8 kWh Battery pack advice

We are currently in the process of creating a electric snowmobile, for a SAE (Society of Automotive Engineers) Zero-emissions competition.

We are limited to 8 kWh pack size.

The current battery pack we are using consists of Headway liPO4 in a 6P setup, with 144V total.
Our motor is a Warp series (7-9 cant remember off the top of my head)
The snowmachine is based off of a 2006 300cc Ski-doo tundra.

The issue is, the way we built our battery pack come to find out is against the rules of the competition.

So we have to tear down our packs and re-assemble them differently in order to make them legal.


But since we already have to tear them down, we are considering other options for batteries since its going to be a lot of work no matter what we do.

We have $4000 via a grant available specifically for buying more batteries, but at this point we haven't found any better alternatives as of yet that fits in our price range.

Do any of your have some advice for some new batteries?

- The battery will only really be used a few times for the competition, and a couple show-off events so life isn't that important of an issue, but it would be nice if it lasted a relatively long time. Since we hope to reuse the pack in future competitions
- High C rate is very importance since two of the events in the competition are an acceleration event, as well as a Draw Bar Pull.
- Light weight is probably the most important of all since the assembled machine as is weighs 550lbs, and we are suffering from it flattening out our suspension.
- We have a actual battery welder to connect our batteries together, as well as plenty of nickel strips.
- We are willing to work with just about any battery technology, but we do have to pass a safety inspection, so if we can avoid liPo's it would make everything easier.

We are on a short time limit since we have to ship the machine down to the competition in the first week of march, so if the packs came pre-assembled we wouldn't mind.
I was quite excited today, since we took it out on its maiden voyage today through some powder and it worked amazingly, but sad because we have to tear apart our batteries.

I am interested in any advice you guys have on the matter, since this place seems to be teeming with electric vehicle knowledge.
Feel free to ask any questions.

(PS: If you feel like sponsoring us a battery pack at a discount, we are a Tax deductible non-profit organization 8) )

Why do you have to pull apart your pack? And what does the safety inspection cover?

From what you are asking LiPo would be the best bet. It will give you the best bang for buck.

For example for $3900 you could get about 80 of these packs, that will give you about 8kwh, weighing in at 64kg, or 140 pounds.
And it will deliver as many amps as your controller is likely to handle.
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=16207

This thread might be of interest.
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=23832

Good luck.
- Adrian

In a performance application, if you don't use LiPo, you're just wasting time.

Nano-techs are what you want.

Sell the dead-ways pack to make up the price difference.

There were a few reasons, first, they didn't like how our pack was arranged, it is currently 4 separate packs, each at 36V, and inside each pack there are six batteries in parallel with 11 of these in series.

What they want is for each pack to be at 144V, with no parallel batteries within each pack. Then just string the packs in parallel.
We originally had a 12.4 kWh pack, but they changed the rules to 8 kWh max. So our target voltage is ~180V

The other reason is, they want fusible links to be wired in-between each individual battery, we were just counting on the resistivity of the nickel, and the non-explosiveness of the headway's, but that's not going to cut it.

The full rules for the competition, are here: http://students.sae.org/competitions/snowmobile/rules/rules.pdf (it's 73 pages)


I have briefly glanced at those Turnigy's before, but on closer inspection they look pretty interesting. I was slightly worried about the voltage increase we would put them through, but seeming how much higher voltage EVDragRacer is using, it doesn't look like it will be a problem.


I just need to go through the rules to make sure those can be legal.

Thanks for the input Adrian, if anything else pops into your head let me know!

I'm not you sure you want to be sitting on top of 144 - 180V DC with the possibility of melting snow in the equation. This is one application where I'd want to keep the system voltage at ~50V max and gear to that.

liveforphysics said:

In a performance application, if you don't use LiPo, you're just wasting time.
Nano-techs are what you want.
Sell the dead-ways pack to make up the price difference.

Click to expand...

hahaah, i knew that was coming!
If you have a short operating range and want to show off extreme power, i agree with him. The nano-techs have extremely low voltage dive and sustain their high voltage for a very long time, similar to A123's... maybe better !

If you need to run for a long time, i would say 25-30c hobbyking lipo is a more economical choice as it cost half the price as nano-tech.

Either of these would be a huge improvement over headways. They would both be lighter, and the headways only output 5c whereas these bad boys output 20c-90c+.

A fusible link between each battery sounds like the tabs in lipos to me.

Not wanting you to paralell the batteries is just stupid. Somebody has a degree, but no commonsense.

Nano's arent going to thermal runaway discharging. But for charging, you do want to paralell as many as possible first. Tell that dummy it makes the charging safer, if the pack is bigger and charges at lower voltage. Charging is when safety is important.

144v charger is going to be safer? I sorta doubt that.

neptronix said:

liveforphysics said:

In a performance application, if you don't use LiPo, you're just wasting time.
Nano-techs are what you want.
Sell the dead-ways pack to make up the price difference.

Click to expand...

hahaah, i knew that was coming!
If you have a short operating range and want to show off extreme power, i agree with him. The nano-techs have extremely low voltage dive and sustain their high voltage for a very long time, similar to A123's... maybe better !

If you need to run for a long time, i would say 25-30c hobbyking lipo is a more economical choice as it cost half the price as nano-tech.

Either of these would be a huge improvement over headways. They would both be lighter, and the headways only output 5c whereas these bad boys output 20c-90c+.

Click to expand...

Nano-Tech 5Ah cells have an Ri of 0.8mOhm, or a specific Ri of 4mOhm/Ah.

A123's best performance cells, the old M1's (which they no longer make), have an Ri of 7mOhm per 2.3Ah cell, or a specific Ri of 16.1mOhm/Ah.


Therefore, and equal sized pack made from Nano-Tech's has 1/4 the voltage drop, and for a given acceptable amount of heating to be produced, can discharge at 4x the current level of the best A123's.

They also have drasticly higher energy density and specific energy.

Nothing you do is going to effect them under discharge. Hit them with a 200C load, they shrug it off. Hit them at 100C continuously (which is way higher than they are rated for), and they laugh it off.

liveforphysics said:

Nano-Tech 5Ah cells have an Ri of 0.8mOhm, or a specific Ri of 4mOhm/Ah.

A123's best performance cells, the old M1's (which they no longer make), have an Ri of 7mOhm per 2.3Ah cell, or a specific Ri of 16.1mOhm/Ah.

Click to expand...

Holy smokes. I now understand why the interest in A123 has plummeted. I wonder who will be first to use these in a competitive racing application. I for one am tired of seeing slow electric drag cars and races.

About the LiPo charging, during the competition we will only have around ~7 hours to charge our pack, granted it most likely wont be depleted, but I don't really want to rely on assumptions. Id like to go with the iCharger 1010B+'s, but we can only afford a few of them, and with 72 packs, that would take a lot of time to charge each of them.

Are there any chargers our there that you guys would recommend that could charge multiple packs at once and balance them relatively accurately, so we could easily charge our packs in a reasonable amount of time?

Also, the rules mandate that we have a BMS for our batteries, and with 72 packs of 6s that would be around 504 wires to connect. We are currently using Mini-BMS's and these do scale well, but that would be an awfully large bms stack. Is there anything out there that could do the job in a more efficient manner?

flip_normal said:

I'm not you sure you want to be sitting on top of 144 - 180V DC with the possibility of melting snow in the equation. This is one application where I'd want to keep the system voltage at ~50V max and gear to that.

Click to expand...

In the rules themselves, if the batteries aren't "rain approved" aka, your packs aren't well sealed you wont be allowed to compete. Besides, the batteries are far enough removed from the rider, that if water did cause the pack to short, you could easily jump off the machine before all goes to hell and it bursts into flames. In addition all of the packs are surrounded with fire resistant material. As well as placed in a insulated non conducting battery box.

Oh man, if you can't connect the BMS system in parallel, forget it. All those little 5ah serial packs will explode your price and complexity factor.

This competition sounds like a pain in the ass.

A hyperion net duo will do 14s at once, so that can handle two 6s packs at once, cutting down the complexity of charging. It also doesn't cost much more than the iCharger 1010b+ and can spit out more watts as well.

If you get nano-tech batteries, those things can charge quick as hell. Regular lipo likes to be charged at 1-2c. Liveforphysics probably knows what insane rate the nano techs can be charged at..

neptronix said:

Oh man, if you can't connect the BMS system in parallel, forget it. All those little 5ah serial packs will explode your price and complexity factor.

This competition sounds like a pain in the ass.

A hyperion net duo will do 14s at once, so that can handle two 6s packs at once, cutting down the complexity of charging. It also doesn't cost much more than the iCharger 1010b+ and can spit out more watts as well.

If you get nano-tech batteries, those things can charge quick as hell. Regular lipo likes to be charged at 1-2c. Liveforphysics probably knows what insane rate the nano techs can be charged at..

Click to expand...

Alright, I just read through the section of the rules about this issue a half a dozen more times. And it seems we only need a monitoring system when we are Charging the batteries. Which is good, since the chargers (hopefully) will do that for us properly.

But one of the events is a range event, where you drive the machine until it cant drive anymore. I know completely discharging a Li-Po is abhorrent to their further health. Granted we would only completely discharge the pack 4-5 times over the next several years, so if all it does is reduce the batteries cycle life, that wouldn't be that huge of an issue.

But it still would be nice to not put the packs in danger, what would be a relatively simple way to keep from flat lining a pack of this size? With our old Headway's we weren't all that worried about the issue, since they are rather robust, and could handle a complete discharge without too many ill effects.

Dont worry, discharging the battery below normal wont hurt it as much as everyone makes out, 5 times in a few years and you will still be golden as most of the time when you are using the batteries you would be unlikly to drain them much and be charging them straight after.

To make things alot simpler with building your pack you could get some really large cells http://endless-sphere.com/forums/viewtopic.php?f=14&t=23029. They are not at the same quality of Turnigy, but much larger and cheaper I have discharged these afew times below 3V, but so far they have not shown any problems with that. Their actual ratings of the batteries is probably closer to 16-17Ah and 10C. If you have the money certainly go for the Nano batteries....they are insane.

Holy chit.

A BMS for a race vehicle is like the essence of retarded fail.

Charging a pack that size with RC chargers almost rivals it in pushing the stupidity boundaries.

Get the 45-90c cells, and you will be capable of a 6min charge time if they have an appropriate generator.

To charge, use 48v 3kw. HP server supplies. They go for $31 shipped as surplus. Get 8 of them if they are providing a gen that can chug a 25kw load.


When its time for the range test, charge to 4.35v/cell. You can stuff over an extra amp-hour into each 5ah cell that way, zero risk of venting if you only charge to 4.35v, but don't get greedy and go higher.

To monitor, combine all paralleled balance tap leads, and monitor each 6s group with a cell level monitor like a chargery bm6. This way all monitoring for a 200v pack costs under $100. And it gives you programmable cell imbalance, high voltage, and low voltage alarms.

Do NOT charge cold cells (causes damage, (fire risk from metallic lithium formation). Do not run cold cells (no risks, but performance sucks badly).


Lastly, the key to winning these type of contests is all about who gets the most real-world testing and practice.

Xrain, what is the max amp draw / voltage of your controller? A smallish booster pack of 90C nanotech's may win your drag race without breaking your budget. Do the math on capacity, buy enough to win.

Then, do a little cost analysis on cells to find the best Energy Density / Dollar. If you're over-budget, that's your optimal solution. But if you still have budget to spend, then go for cells with the best Energy Density / Weight (as long as they can provide the required continuous discharge amps).

A word of caution, since you're are facing some time pressure: make sure to break in your batteries BEFORE the competition. This is at least as important as the battery chemistry you choose. Probably 6-12 cycles for most battery types should be sufficient. Otherwise, you'll get poor pack capacity, balance problems, and possible cell damage.

Last question. Have you made some provision to keep the batteries warm during the event? To optimize performance, they probably should to be kept between 20 to 25 Celsius. This trick could just win you the range contest...

Good luck, keep us posted! Some pic's be nice, too 8)

liveforphysics said:

Holy chit.

A BMS for a race vehicle is like the essence of retarded fail.

Charging a pack that size with RC chargers almost rivals it in pushing the stupidity boundaries.

Get the 45-90c cells, and you will be capable of a 6min charge time if they have an appropriate generator.

To charge, use 48v 3kw. HP server supplies. They go for $31 shipped as surplus. Get 8 of them if they are providing a gen that can chug a 25kw load.


When its time for the range test, charge to 4.35v/cell. You can stuff over an extra amp-hour into each 5ah cell that way, zero risk of venting if you only charge to 4.35v, but don't get greedy and go higher.

To monitor, combine all paralleled balance tap leads, and monitor each 6s group with a cell level monitor like a chargery bm6. This way all monitoring for a 200v pack costs under $100. And it gives you programmable cell imbalance, high voltage, and low voltage alarms.

Do NOT charge cold cells (causes damage, (fire risk from metallic lithium formation). Do not run cold cells (no risks, but performance sucks badly).


Lastly, the key to winning these type of contests is all about who gets the most real-world testing and practice.

Click to expand...

The only reason we were considering a bms was because of how we interpreted the rules, come to find out we do not need one during discharge.

Using a RC pack in any other situation would indeed be foolish, as spending most of your day painstakingly charging each cell is a PiTA as well as it takes forever, and you couldn't charge your pack in time for any subsequent races.

However for this competition in my mind it makes since, as we have a 7 hour+ opportunity to charge on our battery. As well as the rules requiring us to have a monitoring system during the charge cycle, I really do not think they would allow us to bulk charge it and check it by hand. At foolish as this might seem on their part they make the rules.

Since integrating a BMS into the pack would be a huge PiTA and something we deeply want to avoid, Taking the time to several RC chargers seems to make the most sense.

Another part of it, is they measure the amount of energy we put into our packs, and as far as I know high quality RC chargers would give us the highest electrical efficiency. (certainty not the most time efficient)


I may be wrong but using RC chargers seems to be the best option in this case.

But I am curious about using the HP server power-supplies at any rate, How exactly would you wire them up and charge on your battery with them?


As for the 45-90C nano's, sadly the nano's seem to be out of our budget, as we only have $4000 to make this pack and the cheapest combination I have been able to make that would be acceptable is more in the $5600-6000 range
So as far as I know I'd like to go with the 6S 20c standard li-po's as they fall within our budget.
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=9176

Would the slight overcharge to 4.35 still work with these batteries?


If we could wire our batteries like any other sane person, I would go with parallel bus's and combine the V-sense wires and bulk charge it in a heart beat. But instead we are forced wire it like this.



Because of that, we would either just have to take the V-sense off of a few batteries and hope that they are representative of the rest of the pack, which would be a not so good idea.

Or have monitor every single cell at once. (aka install a bms)

Instead, my plan was to Disconnect all of the wires from the ground bus, then have a enclosed panel with all the v-sense wires and power connectors and manually charge all of them.

Would it be a lot of work to make this panel... Most likely, but its a lot more practical than installing a bms.



So if anyone has any alternative ideas, let me know.


EDIT:

Holocene said:

Xrain, what is the max amp draw / voltage of your controller? A smallish booster pack of 90C nanotech's may win your drag race without breaking your budget. Do the math on capacity, buy enough to win.

Then, do a little cost analysis on cells to find the best Energy Density / Dollar. If you're over-budget, that's your optimal solution. But if you still have budget to spend, then go for cells with the best Energy Density / Weight (as long as they can provide the required continuous discharge amps).

A word of caution, since you're are facing some time pressure: make sure to break in your batteries BEFORE the competition. This is at least as important as the battery chemistry you choose. Probably 6-12 cycles for most battery types should be sufficient. Otherwise, you'll get poor pack capacity, balance problems, and possible cell damage.

Last question. Have you made some provision to keep the batteries warm during the event? To optimize performance, they probably should to be kept between 20 to 25 Celsius. This trick could just win you the range contest...

Good luck, keep us posted! Some pic's be nice, too 8)

Click to expand...

I'd need to check again to see our controller specifications,

The pack I have in mind using the standard Turnigy Lipo's, has a capacity of 7983 Wh, which is a hair below the 8kWh limit. As well as these seem to be in our budget (Although they have to be shipped up to where we are in Alaska which is always expensive). The range event is a large part of the competition, with the machine limited to travelling at 20 mph, so I don't think a small high C pack would be in our best interest. As well as just using the 20C batteries is more than enough to fry our motor (Modified Warp7) :lol:.

The name of the game in this competition is efficiency.

Breaking the batteries in is a good point, I'll do my best in fitting it in the schedule.

As for keeping the batteries warm, the only thing I could think of to keep everything warm would be integrating a battery blanket into the pack and monitoring it with a thermometer before hand. And trusting the insulated pack to keep the heat in during the Event. I'm not sure it would be worth running a heater off the pack while we are running, but I suppose that will depend on how much the pack cools down.


I'll be sure to add some pictures and video's when I get a chance. :wink:

Is there a set amount of energy you're allowed to put INTO the battery pack?

A benefit to higher 'C' rate packs is that their lower internal resistance leads itself to holding higher voltage under load. That higher voltage x your current means more power and more energy. Another way to look at it is that lower internal resistance means less energy is lost as heat while you discharge.

The last thing you want to do in a range competition is throw battery energy away as heat.

Shame you're forced to series packs first and then parallel. Collecting all the current at the end of those long series strings sucks compared to paralleling first.

voicecoils said:

Is there a set amount of energy you're allowed to put INTO the battery pack?

A benefit to higher 'C' rate packs is that their lower internal resistance leads itself to holding higher voltage under load. That higher voltage x your current means more power and more energy. Another way to look at it is that lower internal resistance means less energy is lost as heat while you discharge.

The last thing you want to do in a range competition is throw battery energy away as heat.

Shame you're forced to series packs first and then parallel. Collecting all the current at the end of those long series strings sucks compared to paralleling first.

Click to expand...

No we are allowed to use as much juice as we want, they just keep track of it to calculate our overall efficiency, and award points based on it.

The whole competition is points based, some parts are worth more points than others, but every point helps.

I hear yah on the high C rates, but what comes with those C rates is a higher cost, and considering we fight tooth and nail for every dollar we have available, at the current prices, I think 20C would be fine. Also, it tends to be rather cold while driving a snowmobile, so a little extra heat from the packs might help a bit...


The idea behind the pack arrangements in their eyes, is that if any battery in a string shorts, or has an issue, it completely removes it from the pack when it burns a fuse. Leaving the rest of the batteries to go on their merry way. It just makes wiring them a nightmare.

Unfortunately their method doesn't just take out the bad cell, it takes the entire series string out.

If they would let you do the sensible, normal thing, of paralleling first and then series, you'd only lose one cell when it blew it's link.

Instead, you lose a huge percentage of your pack just because of a single point failure.

Plus, now the load on the rest of the pack just got that much higher, so if any of the rest of the cells were within that percentage of popping their links, they will, and that will probably take your whole pack out pop pop pop pop like a bunch of series wired holiday lights.

Their way is LESS safe to use than the normal way.

With lipo, you don't need to balance charge it every time. If you leave a voltage buffer at the top and bottom of the pack, you will not have issues of cell balance until many dozens of charges.

I bulk charge to 4.15v and stop discharging at 3.5v as the cells start heading off a cliff to 3.0v and below by then.

This means i have about 750mAH left in the pack, which is a standard margin of variance in mAH for the zippy / turnigy cells at the bottom of the charge. So then, i am not worried about overdischarging anything.

When i bulk charge, i have 0.1v of headroom at the top, which is about um.. 250mah tops.

So i effectively have 4AH in each pack, and i can get over 4ah by balance charging when i need to. My guess is that you won't need to do balance charging, you can probably bulk charge the 100 something volt strings at a time and check them with a cell meter of some sort just to be safe.



This is what a year old zippy 20c 5AH battery discharge curve looks like. Notice how the last 500mAH is um.. mostly for show?

neptronix said:

With lipo, you don't need to balance charge it every time. If you leave a voltage buffer at the top and bottom of the pack, you will not have issues of cell balance until many dozens of charges.

I bulk charge to 4.15v and stop discharging at 3.5v as the cells start heading off a cliff to 3.0v and below by then.

This means i have about 750mAH left in the pack, which is a standard margin of variance in mAH for the zippy / turnigy cells at the bottom of the charge. So then, i am not worried about overdischarging anything.

When i bulk charge, i have 0.1v of headroom at the top, which is about um.. 250mah tops.

So i effectively have 4AH in each pack, and i can get over 4ah by balance charging when i need to. My guess is that you won't need to do balance charging, you can probably bulk charge the 100 something volt strings at a time and check them with a cell meter of some sort just to be safe.

*img*

This is what a year old zippy 20c 5AH battery discharge curve looks like. Notice how the last 500mAH is um.. mostly for show?

Click to expand...

I don't think the safety inspectors will allow us to be manually checking the pack voltages, but your idea would certainty be wonderful if we could get away with it. This does give me an idea tho, we could take a BMS, and wire 8 JST-HX connectors to it, so then we could monitor all the cells in a string while its charging.

I wish we could reuse our existing BMS for this purpose, but the MiniBMS is LiFePO4 specific, so I'm not sure it will play happy with a Li-Po, so this might involve us purchasing a new BMS if we went this route.

But thanks a lot for the idea, and the info!

Xrain said:

I don't think the safety inspectors will allow us to be manually checking the pack voltages,

Click to expand...

Who are these people ?
They allow you to build the device ,.. but dont allow you to monitor it during operation, to avoid risk ! WTF ?
together with their dictate on pack design,. they are verging on being irresponsible !

There are two options here. Your rules were written by imbeciles, or you are again making incorrect interpretations of them.

When cells have a parallel connection at the cell level, that behaves electrically as a single cell, can be modeled as a single cell, monitored as a single cell, and depending on your definitions reference (like the UN 38.3 lithium pack shipping designations) counts as a cell/cell-group/module, not a battery.

But, before anyone spends any money or before a single part is purchased, you ALWAYS hash out the rules and ask the questions to the sanctioning body so you understand WTF you are building. Fixing this most critical first step that you've evidently skipped should be mission critical priority #1. No other work should be done or even discussed towards designing to win an event until you understand the boundaries/rules of event you are entering...


Forget RC chargers exist. Also, it's impossible for them to match efficiency of bulk charging with a server supply, as server supplies alone are the most efficient type of DC/DC converter, and your RC charger inherently requires being fed from a DC/DC converter before it has it's own inefficiencies enter the picture. It's also just an absurdity. You do not want to be doing any balancing at the event. You need to cycle and abuse the piss out of the pack well before the event, and if you find weak cells that would need balancing, fix the pack at that point rather than trying to band-aid a BMS into covering up a weak cell. When at the event, start with your known-balanced pack with known-good cells, and batch charge it as a unit.

This is an example of a logical charger:

http://www.serversupply.com/POWER%20SUPPLY/SERVER%20POWER%20SUPPLY/3000WATT%20REDUNDANT/HP-COMPAQ/253232-001.htm


You can adjust the regulated voltage. It naturally behaves as a CC/CV charger. It needs no cut-offs or anything, you just adjust the end voltage to match your pack HVC needs (there are a number of guides for how to adjust the pots inside, what pins to solder to activate them as soon as plugged-in etc). It can handle some crazy input voltage range like 90VAC/VDC up to 300VAC/VDC.


I understand funding is limited. I also understand you have ~8kwhr of headways and some BMS. That should easily convert into the cost difference to get Nano-Tech packs, at least get the lower C-rate nano-tech packs if you can't afford the 45-90C packs.



When charging, and/or before charging is permitted to enable if you're just starting out with a cold machine, you must have the pack warm, or even hot. Thin high power resistive heating pads assembled into layers between the packs are critical. Use the huge volume difference between your old headways pack and your LiPo pack as space for as much thermal insulation as you can manage. During the run itself, your cells will not be making meaningful heating at all if headways would have been able to handle this current load. Keeping it not just warm, but HOT during the long-range test could easily mean another 20% useful energy from the pack.



And above all these things, (except finding out the rules of the contest you are entering...), you need to do real-world test runs. Lots of real-world test runs. Getting a few real-world test runs will be more important than having the best battery design or the best motor or whatever. At the racetrack, money can only buy the best parts, but the guy who wins is often the guy who has just beaten and abused his system harder and more times than the rest of the field, so he all ready found and addressed his inevitable weak-points in the systems BEFORE race day.

Basically, it's the first option.

This is our third year in the competition, but they change the rules each year. Our first year we came to the competition every other team had small little packs anywhere from 2kWh to 7 kwh, since a large part of the competition was a range event, we decided to stuff as many batteries as we could in a snowmachine, and came with a 13kwh pack. We of course won the range event, but we used cheap gigantic lifepo4's, and so the sled weighed around 700 lbs so it was tippy, and not all that fun to ride as there was only 1 inch of foam on the seat.

This year, is a massive improvement with just the headway's installed coming in at 550 pounds. With even the standard Li-Po's installed that would put us down to ~470 pounds, which is amazing.

It would be wonderful to get my hands on any Nano-tech cell.
The problem is, we can't sell the headways. We got the money for these batteries through a equipment request Grant. Another project had also filed a request for a battery pack for their project. Since funding was tight they gave us $4000 with the understanding that we would split it to get battery packs for both projects.

Since they don't have very high current draw requirements, we offered to give them one of our headway packs, which would be a huge improvement over the lead-acid batteries they were using. In addition to this, the only way we were able to get the money we have, is by working on numerous other EV projects, and we just shuffle some of the equipment around to make everything work, so the rest of the headways and BMS's would go to the other EV projects.

I have no doubt that the nano-tech batteries are far superior to standard Li-Po's in every way, but there just isn't enough benefit in terms of how this competition is run, in addition.


I would absolutly love to go with a server power supply and just bulk charge the whole pack. However there are two different rules that prevent us from doing that.

1. Our charger has to be UL certified. And I'm not sure that we would be able to get a waver in time for a server power supply that we modified ourselves, if at all.

All chargers must be UL (Underwriters Laboratories) listed. Any waivers of this requirement require approval in advance, based on documentation of the safe design and construction of the system, including galvanic isolation between the input and output of the charger.

Click to expand...

2. We must have a Battery Monitoring system during the charge cycle

FH-2.5.1 Battery Monitoring
A battery monitoring system appropriate for the battery type is required. The battery monitoring system must monitor the battery to prevent overcharging.
Depending on the battery type, thermal monitoring may also be required.

Click to expand...

In addition we aren't allowed to physically access the pack during charging.

Provisions for charging must follow the same rules as other high-voltage wiring: no exposed connections, proper strain relief, etc. The battery enclosure must remain closed during charging.

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The rule that had been giving me some confusion is this one.

If multiple parallel strings of batteries are used, then each string must be individually fused. If individual fuses are used this will provide a total fusing equal to the number of fuses multiplied by the fuses rating. Any wires conducting the entire pack current must be appropriately sized to this total fusing or an additional fuse used to protect the wiring.

Multiple parallel fuses in a single string are not permitted.

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We had originally arranged our headway pack like any other sane person would, but we were told that we wern't allowed to arrange them this way.


I have just as much hatred towards some of these rules as any of you, for example, the 8 kWh rule completely baffles me, It seems like they felt we were being unfair to everyone else with little 2-6 kWh packs, somehow when we came with a 13 kWh pack, and installed that rule to prevent it.

They seem to want us to focus on making the drive train as efficient as possible, but that just doesn't make sense to me as motor technology hasn't really changed significantly in 100 years, while battery technology changes every 6 months. And all of the limits to widespread EV adoption is related to battery limitations.



This is why I wanted to go with RC chargers, they can act as our BMS since they constantly monitor the cells for overcharging. It has very little to do with keeping the cells balanced, or because do something as tedious as charging 72 batteries. This is just flat out the only way I can think of to do this, without creating a 509 cell BMS.

If anyone can think of a simpler way that is complaint with their rules of automatic overcharge protection combined with the single parallel string fused connection. Let me know.



As for extensive real world testing, I know this is extremely important, around 1/3rd of the people who show up to this competition don't even get to compete in the driving events, since they didn't test their machine beforehand and it blew up when they tried it.
Heck we made that mistake last year and a bad reversing relay blew up our motor and controller.
I also have done circle track racing for the past 8 years of my life, and I can't count how many races I won simply because I kept my car better maintained, so it stayed strong while the other guy's broke down.

Xrain said:

If multiple parallel strings of batteries are used, then each string must be individually fused. If individual fuses are used this will provide a total fusing equal to the number of fuses multiplied by the fuses rating. Any wires conducting the entire pack current must be appropriately sized to this total fusing or an additional fuse used to protect the wiring.

Multiple parallel fuses in a single string are not permitted.

Click to expand...

We had originally arranged our headway pack like any other sane person would, but we were told that we wern't allowed to arrange them this way.

Click to expand...

I would ask for clarification on which rule stops you from wiring the packs in the way you would like, because I can't find it in the rules you linked to. http://students.sae.org/competitions/snowmobile/rules/rules.pdf
I didn't see anything mandating that you use parallel strings. Only an IF you do, fuse them this way.

I think the UL listed charger will be the rule that annoys you the most. As all the RC style chargers I know of will not be UL listed, so this may force your hand in how you charge the packs. You may end up just having to get whatever UL listed charger you can find then having some non-UL listed monitoring, like from here, which should be able to do your HVC & LVC.

Good luck.