ISO 30C to 45C rated pouch cells

[DogDipstick]

What would you propose for a 2200 lbs race car (with all safety items) to accelerate to and average 250 MPH over a measured mile on 5 miles of salt?

I propose you hire an real engineer for consulting. Private engineering consultation company, for hire. For you said it yourself, earlier..

Yes I will be the chief shop sweeper. I am also the owner, chassis builder, and "engineer".

I am only the shop sweeper. Here. I don't know the best. I do know what I would do. If I had any module to pick cost not considered, I know what I would do. If I had enough money. Lots here have more exp. than me... but... we are licensed to engineer, and incorporated.

Honestly, they are out there. You got my vote, ..... At least enough to get started and you can derive the heat load in W/Ah easily, empirically based on IR reading and throughput... get your data in place and do it.. . Gauge the longevity vs performance... ect.. if you are into that kind of thing.

here: Art Razumovsky is a member here and can sell you a warranted pack for not to much money, compared.. and.. if you don't like it, there is always a resale value for a Chevy Volt battery so you can recoup. You can pm me for his email address, if you like. Or just contact him directly, but pls. tell him Jp sent ya. lol. Delivered.

https://greentecauto.com/hybrid-bat...zOvw7F4GVM_cwzd7jnZkKrwG9533eCZBoC6CYQAvD_BwE

The more money ya got the more options there are. This is cheapo budget racing I like^^^^^.


400lbs for a thin, frontal section coolable 396v 2000$ used Volt pack capable of 1000A and proven by some people in the past.. I just like them. They are small.. small enough to fit between your legs.. (g0 second gen, if you were to go this rout. ) (not the 12s 44v ones ) ( they are gen 1) .

 

[SpeedRacer93]

"chief shop sweeper" is a racing term for hired drivers. The saying is all you want them to do in the shop is sweep the floor.

One thing I have learned over the years, is that in order to be successful as a business owner, you need to have a very strong understanding of a complex task even before you go out and hire an engineer/consultant.

 

[SpeedRacer93]

Using my spreadsheet I found that a 5 ah pouch cell with a 30C to 45C should build the lowest weight battery of 201 lbs for cells only. This is for a 320 Kw 400 Volt pack with 80% usage and 80% inverter efficiency.

Found this site via the rcgroups.com forums that has some good testing results for RC pouch cells.

http://www.liporatings.com/index.html

It appears they run their tests out to 45 seconds. I may just buy a couple of the cells they rated with closest to advertised C rating (some are 90-100% exact ) and tear them down to see if I can upgrade the feed wiring like talked about before or discover the cell manufacturer.

 

[john61ct]

Wayne Giles' ESR/IR meter, Rick Distler aka rampman, RCG company Ramp man

Jack Fetter (JFetter) is webmaster

also Mark Forsyth,

John Julian website here at: http://jj604.com/LiPoTool

Dinogy is MarkF

All are brilliant, use the forum search function to deep dive into the relevant threads, you will learn **a lot** of very valuable stuff

 

[DogDipstick]

All are brilliant, use the forum search function to deep dive into the relevant threads, you will learn **a lot** of very valuable stuff

Right on Easy peasy way to get the estimated max Crate. Best Imo. Since I began posting on RCgroups, people have pm'd me to make them custom packs there quite a bit. Quite a bit.

Lol I have actually talked to Wayne about Chevy Volt cells, the max A, and the formula used to build the calculator. Lol. Tried to give em to Mark for testing for free. Denied. All 5,000mAh packs they like, not 15,000. Definitively the go to for hobby grade 5000mAh pack data. They like to fly things. Lol. Love those guys.
Carry on.

(If you ask me, EV cells > Hobby grade cells. )

 

[Hillhater]

(If you ask me, EV cells > Hobby grade cells. )

In many respect yes, ..
... But not much use for flying,...or racing where every extra pound is critical !

 

[SpeedRacer93]

(If you ask me, EV cells > Hobby grade cells. )

In many respect yes, ..
... But not much use for flying,...or racing where every extra pound is critical !

In this project every extra pound needs to be accounted for somewhere else. I would rather no compromise a couple additional tubes in the chassis because the battery pack was cheaper but 250 lbs heavier. Plus 250 lbs heavier battery needs to be supported and contained which requires a stronger chassis.

There is a reason that race cars are built from 4130, carbon fiber and TI bolts to make them lighter plus stronger. You have to accelerate every pound and then slow down ever pound. Plus every pound adds mass when things go bad.

With most EV pack they are not built for cell serviceability in mind. If one cell drops down you have to replace the entire sub pack. With new individual RC style cells you can replace a bad individual cell. Again with racing you are pushing things to the limits by design.

 

[DogDipstick]

Wh/Kg (weight density) and W/L (power density) figures much better than RC lipo. Nowadays fo sho .. ( unless you believe the guy selling you his "100C lipo"... Lol. ) Thats why you want a hybrid pack, partly a reason. The density.

From what I research.

Yall ever destroy a lipo? I killed those hobbylipos, fast. Cheap and quick. Never destroyed an EV lipo. Can/could switch out a "bad" cell just as easily ( easier) than trying to take apart a hobby grade lipo, IMO. Safer too, by leagues. I hit my Hobby lipo on a Kweld.. Dead. Burrnt. Just look at tha tabs and calculate the Kcmil and you see they are a sale: a joke: a weak spot that burns when high current fuses it. See it all the time with controlled, measured short circuiting. Hit my EV lipo.. Over 2000A. A few hundred times plus for 1800A+. Reliability, repeatability, consistency. Can replace a cell or group of cells in an afternoon if you know what you are doing.

My "thing Ifly" can fly with EV lipos, just as easy, better even... . Lol weight. Punch out at the same 100mph. Some Hobby king lipos dont near 140Wh/kg... even... and some EV lipos go over 220Wh/Kg... power density well into the regions greater than any Hobby lipo I ever had.. Honest empirical experience.. An ( early) Chevy Volt cell weight: 380g.. Power.. 1Kw easy... ... 300A... single cell.... Try that with a hobby grade lipo. That is good power to weight. Volume is a problem though with these.. they made better ones in the last 8 years... I know alot of them, yall know.

The measurements for power density are right there on the RCgroups threads. Vs the Manufacture data from the EV market.

Wh/Kg and W/L ARE higher. I would bet ( my bike) on it. In ev lipo, nowadays. Certain.


Consistency and reliability are built by people who know what they are doing... studied to a T... , not bought in a catalog with seller claims.."100C"... but huge quality controlled contracts that have millions ( of $) relying on the product to meet the design ( exceed ) criteria... ( usually build much better by teams of OEM battery engineers... than .. hobby grade lipo sellers.. ) .

Look forward to see what you come up with , regardless. I know what I would do.

 

[John in CR]

Some of things I believe I may need to also take into account:

• Locked rotor phase amps effect on the max draw (ie: AMPs needed for max nM)

• 
  
  Brushless motors won't draw max battery current at 0 rpm. It requires some rpm before you can build up to max battery current. I see it all the time with my highest powered rigs regardless of the controller tuning for current ramp up, so the only times I see peak current limits is when I launch at or near zero rpm and hold the throttle wide open to accelerate through about 1/3 of top speed.
  
  Max torque does not mean max battery current. It's at the point where max phase current is, and at that point PWM creates a low apparent voltage to the motor. Max power input occurs much later in the run but before the BEMF gets high enough to be the current limiting factor.

 

[SpeedRacer93]

Some of things I believe I may need to also take into account:

• Locked rotor phase amps effect on the max draw (ie: AMPs needed for max nM)

• 
  
  Brushless motors won't draw max battery current at 0 rpm. It requires some rpm before you can build up to max battery current. I see it all the time with my highest powered rigs regardless of the controller tuning for current ramp up, so the only times I see peak current limits is when I launch at or near zero rpm and hold the throttle wide open to accelerate through about 1/3 of top speed.
  
  Max torque does not mean max battery current. It's at the point where max phase current is, and at that point PWM creates a low apparent voltage to the motor. Max power input occurs much later in the run but before the BEMF gets high enough to be the current limiting factor.

• 
  
  Yeah I need to get a better understand of the relationship of phase amps vs battery amps if there is one.
  
  The plan is for the motors to be about a 1.2:1 gear ratio to the wheels. With a 21.3" diameter LSR tire that will put us at about 4800 RPM at 250 MPH. We can use a push truck for the first .5 to .75 miles to get us up to about 50 MPH (~950 motor RPM) before we pull away. My idea would be to use regen to hold the car against the push truck. You don't want to use the hydraulic brakes because we work to get the pads off the rotors for less drag.
  
  Here is the stock BMW i3s motor as an example. (BTW the i3s and i3 both use the same motor and controller. The i3s is programmed with a different power curve.) (Hmm Axiom may work really nice with these motors. I may need to talk with someone.)
  
  
  
  Back to batteries: I have ordered a couple of the RC batteries that ranked the best in the LiPo tests. HK Graphene 5000 45C which is true rated at 45C (100% as advertised) on their 90 second test and a Rhino 5000 50C which is true rated at 45C (90% as advertised) As stated earlier my maths show a 30C to 45C cell may fit the load and give a good pack weight. I am mainly really looking to see if I can find the source of the cells and maybe double up or replace the feed wires.
  
  On the testing thread there is some talk about the feed wire/connector size. The main thing they talk about is the failure of the solder joints not the wire. A 10 AWG wire at 225 A for 90 seconds should about be a "fuse-able link" but appears not to be the case. Still more reading to do.

 

[john61ct]

The burn point for fusible links needs to be tested in each case, not at all precisely predictable for general use beyond **very** rough guidelines.

 

[John in CR]

An efficiency overlay of that same graph would let you know battery current. For a smooth transition the controller is likely to be hitting battery current limiting as it hits that knee in the torque curve. For a guesstimate since I'm sure that's a high efficiency system, I'd think it is at 85-90% efficiency at peak power output, so 150-160kw peak power draw from the batteries.

Also, to get the most power through the system, I'd really chill things down as much as possible just before the run, and don't use regen braking to hold it against the push truck, since that will create heat. Instead use stronger springs to open the brake pads in order to avoid unnecessary drag, so you can use the mechanical brakes.

I don't understand the preoccupation with weight. It's not going airborne or the repeated accel/decel of a racing application, so I think the added weight of more battery will be useful to keep it on the ground at those speeds with no real detriment as long as you allow enough space. Quality cells with accurate specs and big tabs for good connections will not only be far safer, but you'll be able to accurately predict voltage sag to get to the voltage you want under load for best performance.

 

[amberwolf]

Yeah I need to get a better understand of the relationship of phase amps vs battery amps if there is one.

That depends on your specific controller and whatever settings it may have. Some have a fixed relationship, some have a programmable one, and some you separately set each one, and some are a mystery with no programmability.

My idea would be to use regen to hold the car against the push truck.

You'll have to start with your battery at less than full charge to do this, or the regen can overcharge it.

How much less than full depends on how much energy the regen is recapturing, which you would have to test with a wattmeter under the same conditions it would happen in the run.

Additionally, some controllers won't even allow regen unless the battery is below a certain percentage of HVC, or a specific voltage limit, etc.

Some will roll off the amount of regen available the more full the battery is.

You'd have to check your specific controller for all that information.

On the testing thread there is some talk about the feed wire/connector size. The main thing they talk about is the failure of the solder joints not the wire. A 10 AWG wire at 225 A for 90 seconds should about be a "fuse-able link" but appears not to be the case. Still more reading to do.

If you can eliminate solder joints and use crimps to the cell tabs, or welds, it could give a better (lower resistance) connection (less voltage sag, more power to the controller and less wasted as heat). (and less likely to fracture from vibration).

There are a number of threads around the forum about DIY pouch cell packs, mostly A123, that have ideas on crimping or clamping the tabs together or to busbars. Or JonesCG's methods using the LiPo cells.

 

[amberwolf]

I don't understand the preoccupation with weight.

More mass means slower acceleration for the same given power, and so a longer time for the distance of the run.

Less mass means less power needed for same acceleration, or faster acceleration for the same power, and so a shorter time for the distance of the run. Better record.

 

[SpeedRacer93]

I don't understand the preoccupation with weight. It's not going airborne or the repeated accel/decel of a racing application, so I think the added weight of more battery will be useful to keep it on the ground at those speeds with no real detriment as long as you allow enough space. Quality cells with accurate specs and big tabs for good connections will not only be far safer, but you'll be able to accurately predict voltage sag to get to the voltage you want under load for best performance.

I totally hope I don't get airborne.

The car can weight a max of 2200 lbs with out driver. Here is a quick breakdown of the weights.

Max weight 2200
Frame 350
Hubs 22
Wheels 80
Tires 56.8
Brakes 85
Gearbox 100
Motors 370.4
Controllers 100
Batteries 500
Body 100
Parachutes 40
Cooling 196.8
Seat 15
Padding 10
Safety Belts 5

Remaining 169

Cooling system is 25 gallon closed loop ice and water (no radiator because the body is totally enclosed). I am looking at other options also for lighter weight cooling system.

I like to have about 5-10% of the car weight as movable ballast weight to deal with CG vs CP adjustments.

With the batteries being the highest percentage of the total weight it is where I am current looking to get straight. Remember I am looking to having two battery packs (one for front and one for rear) so the 220 lbs weight will be doubled plus needs mounting and wiring etc.

 

[Hillhater]

DD.. why dont you propose a pack using your best shot at available EV cells , targeting 400v, 1000A continuous, to estimate the weight and size. ?

 

[SpeedRacer93]

DD.. why dont you propose a pack using your best shot at available EV cells , targeting 400v, 1000A continuous, to estimate the weight and size. ?

Not very many EV cells can pull that rate for 90 seconds. I personally have tested and use Chevy Malibu Hybrid packs on the bench. They have a higher C rating then the Volt 48v modules but my luck with them on the used market is not been good.

The Volt modules are only rated for 250 A continuous and 350 A peak. A 9 unit - 48v Volt modules would weight about 405 lbs. Yes people say they will pull 1000A but they are not rated for that level. Also people who have used the Volt packs state they sag when pushed hard.

Tesla modules would be about 1200 lbs.
Kia Soul EV are good cells but have issues getting hot when pushed so cooling would be required since they are air cooled.
BMW i3 modules are only 1C continuous/2C Peak.
Chevy BOLT battery if I remember right are low C rated also.

I really don't want to get into tear down a factory modules to create a custom module.

 

[DogDipstick]

Cooling system is 25 gallon closed loop ice and water (no radiator because the body is totally enclosed). I am looking at other options also for lighter weight cooling system.

I have a couple a degrees in water management and the only thing they told me is... Water is heavy. 8.34 lbs. 25 ( gallon) * 8.34 = 208 lbs.

Now upon a whim:


You would be better off in cutting that weight in half or more with dry ice..? Dry ice sublimates at 194.65 K (−78.5 °C; −109.3 °F) at Earth atmospheric pressures. Might be able to take advantage of this... if want, where weight is involved... if the runs are only 90 seconds and a finite dimension... easily calculated for.. ( and gets lighter as you go! lol. ) Not practical with an internal combustion heat engine... but very well may be usable for electrical power traction, traction pack and the cooling loads of such.... I have seen a few dragsters built with diy EFI, and fuel dry ice coolcans, intercooler misters with Co2... and stuff.. back in the ol EFI fun days. Who know if it would make a difference... but heat transfer is like 99% of what puts food on my table, we know it well.

The Volt modules are only rated for 250 A continuous and 350 A peak. A 9 unit - 48v Volt modules would weight about 405 lbs. Yes people say they will pull 1000A but they are not rated for that level. Also people who have used the Volt packs state they sag when pushed hard.

I do wonder where you get the data from. I could show you a datalog of a fellow pulling 700A on one for a good 45 seconds or more, and the associated "sags" and "droops". I do not know of a single "48v" module in production used on the Chevy Volt. I do not know where this "48v" figure comes from... seems ambiguous... To me.

We are looking at 50 to 100 cycles

"Rated " spec certainty have longevity in mind. Very easy to empirically derive the load on both the cooling system and the cell to keep it cool and pushing current. My hobby is measuring IR of available EV cells in the market today.. and I will tell you .. they are powerful... but since you are ot gonna tear down and whatnot... worry about " replacin cells", I guess I do not have much more to suggest here.


I want to buy a "100C" sleeper cell x 5, assemble a battery, and sent it to the RCgroup guys.. so they can really test the claimed "100C" .. from an independent standpoint, with hobbyist accepted, standards for "longevity" and " empirical data"... If I were thinking about investing alot.

Fact is I have ( already ) cells on that ( RC groups) list..many of them.. I have measured the Graphene packs, right nest to the Chevy cell, right next to Kia cell, right next to LTO cells.. Right next to LifePo cells.. and the EV cells are much safer, more powerful, and longert lasting from my experiences.


Or an Ioniq...... have you looked at Ioniq, the cells? they would be what I would like to get my hands on next.

DD.. why dont you propose a pack using your best shot at available EV cells , targeting 400v, 1000A continuous, to estimate the weight and size. ?

Already did, friend. Also stated a seller, offered an email, and asked for a referral, should he call, Art, and buy something, warranted and resell able. For starters.

......But he said he got the data of the "rated " module as a "350A" module.. ( I have much different empirical data, tho.. from my researching, but who am I to say one way or another.) ( and if you are only expecting 50 cycles.. well then.,..guess we will never know, for the reference of "350A peak" is restricting ( dont know where it came from, I know, from looking at data, a Volt module can push alot more than that in a sprint... with a cooling load and a good flow pump.. )

Plan to run the motor "above rated spec"... ( 380Kw) but... the battery cell not " above rated spec"? IDk If I can help much more.

. I am mainly really looking to see if I can find the source of the cells and maybe double up or replace the feed wires.

Good luck doing that safely and reliability, friend. ( Or at least without someone to sue in case your installation fails. .. ) God bless you in reaching your targets.

 

[Sunder]

Back to batteries: I have ordered a couple of the RC batteries that ranked the best in the LiPo tests. HK Graphene 5000 45C which is true rated at 45C (100% as advertised) on their 90 second test

A couple comments here. I use these cells for my electric bike.

1. 109S4P of these cells would weigh 137lbs. Roughly. I didn't calculate the weight lost by removing all the extra wires and connectors. So, you're way under your 500lbs budget. Even add in the box, big bus bars, BMS, etc, and you're not even coming close.

On the testing thread there is some talk about the feed wire/connector size. The main thing they talk about is the failure of the solder joints not the wire. A 10 AWG wire at 225 A for 90 seconds should about be a "fuse-able link" but appears not to be the case. Still more reading to do.

If you can eliminate solder joints and use crimps to the cell tabs, or welds, it could give a better (lower resistance) connection (less voltage sag, more power to the controller and less wasted as heat). (and less likely to fracture from vibration).

2. I don't have the original context of that quote, but I suspect they would be referring to a wire that was too small, heating up to a temp melts the solder that connects the wire to the cell. 60/40 Solder melts at 183*C, where as silicone wire I've seen rated variably to 200/220/250*C, so the solder would melt before the silicone wire would burn.

Given the contact area for inter-cell connections, and the very short distance between cells, it seems unlikely that the solder there would be the first failure point.

Those packs also use some kind of high temp solder that doesn't melt with a standard iron. I had to buy a higher temp soldering iron to work with these packs.

 

[DogDipstick]

Given the contact area for inter-cell connections, and the very short distance between cells, it seems unlikely that the solder there would be the first failure point.

Those packs also use some kind of high temp solder that doesn't melt with a standard iron. I had to buy a higher temp soldering iron to work with these packs.

Those packs love to blow the tabs right there. Anywhere from 1000A to 2000. Even much less. Desoldering in a three min flight is not unheard of pulling 200A on 14s. I have melted a few ( 6mm) bullets and had to autorotate a 12lb heli unpowered to the ground from a few feet up.

I can tell you exactly the elemental composition of those tabs, using molecular spectometry... If you would really want to know. The exact alloy of the copper, the aluminum.. and the solder.. individually. One can count the Kcmil.... Some cells are built very differently from one and another here... A reason that EV cells have very wide and thick tabs also.. comparatively.

The busses must be sized for the load. Or they will fail, be a danger, and create a hazard. That is what Hobby grade(designed) lipos do.. size for the load ( of an RC plane)... Noone ever pulls hundreds of Kw from RC lipo in practical applikcation....

Look at Chris Jones's builds.. High number of cell level interconnections... High power levels.. cooling.... Some of the biggest, most powerful handbuilt racing applications... how he handles the current busses. Zero skimping. There.

Or the way J. Metric does it. In his purchasable batteries.


Failed ( underspecced, untested, or otherswise misdesigned power busses in the emergency generators) have bought 200 million dollar+ power plants to the brink of catastrophe, destruction, and their knees, before, more than once...

 

[Hillhater]

Just buy a recycled Volt module.... tested and proven to be 1000A capable, .....

? But for how long ??. 10 sec ??.
Indepemdent testing b the US Department of Energy, of the bare volt pack showed its maximum 10 sec PULSE capability was somewhat short of that (300kW ),..at full charge. And reduced significantly as capacity reduced (<250kW ) @50% SOC and 150 kW at 20% SOC.....remember those are 10sec pulse tests. !
I wonder what it does under a continuous 90 sec max discharge test. ! ?

 

[Frank]

Sounds like a fun project. Can I ask how you estimated 90-sec. run time?

While motor current will ideally be maxed for an entire run, battery current will not be. I know of one team who ran the long course who used 4-6 kWh total energy, including driving 3 miles from the pits, returning 5-6 miles up the return road (to the start area) then driving the 3 miles back to the pits. I suspect 10 kWh design energy is reasonable.

I drag race with Sleeper cells and have also exceeded 200 mph on a pavement course using them. I always design for much less than the rated C-rate ex. the 4P pack on the LSR bike drew a max of 500A and that for only a short time. I run the 3P pack in my drag bike a bit harder but typically not exceeding 900A peak draw. The 6S packs use 8 AWG wires but the new 12S packs have dual 8 AWG (I think it's #8) and would be a better choice for a longer run. Some of the cells in my drag bike are from 2014 and still hanging in there really well. These are expensive but very good cells. I just took apart my drag bike pack and with the exception of 3 bricks, the entire pack was sitting < .013 V total difference (about what I balance to); last time they were charged was August '19. The 3 bricks were at .05 V difference which isn't too bad but makes me wonder if one or 2 cells in the bricks aren't getting weak.

Something to think about is that LiPo's typically sag at a fairly constant rate. If you could identify a cell with more linear sag characteristics (A123?) so that after the initial "knee" drop-off the discharge would occur in a more linear fashion - you would preserve voltage for the end of the run where it really counts. You could oversize the S-rating of the pack and partially discharge the pack before a run (ex. using baseboard heaters) so they would be sitting at a more linear part in the discharge curve.

Regardless, nothing says you can't start with hobby level cells and see how things go, you'll have plenty to worry about with a new build anyway. I don't know much about the Volt cells but access to liquid cooling might be important. Also, have you looked at Enerdel? I don't even know their business status at this time but their cells are pretty robust (NMC) but sag a bit more than other types of lithium. Given the state of the salt I'd also be thinking about running on pavement.

 

[SpeedRacer93]

A couple comments here. I use these cells for my electric bike.

1. 109S4P of these cells would weigh 137lbs. Roughly. I didn't calculate the weight lost by removing all the extra wires and connectors. So, you're way under your 500lbs budget. Even add in the box, big bus bars, BMS, etc, and you're not even coming close.

The pack is up to 109S7P taking into account a mistake in my formula, 80% usage, and 80% efficiency (yes I am using a low rating because of pushing to the limit.) With this new size it would 201 lbs cells only with individual cells and 248 lbs using new pre-made packs. There will be two complete packs in the car. One for the front motor combo and one in the rear for the another set of motors. The motor combo is not set in stone yet. I have been testing i3 motors on the bench and working on a mounting system for our eddy brake engine dyno that I just reassembled after the move to test under load. (I own an i3 and having been a BMW owner, racer, and fanboy for decades.) But I am also looking at other motors from YASA and EVO due to better packing in the car.

2. I don't have the original context of that quote, but I suspect they would be referring to a wire that was too small, heating up to a temp melts the solder that connects the wire to the cell. 60/40 Solder melts at 183*C, where as silicone wire I've seen rated variably to 200/220/250*C, so the solder would melt before the silicone wire would burn.

Given the contact area for inter-cell connections, and the very short distance between cells, it seems unlikely that the solder there would be the first failure point.

Those packs also use some kind of high temp solder that doesn't melt with a standard iron. I had to buy a higher temp soldering iron to work with these packs.

My context was that I feel that 10 AWG is under rated for the 225 A load but the only failures they are reporting on the RCGroup tests is with a couple pack with poor solder joints that failed. It was low temp solder and may have been a poor production.

They do talk about the higher temp solder being used on some of the cells. I am still reading the posts to get a better understanding of how they are testing and how the packs are built. (I think my wife is onto me because instead of reading a book at night I am reading on my laptop. She knows how I get before taking on a wild project. I go into intense study mode.)

With the pre-built RC packs it messes a little with the normal P first then S build design. I could do S first them P the modules. I would like the complete battery packs built from sub modules. That way if there is an issue, we can swap a sub module instead of the full pack.

Still need to get an understanding of the pack cooling needs since they will be enclosed with no air flow.

 

[SpeedRacer93]

Sounds like a fun project. Can I ask how you estimated 90-sec. run time?

Yeah it has me excited. 90 sec is a "common" average for a long course run at 250-275 mph with a constant 0.5 to 0.6 G acceleration rate. They like streamliners to do record runs on the long course so building the car with that in mind. Once we complete our proving runs up to 175-200 MPH, I think we mainly would be on the long course.

While motor current will ideally be maxed for an entire run, battery current will not be. I know of one team who ran the long course who used 4-6 kWh total energy, including driving 3 miles from the pits, returning 5-6 miles up the return road (to the start area) then driving the 3 miles back to the pits. I suspect 10 kWh design energy is reasonable.

I drag race with Sleeper cells and have also exceeded 200 mph on a pavement course using them. I always design for much less than the rated C-rate ex. the 4P pack on the LSR bike drew a max of 500A and that for only a short time. I run the 3P pack in my drag bike a bit harder but typically not exceeding 900A peak draw. The 6S packs use 8 AWG wires but the new 12S packs have dual 8 AWG (I think it's #8) and would be a better choice for a longer run. Some of the cells in my drag bike are from 2014 and still hanging in there really well. These are expensive but very good cells. I just took apart my drag bike pack and with the exception of 3 bricks, the entire pack was sitting < .013 V total difference (about what I balance to); last time they were charged was August '19. The 3 bricks were at .05 V difference which isn't too bad but makes me wonder if one or 2 cells in the bricks aren't getting weak.

Something to think about is that LiPo's typically sag at a fairly constant rate. If you could identify a cell with more linear sag characteristics (A123?) so that after the initial "knee" drop-off the discharge would occur in a more linear fashion - you would preserve voltage for the end of the run where it really counts. You could oversize the S-rating of the pack and partially discharge the pack before a run (ex. using baseboard heaters) so they would be sitting at a more linear part in the discharge curve.

Regardless, nothing says you can't start with hobby level cells and see how things go, you'll have plenty to worry about with a new build anyway. I don't know much about the Volt cells but access to liquid cooling might be important. Also, have you looked at Enerdel? I don't even know their business status at this time but their cells are pretty robust (NMC) but sag a bit more than other types of lithium. Given the state of the salt I'd also be thinking about running on pavement.

I am specing the pack at 3.7 cell voltage and only going to 80% usage. I have it on my list that I need to look at the voltage drop at the end of the run and take that into account also because, like you said, that is where it is really needed. This battery pack design is a work in progress. But I tend to be a deliberative overthinker, sometimes to a fault.

We plan to do some proving runs on the pavement before moving to the salt. The long term state of the salt is a big worry thanks to the Bureau of Land MIS-Management.

My plan is to over build the car so that if things are going well, we can add a 100 Lbs or so and move up to the E3 class also. But that is a wild thinking way ahead dream. Heck we never have pulled injector wires and spark plugs to run a different class. We would never do something crazy like that... :wink:

 

[hallkbrdz]

Any update over the last month? Interesting project...

I'm looking at similar cells for racing with weight being very important as well, just over 8 minutes not 90 seconds. I've worked up quite a spreadsheet for my calculation, could be helpful if you are interested.