24S 10P A123 Pack. Worth the effort?

[njs]

I am currently working on a hybrid race car to compete in the 2010 SAE Formula Hybrid competition.

We have determined we will need 72 Volts, hence the 24S.

We have also determined we would like to use an Agni motor. The 95 reinforced is rated at 400 amps. Our plan is to goto about 350A for brief periods.

To produce 350A we really only need about 4 batteries in parallel (50C). Problem is that's only enough for 2 minutes, and it will drastically reduce cycle life of the batteries pushing them that hard.

I have heard A123's maintain cycle life extremely well up to about 17C and 140 degrees F. 17C * 2.3 = 39 Amps. Going with 10P to be on the safe side seems like the best choice. The endurance race is 22km, and is typically completed in around 20-25 minutes. 17C is about 4 minutes of raw full power @ 350 A. I figure this will only be used about 1/4 of the time giving more like 16 minutes of use. An ICE assist and intelligent use by the driver will put us the rest of the distance.

I have decided for weight displacement reasons splitting the pack into two and putting a 24S5P on each side of the car would yield the best results. This would allow for one BMS per side. I have seen other discussions on having a hard switch to put all packs in parallel and rigging a charger that can put out 3.7 V and 200A. Is this at all viable if we can build such a charger? I'm not holding my breath on this.

To be clear, we will build small packs of 5 batteries in parallel. 24 will be connected in series and stacked on each side of the car. One BMS for each side.

Main Questions:

1) Is it at all feasible to build 48, 5P packs? Shouldn't take too long if our team can get an assembly line set up.

2) What size wires? ~200A between the packs on each side. ~400A into controller, I assume we are going to need some really fat pieces of copper.

3) How much ventilation? I was thinking of opening the front and back of the battery enclosures and putting small fans to facilitate airflow would work fine. Would this even be overkill?

4) Should two 12S 10P packs be considered (rather than two 24S 5P packs)? This would allow us to use one BMS for the whole system. Or do we even need to go the opposite way? 3-5 BMS?

5) Any soldering advice? I've only watched a few videos on the subject. 100 W soldering gun to prevent too much heat going in. Cleaning all connections thoroughly before starting. Flux, all that jazz.

6) Would you guys recommend cutting off the green covering? My guess is it just slows heat transfer. In general you want the battery in the center of the 5P pack to dissipate through the metal into the other batteries. Would shrink wrap also hinger this process? Is shrink wrap necessary to prevent some serious shorts?

7) Anything I'm completely missing?

Please don't tell me I'm dumb and should just use Headway, Thundersky, Kokam, BMI, or LifeBatt. Those are great batteries for electric only applications, but the point of this pack is a lightweight high power assist for the ICE's abysmal torque curves. Kokams are absurdly expensive, and I know LifeBatt produces a high power battery also, but I can't find a single price quote and I have people on Ebay practically giving away the A123 26650's (Yet to be determined if these are fakes. Some test will be performed next week once bigger resistors arrive).

I plan to document our progress and will try to update with pictures as the process goes on. Thanks for any help.

 

[voicecoils]

1) Is it at all feasible to build 48, 5P packs? Shouldn't take too long if our team can get an assembly line set up.

Not the end of the world. Even better would be to enlist a locally battery centre to use a proper tab welder to build the packs in return for sponsorship promotion.

3) How much ventilation? I was thinking of opening the front and back of the battery enclosures and putting small fans to facilitate airflow would work fine. Would this even be overkill?

Heat should be minimal from healthy cells if they're not pushed past their specs.

6) Would you guys recommend cutting off the green covering? My guess is it just slows heat transfer. In general you want the battery in the center of the 5P pack to dissipate through the metal into the other batteries. Would shrink wrap also hinger this process? Is shrink wrap necessary to prevent some serious shorts?

The cell can is live and removing covering may be un-necessary (see above comment on heat).

Please don't tell me I'm dumb and should just use Headway, Thundersky, Kokam, BMI, or LifeBatt.

If you're racing you should be using the highest energy and power dense options available to you. Therefore you should seriously consider lico lipoly. If you want lightweight, lifepo4 chemistry is currently not the answer. Kokam's are one option for large format lipoly but I'd have a look at what could be done from hobbycity.com 5S1P or 6S1P 5Ah blocks. Do you have a weight or energy storage limit in your regs?

 

[liveforphysics]

You can reach your goal with LiPoly for about 1/2 the cost, 1/2 the weight, and about 1/20th of the pack building effort/time.

The pack I normally run on my E-bike would more than meet your needs, cut your budget, and most importantly, in a game where dropping weight is everything, you would find no lighter weight battery solution.

Go to hobbycity.com

Click the tab on the left for batteries, go to LiPoly, go to 30C, look for 6S 5Ah packs, order 12 of them. That will give you 84v nominal and 15Ah (more like 18Ah with the way they de-rate cell capacity). Buy 24 of the 6S JST extension balance tap harnesses, a GGoodrum balance/HVC board for charging, and a pair of 48V meanwell power supplies off Ebay. Next pick up 8ft of 0-awg supple fine stranded car audio power cable off ebay, and a two pair sets of the big 300amp anderson connectors (one for motor to controller, one for controller to battery), and heat shrink and solder for combining all the LiPo pack leads together.

Combine these parts with about 4-8hrs of skilled labor, and POOF! The battery, charger, wireing, connector task is completed, and likely all completed for less money than the A123 cells would cost alone.

Best wishes,
Luke

 

[njs]

I've seen you multiple times hating on the A123's but never really backing your claims up with numbers. At $6.55 a cell (shipped) I would spend about 80 bucks to get as much power as in the $100 + 10 S&H Turnigy pack. Hardly 1/2 the price. I do agree on the weight. Even with the wires and housing the Turnigy post much better energy per mass especially if actual cell amperage is closer to 5.5 Ah.

I think bumping the size up to 20-25Ah would be around the right level. More energy = more use out of the e-motor and less stress on the pack.

I'm just worried after a month of testing the Turnigy batteries will be gone because 35C is and absurdly overstated number based on 50 charges. With the A123 I know they are so overpowered (50 kw), and abuse proof (2000+ cycles) that we will have a solid battery for a long time using them at 15C. I don't get that same feeling with the Turnigy packs. Especially with people complaining of loose solders. There is no cycle life rating, and I can't find a single discharge plot. Really, there is practically no information on them.

I've yet to hear of one actual case of someone receiving fake/used A123 cell from ebay which seems to be the only argument against them.

I really am interested on your thoughts and if you have noticed any power drop in the cells you use.

Thanks

 

[njs]

O yea, to answer voicecoils. Total vehicle energy limit is 20MJ. Any battery added reduces the amount of gas allowed. I assume i'm going to be reducing performance trying to push the upper envelope of that. Of course, pure battery would eliminate the ICE weight, but that's not allowed (weird rules).

I can't find any tab welders close by. I see plans to build a $100 tab welder online. Might destroy a few in the process, but superior results compared to normal soldering.

I like the 6S 5Ah blocks more than any other alternative to the A123, and will continue looking into them more.

 

[liveforphysics]

You will find yourself hard pressed to find a more abuse-able cell than inside those 30C 5Ah. Ducted fan jet guys beat those packs with a constant 100-150amps straight out of the box with no break-in, recharge at 2C-5C, and fit in 10+ cycles in a day of flying. Lots of guys do that every weekend in heli's and planes, and rack up cycle numbers into the 4 digit range.

Charge them right, don't discharge below the safe point, and they will give more life you will ever need for any SAE racer. Some of us have been commuting on the same LiPo packs with a couple cycles a day for over a year with no failures or issues. Its not for lack of abusing the cells either

As far as prices go, I forget you guys don't have deep discounts there. 30c 6s packs in your desired quanity would be around $50. Contact HC, tell them how many you wana buy, and they will hook you up. Those guys love to dicker, and you will never get the same price in a quote twice. Lol, keep getting quotes until you find the lowest price, then buy. Lol

 

[jag]

Nice project. I found this web page http://www.formula-hybrid.org/ Is it the right one?

the 20MJ max energy you mention is the combined gas thermal energy and battery electrical energy right?
(I didn't find a definition in the rules, though I only skimmed the 108 page rule book in about one min)

Here it looks like you would gain "energy at the wheels" by upping electrical storage. (Since the efficiency of converting the gas to wheel power is maybe 10-25% while the electric system can easily manage 60-80%). Of course the performance penalty from adding battery weight need to be counted.

I did stumble on the following in the rules:

4.5 Accumulator Type and Size
Total accumulator voltage may not exceed 400V.
Accumulator capacity may not exceed 4,449 Wh.
Teams must state, as accurately as possible, their accumulator capacity. There is a $6,000.00
limit on the “standardized” cost of the accumulator system. Energy accumulators must be of
an approved type. At this time only batteries and capacitors are permitted.

So looks like you can make up to 16 of the 20 MJ electric:
octave:1> 4449*3600/1e6
ans = 16.016

 

[jag]

2010 Formula HybridTM Rules
<snip>
6.8.6 Parties
Disruptive parties either on or off-site should be prevented by the Faculty Advisor.

 

[liveforphysics]

People always have such low efficiency numbers for ICEs. Makes me wonder what engines they are using.

0.347lbs/hp-hr is around the BSFC's of a Winston cup racing engine. That's a dumb pushrod 2v 800hp V8 engine with a carb.

0.213Kg/Kwh

4.69Kwh of mechanical work per Kg of gasoline

12.5Kwh/Kg heating energy for gasoline.


That's 37.5% efficient for a dumb hick engine based POS pushrod V8 with all sorts of lame rules and restrictions to cripple the potential performance.

 

[njs]

ICE's are assumed to be 27% efficient by the rules. So 20 MJ of energy is more like 80 MJ's of actual gasoline. The 16 MJ limit wasn't important as that is more than we would like to have weight wise. The engines are limited to 250cc. The Ninja EX250 is what we are using.

Also, batteries are assumed to be 80% efficient. Theoretically 20 MJ of these Turnigy cells gets rated at 16 MJ. And if they are closer to 5.5 Ah we would gain an extra 10%. So pushing 22MJ but only subtracting 16 per rules. I guess what they consider "nominal" voltage is important. Anything over 3.3 and they are really screwing you on total energy capacity ratings.

 

[liveforphysics]

Request your packs to have 3Ah labels put on them. HC will do it, they are pretty cool like that.

 

[liveforphysics]

ICE's are assumed to be 27% efficient by the rules. So 20 MJ of energy is more like 80 MJ's of actual gasoline. The 16 MJ limit wasn't important as that is more than we would like to have weight wise. The engines are limited to 250cc. The Ninja EX250 is what we are using.

Also, batteries are assumed to be 80% efficient. Theoretically 20 MJ of these Turnigy cells gets rated at 16 MJ. And if they are closer to 5.5 Ah we would gain an extra 10%. So pushing 22MJ but only subtracting 16 per rules. I guess what they consider "nominal" voltage is important. Anything over 3.3 and they are really screwing you on total energy capacity ratings.

You could shave a whole boatload of weight, like maybe ~80lbs, AND improve thermal efficiency and lower BSFC's by going to something like a CRF250R engine over a Ninja 250 engine. They are both dirt cheap. The CRF250R engine does about 45-55hp (depending on tune), and engine and tranny is seriously like 1/3rd of the bulk and weight of a ninja 250 engine.

EDIT*

I did some research. You could expect about 15-20bhp more than your Ninja engine, and about 55lbs of weight reduction in the engine/tranny system. It would also drop the center of gravity by a large amount, and it has exceptionally small packaging.

You could do a cheap and easy cro-moly frame, use a CRF250 engine over the Ninja, and be lighter than something with an absurdly costly and labor intensive carbon fiber monocoque frame and a Ninja 250 engine. You would also have more power and better economy due to a lot better thermal efficiency design and higher compression.

I was on a Formula SAE team years ago in college. I set them up with a wicked powerplant (my job on the team), the rest of the team wasted time and the budget on exotic materials, totally failed at getting a chassis together, and we didn't even attend the competition. lol FAIL! lol

 

[njs]

It has to be 4 stroke.

We looked into dirtbike engines first. Most are disallowed in some way. They generally have superior power to weight. I'm not sure where the efficiency is.

 

[jag]

For the batteries, before committing to the full pack, buy a small sample set of each LiPi and A123. Run both through the rigours of your high current discharge and charge to get some real world data specific to your use.

I'm also thinking that you will benefit from some smarter control system than a simple BMS. You have to consider what range to cycle the cells, e.g. 20-90%, and since the race course is known for each event you could try to do globally optimal control given the trajectory to make sure you have the appropriate charge state e.g. when starting to accelerate on a straight, or when expecting to slow down in a curve. (I'm assuming you will be using regen)

What are your plans for electric motor and drivetrain? Any pics of current state?

 

[Lapwing]

http://wikileaks.org/leak/toyota-prius-a123-car-fire-investigation-report-2008.pdf is well worth the read on page 22-23.

I would think that http://www.proev.com/ the electric imp strategy would be a better approach. Simpler with fewer cells = less complexity.

This was also the approach Agnimotors took on there winning TTXGP bike earlier this year.

 

[njs]

Kokam's are about twice as expensive as A123, and even their "high power" battery isn't very impressive.

 

[liveforphysics]

It has to be 4 stroke.

We looked into dirtbike engines first. Most are disallowed in some way. They generally have superior power to weight. I'm not sure where the efficiency is.

It is 4 stroke. I looked through the rules and saw nothing to exclude it. In fact, it seems like a very obvious choice. Major weight reduction, more power, less friction, better thermal efficiency, better powerband, etc etc.

You would very likely have better results dumping the whole electric side and just running the CRf250 engine. Your vehicle could be about half the weight with more power than the ninja 250 + electric setup. But, if you must have a hybrid, it would sure make a hell of a lot more sense to do a crf250 for the engine than a 1987 designed (and largely unchanged to present year) ninja 250 heavy turd engine with a terrible powerband.

 

[liveforphysics]

3.5.4.1 Engine Limitations
Engines must be Internal Combustion, four-stroke, with a maximum displacement of 250cc for
spark ignition engines and 310cc for diesel engines and be either:
1. Modified or custom fabricated. (See section 3.5.7)
Or
2. Stock – defined as:
• Any single cylinder engine, or
• Any twin cylinder engine from a motorcycle approved for licensed use on public roads,
or
• Any commercially available “industrial” IC engine meeting the above displacement
limits.
Note: If you are not sure whether or not your engine qualifies as “stock”, contact the organizers.

Permitted modifications to a stock engine are:
• Modification or removal of the clutch, primary drive and/or transmission.
• Changes to fuel mixture or timing settings.
• Replacement or modification of any exhaust system component.
• Replacement or modification of any intake system component; i.e., components
upstream of (but NOT including) the cylinder head. The addition of forced induction
will move the engine into the modified category.
• Modifications to the engine casings. (This does not include the cylinders or cylinder
head.

 

[liveforphysics]

I can also confirm that it will comply with the rule for fitting the stroke measurement tool down the plug hole, as they do that to every bike that places at every event.


If you want to use the Ninja engine, and go for weak, peaky, and heavy, you should go with A123 as well. They compliment each other well.

Or, if you wanted to make something that performs well, the CRF250 engine with LiPo packs FTMFW.

Being a restricted engine, I would highly recommend a 1/4D simple radius inlet to feed a large plenum upstream of the carb. You could play with a reed valve, or perhaps electronically controlled rotary disk valve just upstream of the transition between the plenum and the carb. If you are looking for every last bit, reduction porting that cylinder head to be optimized for keeping a good velocity/mass balance in the intake charge would be ideal. Don't try to do it on your own, it's a dark art, farm it out to shop that specializes in CRF250 cylinder heads.


Best Wishes,
-Luke

 

[njs]

I guess I was looking at older engines. When did they make it 4 stroke?

Also, how much will this cost? I see a 2007 for $1,500 on ebay.

 

[liveforphysics]

CR250R = 2-stroke
CRF250R = 4-stroke


I've seen them on craigslist in the $500 range. If you look in the right places, you should be able to find a 2009 for under $1,000.

Each year they have managed to further shave weight down on them, which is an amazing feat in itself, as you can all ready hold the entire engine/tranny assembly easily on 1 arm. lol

 

[tostino]

I'll agree with LFP on the CRF250 engine. It is a really nice running, powerful motor. The newer engine you get, the better, however, I had a 2005 and it was still a very nice bike and engine. It's hard for me to compare to a street bike, because sadly, i've never had the chance to ride one.
I can compare it to my dads old 2001 suzuki DR-Z 400 (four stroke 400, dual sport bike), and my bike had a MUCH better power to weight ratio. I can't say for sure that my 250 motor produced more power than the 400 motor, because I didn't ever put it on a dyno, but I wouldn't doubt if it did.

 

[njs]

I emailed Hobby City. No answer for 3 days. Not sure what the deal is. I'm wondering if the 5S batteries are better. The 6S only gives 66.6V or 88.8V.

5S could give me a 74 V pretty easily.

Assuming I can get these for 60-70 a pack I'm down for them. Otherwise A123's it is.

 

[vanilla ice]

I was on a Formula SAE team years ago in college. I set them up with a wicked powerplant (my job on the team), the rest of the team wasted time and the budget on exotic materials, totally failed at getting a chassis together, and we didn't even attend the competition. lol FAIL! lol

That sucks about your team phys. :?

Njs, even retail isn't too bad at HC prices.. the 5s flightmax 15c packs are $43 right now. Not sure if 15c is enough power for what you need to do, but its cheap! If not go for the higher rate ones.

To second what phys says, even if you have to go back a ways to fit a motocross single motor into your budget, it would be worth it. The older ones aren't too much different. They've been running four strokes in motoX forever. You can maybe get 30 or 35hp max out of a ninja 250r, and even then its going to be unreliable. Basically its an economy commuter/playbike. The yzf, crf, kxf, rmz.. they are competition motors from the drawing board.

If your budget doesn't allow motor work I wouldn't worry too much about it. At least you will be starting with something that has its hp/cc optimized from the factory.. unlike with a street model. There's not really any big easy gains to be made with motocross motors. They've taken care of things for you!

 

[njs]

Basically its an economy commuter/playbike.

I agree, which is part of the reason we are using it. If we were to use no batteries at all the car would be allowed 2.25 liters of fuel. Assuming we have a 6-8 MJ pack we are only allowed around 1.5 liters of fuel. I don't really know the efficiencies, but I know that's not parameter number one when designing a dirt bike engine. The ninja may feel anemic but that is where the e-motor will come in.

Of course there is the other side of this argument that a lighter car will be able to maintain higher speeds in corners and accelerate more quickly. I really have few problems with using the crf 250, and if I could easily find one I would send it over to the guy working on the engine.

Thanks for the head up on those Flightmax 15C's. That price is very persuasive. The 25C is only 66. Weird that it jumps to 99 for the 30C. I guess idiots like me usually jump on their website and buy the largest one.