearly the most efficient as a system (including driver!) That car may or may not have the highest combined drive efficiency. Remember again that gasoline has a FAR higher energy density than any battery. Even with a poorly tuned engine that's say...10% efficient, you STILL have more energy available. Until batteries get to a couple/few thousand Wh/kg, gas will go further in this competition.


Sorry, that was a misunderstanding on my part. I was looking at this stuff all day yesterday and I was getting burnt out. Yes, you are completely right in that gasoline is much more energy dense than any sort of battery. It makes sense that a parallel is probably the better route to go.



coleasterling said:


> OptimumLap isn't a bad resource to use. It is fairly easy to estimate with a series car, maybe not as much with a parallel, depending on how you use the motor. You've said you'll be holding your engine at the max efficiency point. You know what the fuel consumption is and the conversion efficiency through the generator, so you know how much energy you'll have available from gas. I don't remember if OL gives you energy used, but from there, you can estimate how much further you can go with that energy. Rough, but at least you have an idea.
> 
> I'll throw this out there...Do you think using gas to convert to electricity and feed your accumulator is more efficient than using it to directly drive the wheels?


Okay, so I just went through and did some more lap simulations. One with the CBR250R and one with the EMRAX 207 both with their optimal final drive ratios for fastest laps. I then exported the data to excel and did some approximations on energy consumed by taking the power vs time data. From that I was able to estimate about 235Wh per lap used on the ICE and 537Wh on the EM. Which I'll admit was a shock, we had never done any sort of energy usage calculations before so we have nothing to compare it to, but this should at least give us a decent estimate. Well, combined that only allows us for about 31 laps assuming perfect conditions on everything. 

I'm starting to see what you and everyone has been trying to tell me... 



coleasterling said:


> I don't know what your ME's are telling you, but you don't need carbon to be a lightweight car in FSAE or especially FH. I believe, although I could certainly be wrong, that our FH car was and is the lightest car to run in the hybrid class at 441lbs. We used a steel tube chassis and suspension that wasn't particularly optimized just because we knew we had an extreme time constraint (our team doesn't even meet each other until September every year).
> 
> You've run the lapsim and my guess it that it has showed you that power matters more than weight for times...slightly. No? What happens if you lower your power, but lower the weight quite a bit, too? Our car was heavier than it should have been by at least 35lbs. We ran a Perm 132 at comp, but had a gorgeous little custom motor from Astro we originally planned on running. 6.8lbs and we put 48kW into it. It didn't like that for long
> 
> Point is, 500lbs should be pretty doable if you don't have a hundred pounds of batteries. Have you looked into running another motor or having a custom wind of the EMRAX done to run on say, 100V instead of 300V? Maybe use OL to estimate how much power you need to keep similar lap times when you lower the weight of the car to mid 4-500 lbs.


I'm going to do some more lapsim calculations while varying weight and other motor aspects to see where it gets us.

Also, that motor looks awesome, to bad it didn't like be pushed that hard... 



coleasterling said:


> This is mostly true. You're carrying the extra 25-30lb generator weight and you don't HAVE to run a giant accumulator with a series car, but then you've got to put more energy more quickly into it from the generator, which means upsizing the (or cooling) the generator and running the engine harder. I don't know where the balance point is. There's probably a nice local min somewhere that looks pretty nice. I really should have framed the argument around carrying more battery vs. less, not series or parallel. My apologies. You're very correct in that you could use a lot of batter in either configuration. It has been my experience in FH that the series cars all use more battery, however.


 Finding the best middle point is something that I'm certainly going to have to look at now, in light of the recent developments with the lapsim.



coleasterling said:


> I've got to reiterate...That company is very difficult to deal with. They lied to me numerous times in our dealings and sent me modules with several defective cells that they claimed were good. Know that modules may have bad cells going into it and that you'll have to get extra modules and test every cell if you end up going that direction. I can send you a cell to test if you'd like before dealing with them at all.
> 
> I'll post some pictures of our 2011 car here in a couple hours.


Well, as nice as it could be for the batteries it seems like it might not be quite worth it, so I'm thinking we might be better off sticking with the other options. Right now a guy over at the Endless Sphere forums has something that looks like it'll be perfect for us in terms of our battery pack.

Also, you're car looks awesome! Amazing that it was so light yet built with a steel frame. Our old car is a hulking mess. I liked how it looked since it was the biggest car at the competition last year, but it just isn't practical. 

Here's a pic of our car at competition last year:


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## coleasterling (Feb 29, 2012)

arbartz said:


> Okay, so I just went through and did some more lap simulations. One with the CBR250R and one with the EMRAX 207 both with their optimal final drive ratios for fastest laps. I then exported the data to excel and did some approximations on energy consumed by taking the power vs time data. From that I was able to estimate about 235Wh per lap used on the ICE and 537Wh on the EM. Which I'll admit was a shock, we had never done any sort of energy usage calculations before so we have nothing to compare it to, but this should at least give us a decent estimate. Well, combined that only allows us for about 31 laps assuming perfect conditions on everything.
> 
> I'm starting to see what you and everyone has been trying to tell me...
> 
> ...


That's a GREAT start. Make sure you keep and file all of your analysis so to show the judges in Design. 



arbartz said:


> Also, that motor looks awesome, to bad it didn't like be pushed that hard...


Thank you! The testing that day was being done without the Motor and Controls group there and of course specific directions were not followed on motor temperature measurements or power limits. If I had my way, we would have tested it before comp to make sure it gave us a reasonable e-only Accel time and maybe a couple of reduced power launches for combined Accel testing and then not run it at all until comp. I did not get my way, haha. 







arbartz said:


> Well, as nice as it could be for the batteries it seems like it might not be quite worth it, so I'm thinking we might be better off sticking with the other options. Right now a guy over at the Endless Sphere forums has something that looks like it'll be perfect for us in terms of our battery pack.


I've wondered how the rules committee will/has handled salvage battery packs. They are clearly safe enough, but still don't meet all the rules. The Leaf modules are 2s2p. Will they count that as one "cell" or do you need to tear them apart and fuse?



arbartz said:


> Also, you're car looks awesome! Amazing that it was so light yet built with a steel frame. Our old car is a hulking mess. I liked how it looked since it was the biggest car at the competition last year, but it just isn't practical.


Thank you again! As I'm sure you've done with yours, we all put an insane amount of time into it. We were rewarded with winning and it remains one of the highlights of my college career. 

We mainly saved weight through switching to 10" tires and a minimal accumulator. The previous cars were around 550lbs with the same perm 132, same engine, and same differential. Our entire Accumulator box weighed less than 40lbs. I de-hybridized the car for testing as we went to FSAE after 2011 and it weighed I think around 330lbs. If you were to try to make it competition ready, it would probably be 340 or so? Now with a full set of wings, it weighs ~370lbs. I think you could make a fully legal FH car now under 400lbs. 

Your car looks good! I hate how they require so much structure around the accumulator boxes now. I think they probably needed more than when we competed, but crap. You just need to make it lighter, now! How much did it weigh last year?


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## arbartz (Sep 27, 2013)

coleasterling said:


> That's a GREAT start. Make sure you keep and file all of your analysis so to show the judges in Design.


I'm saving everything I'm doing and presenting it to the team so we're all on the same page. Quite frankly though I'm a bit frustrated that I'm doing all of this when it really isn't what I should be focusing on. I'm electrical team lead, not powertrain lead. Nobody else will step up and do it though.



coleasterling said:


> Thank you! The testing that day was being done without the Motor and Controls group there and of course specific directions were not followed on motor temperature measurements or power limits. If I had my way, we would have tested it before comp to make sure it gave us a reasonable e-only Accel time and maybe a couple of reduced power launches for combined Accel testing and then not run it at all until comp. I did not get my way, haha.


I understand that all too well. My freshman year and sophomore year I was the only EE on the team and ended up doing 100% of the electrical work on the car. In my freshman year nobody trusted that I knew what I was doing (and I didn't 100% know either, but I did know more than most of them about electrical stuff). So when I wasn't there and before I got the BMS system working they took the car out under EM power only and ran it until it wouldn't move anymore. So those batteries are pretty much shot now.



coleasterling said:


> I've wondered how the rules committee will/has handled salvage battery packs. They are clearly safe enough, but still don't meet all the rules. The Leaf modules are 2s2p. Will they count that as one "cell" or do you need to tear them apart and fuse?


I'm actually not going with the Leaf modules, a guy there PM'd me who is making custom packs with 18650 cells. It's an actual company he's starting, so as far as the rules committee is concerned these will just be off the shelf packs.



coleasterling said:


> Thank you again! As I'm sure you've done with yours, we all put an insane amount of time into it. We were rewarded with winning and it remains one of the highlights of my college career.
> 
> We mainly saved weight through switching to 10" tires and a minimal accumulator. The previous cars were around 550lbs with the same perm 132, same engine, and same differential. Our entire Accumulator box weighed less than 40lbs. I de-hybridized the car for testing as we went to FSAE after 2011 and it weighed I think around 330lbs. If you were to try to make it competition ready, it would probably be 340 or so? Now with a full set of wings, it weighs ~370lbs. I think you could make a fully legal FH car now under 400lbs.
> 
> Your car looks good! I hate how they require so much structure around the accumulator boxes now. I think they probably needed more than when we competed, but crap. You just need to make it lighter, now! How much did it weigh last year?


With driver and a full tank of gas it weighed in at 1100Lbs... It was an absolute hulk of a car. We managed to take 4th with it though, which we're happy with. But the rules are so insane now that less than 10 teams of the 30 that show up even get to start their car.

I'm happy though that we were able to run it, albeit under engine only and for no points, but we still were the only ones to get to the acceleration event. That's where we found out that the senior design from last year (that being the coupling transmission) forgot to include the 2.808 final reduction in the CBR250R, so we would top out at about 25MPH, but we had something like 2000Lb-ft of torque at the wheels in 1st gear, which caused us to snap a driveshaft.

Thanks BTW, I really do appreciate all the help!


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