8 kWh Battery pack advice

[texaspyro]

Frankly, the rule makers are REALLY clueless dumbasses. Their rules are down right dangerous. I would recommend bailing out of the competition and going it your own way... properly.

 

[Xrain]

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.

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

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.

All of the EV rules are in Appendix G, That specific rule is Appendix G, FH-2.4: Fusing

I believe we can get a UL listing waver for a RC charger relatively easy, since we wouldn't be modifying it ourselves. Considering all i have really been able to find that is a UL listed charger in my quick search so far is this: http://www.batteryspace.com/smartcharger18afor111vli-ionpolymerrechargeablebatterypack--ullisted.aspx

I think that rule is more they want to know what we are charging with in advance, than they only trust UL listed chargers.

Edit:

Frankly, the rule makers are REALLY clueless dumbasses. Their rules are down right dangerous. I would recommend bailing out of the competition and going it your own way... properly.

The rules may not make all that much sense all the time, but without some sort of competition like this, we might not have been able to do half as much as we have been able to.

 

[adrian_sm]

All of the EV rules are in Appendix G, That specific rule is Appendix G, FH-2.4: Fusing

Yep had a look through all of those rules, and there is nothing in there mandating you to wire your pack up in parallel strings, or any specific manner for that matter.
Ask whoever told you to rewire your pack, which specific rule you are in breach of. There is only a reference to "if you have multiple parallel strings ...." nothing there that says you must have ....

You aren't break any rules as far as I can tell.

 

[dogman dan]

Looks that way to me too. It seems to me, that quote just says wire sizes need to be big enough, if you paralell a lot of cells. Then a fuse on the paralelled string.

Of course, rules can also be intrerpereted by imbeciles..... who can rule your build ineligible.

 

[x88x]

And just the fact that the rules mention something you have to do if you have parallel strings says to me that you are in fact allowed to have paralleled strings.

Good luck with the project, this looks cool.

 

[E-racer]

How I would win an electric snowmobile challenge. Convert this:

 

[John in CR]

Xrain,

I think you've overlooked a key advantage of those nanotech packs. Lower weight is great, but compute the size of your 8kwh nanotech pack and you'll cream in your pants. For the real racing part the more than 50% smaller volume of the nanotechs compared to the dead-ways would give you so much more flexibility in battery placement.

 

[Xrain]

Xrain,

I think you've overlooked a key advantage of those nanotech packs. Lower weight is great, but compute the size of your 8kwh nanotech pack and you'll cream in your pants. For the real racing part the more than 50% smaller volume of the Nanotechs compared to the dead-ways would give you so much more flexibility in battery placement.

Nanotech 45-90C 5000mAh 6s1p 22.2V,
22.2V * 8 = 177.6V
5Ah * 9 = 45 Ah ( 2025A Cont, 4050A burst)
177.6V * 45Ah = 7992 Wh

Nanotech dimensions:
160mm x 50 mm x 52mm = 416000 mm^3 = 0.000416 m^3
72 * .000416 m^3 = .029952 m^3
842g * 72 = 60624g = 60.624kg * 2.2 lb/kg = 133.37 Lbs

Turnigy 20C 5000 mAh 6s1p 22.2V
22.2 * 8 = 177.6V
5Ah * 9 = 45 Ah ( 900A Cont, 1350A Burst)

20C Dimensions:
152mm x 50mm x 51mm = 387600mm^3 = .000388 m^3
72 * .000388 m^3 = 0.027936 m^3
793g * 72 = 57096g = 57.096kg * 2.2 lb/kg = 125.6112 lbs

Comparison: 45C nano-tech vs 20C lipo (45C / 20C)

Weight: 106.2%
Size: 107.2%
Cont. Discharge: 225% Burst: 300%
Price: 189.7%


So overall, the problem is first, we flat out don't have the money to upgrade to a Nano-tech pack. Second, we don't need all that extra discharge power, the 20C's already can put out enough juice to burn out both our motor and controller.

We are already in the process of buying 20C lipo's so the headways aren't even in consideration anymore.

However we have run into a slight problem ordering from hobby-king. Their USA branch for whatever reason does not ship to Alaska (apparently they don't feel we count as living in the US). Their China branch does, but by the time the batteries got here it would be too late, as well as it would cost an ungodly amount to ship it up here.


We got a pretty good offer on these KoKo 25C 5000 mAh 6S batteries. And could get them here in a reasonable amount of time.
http://www.koko-rc.com.cn/product/352065951-200256164/22_2V_5000mAh_25C_YKS_lipo_battery_for_rc_toy_C1.html

Has anyone here heard anything about them?

How I would win an electric snowmobile challenge. Convert this:

Good luck on the draw bar pull!

 

[E-racer]

Wow I'm surprised the US warehouse wont ship to y'all!Excuse my ignorance on the clean snowmobile competition. I live in Georgia and compete in Formula SAE. Wish my school would bank roll a formula hybrid car

 

[John in CR]

Xrain,

I agree that you probably don't need that nanotech's power density, unless you wanted a sure win on the drag and only carry a portion of you pack.

I didn't realize you already were committed to the RC lipo route. It's the lipo to cylindrical lifepo4 volume comparison that I wanted to be sure you appreciated, because that difference is huge, and as you pointed out, it's actually a bit better for the lower power lipo, as is gravimetric energy density.

 

[auraslip]

Bill Dube The owner of the Killacycle heat the 1210 A123 cells to around 70 celsius before each run.. That lower the cell internal resistance a lot and decrease the voltage drop of the pack. These cells have been tested to 250 celsius before they become weak so it’s no worry.

- doctor bass http://endless-sphere.com/forums/viewtopic.php?f=14&t=16775#p246707

Maybe the heat from from the headways IR will help you in the long run

What sort of amp draw are you expecting?

 

[auraslip]

I was thinking more about your pack, and the fact that your buying rc lipo from an alibaba site.

Perhaps more people post about problems than they do about good products, but batteries in general have a pretty high DOA. I'm sure the top of the line nanotech stuff is QC'd properly, but even the 20c hobbyking stuff has seen a few of people complaining about bad cells. The fact that you are buying even CHEAPER cells from an ALIBABA site worries me. I hope you have researched that brand very well! ALSO be sure to look of lipo break in and priming procedures.

As far as bulk charging with HVC protections: I would do bulk charging on each parallel group with the HVC on the bulk charger. Since you can't watch the cells as they charge, I would check all cells to be sure they are bottom balanced before you charge. Then I would set the HVC in the charger much lower than you need it(for example 3.9v instead of 4.2v). This shouldn't effect range much, and will (hopefully) keep the cells from overcharging.

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.

Can you watch the battery charge? You could get some battery medics/battery loggers and be your own single cell HVC. OR check out ggoodrums lvc/hvc boards http://www.tppacks.com/products.asp?cat=26

Bmsbattery sells 2000w chargers for $200 each, and they are voltage tuneble. I don't know if they are UL registered.

 

[Xrain]

I was thinking more about your pack, and the fact that your buying rc lipo from an alibaba site.

Perhaps more people post about problems than they do about good products, but batteries in general have a pretty high DOA. I'm sure the top of the line nanotech stuff is QC'd properly, but even the 20c hobbyking stuff has seen a few of people complaining about bad cells. The fact that you are buying even CHEAPER cells from an ALIBABA site worries me. I hope you have researched that brand very well! ALSO be sure to look of lipo break in and priming procedures.

As far as bulk charging with HVC protections: I would do bulk charging on each parallel group with the HVC on the bulk charger. Since you can't watch the cells as they charge, I would check all cells to be sure they are bottom balanced before you charge. Then I would set the HVC in the charger much lower than you need it(for example 3.9v instead of 4.2v). This shouldn't effect range much, and will (hopefully) keep the cells from overcharging.

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.

Can you watch the battery charge? You could get some battery medics/battery loggers and be your own single cell HVC. OR check out ggoodrums lvc/hvc boards http://www.tppacks.com/products.asp?cat=26

Bmsbattery sells 2000w chargers for $200 each, and they are voltage tunable. I don't know if they are UL registered.

We lucky managed to find an ebay seller located in the US who could sell us Turnigy packs at a reasonable price, and ship them fed-ex who we have a big discount with. So thankfully we wont have to risk the re-branded no-name packs. We are also ordering a few extra so a few dead packs wont be a show stopper.

We will have a pack voltage monitor at least, and possibly a few battery monitors on some of the packs, We just cant open the pack up and monitor every battery.

How the actual pack wiring is going to be done is still up in the air a bit, As I'm considering just building the pack the intelligent way and just do some Cadence Pspice simulations, as well as a few circuit calculations to show why the way I did it was safer. So I'm going to talk with the team-lead today and get everything ironed out. This way we can combine the parallel V-sense leads use a real bms (without requiring 500 channels), and bulk charge the whole pack.

The UL listed requirement is more they just don't want us creating/modifying our own charger, and they want to know what charger we will be using so they can determine if its safe or not. A charger from a EV company who makes several hundred a year probably would have not much problem getting a waver for it.


PS: TailSpin is the best show ever.

 

[x88x]

The UL listed requirement is more they just don't want us creating/modifying our own charge

Ironic that in a competition to create an entire electric vehicle, they don't trust you to make a charger for it..

 

[Xrain]

Well we have our charger situation sorted, and we have 77 5ah 20C 22.2V Turnigy's on the way to us.

I also have a pack wiring design mostly complete.


The main sticking point we have now is a BMS
I've done a bit of searching around and so far I've found:

Elithion Lithiumate BMS for around $900 for everything.
TPpack, for around $200 if I went the easy route and got all the components from them.

I need a 48 Cell Bms, with HVC, at least.

Anyone have any suggestions?

We will be getting the batteries early next week, and I can probibly get some picture of everything up by then.

 

[liveforphysics]

BMS is such such such a terrible idea for a race vehicle....


Don't you just need an HVC/LVC (which can be done with a simple voltage divider and a uController that cuts the controller enable or opens a relay etc) and a way to monitor the paralleled cell groups? Like I had proposed earlier, Chargery BM6's that will monitor each paralleled cell group, and alarm if anything goes wrong (and you can route that alarm output to do anything you like).

 

[auraslip]

Like I had proposed earlier, Chargery BM6's that will monitor each paralleled cell group, and alarm if anything goes wrong (and you can route that alarm output to do anything you like).

Yes! Make this your bms. Route the output of the LVC alarm to a relay that breaks the main packs connection to the controller. Dead simple.

I've been thinking about doing the exact same with the "smart" programmable bms that bmsbattery.com sells. If you do it this way, you can use a bms rated for any amps. Use the BMS just to make or break the relay connection.

 

[Xrain]

BMS is such such such a terrible idea for a race vehicle....


Don't you just need an HVC/LVC (which can be done with a simple voltage divider and a uController that cuts the controller enable or opens a relay etc) and a way to monitor the paralleled cell groups? Like I had proposed earlier, Chargery BM6's that will monitor each paralleled cell group, and alarm if anything goes wrong (and you can route that alarm output to do anything you like).

Like I had proposed earlier, Chargery BM6's that will monitor each paralleled cell group, and alarm if anything goes wrong (and you can route that alarm output to do anything you like).

Yes! Make this your bms. Route the output of the LVC alarm to a relay that breaks the main packs connection to the controller. Dead simple.

I've been thinking about doing the exact same with the "smart" programmable bms that bmsbattery.com sells. If you do it this way, you can use a bms rated for any amps. Use the BMS just to make or break the relay connection.

I think this sounds like an excellent idea! The chargers we are using are actual EV chargers, and have a enable/disable wire, so that makes disabling the pack with the BM6's quite simple.

Rather than Spending $100 on another Contactor, it would be easier just to activate a relay to add a resistor in parallel with our throttle, that way it would significantly cut the sled's power, but we might still be able to move at ~5mph, just in that off chance that the other person managed to go just another 500 feet than where we are :lol: .


I've given some thought to how to keep our pack warm as it has to sit out overnight and do a cold start up. I'm just going to make a simple thermostat using a thermistor and a comparator to toggle a relay when the pack temperature reaches our desired level; then have the heating pads run off our 12V system which isn't constrained by the 8kW pack limit. That way whenever the key is on, the temperature keeping devices are also active. For the cold start all it has to do is move ~150 feet, so it's alright if it does this cold. Then we will have an hour or two to bring our pack up to full temperature for the acceleration event and draw bar pull.

I'm going to try and see if I can find some silicone pad heaters locally, otherwise I'll order some of those Nos bottle warmers Liveforphysics mentioned in another thread, or just some plain silicone heating pads.


I have a Calculus III exam tomorrow I've been studying for so my thermo is kind of shot at the moment, What wattage/how many heating pads do you think we would need to keep our pack warm. The outside temperature will probably be around 20 degrees or more since this is in Michigan. And our pack will be insulated with ~2 inch Blue Construction Foam, So It should retain heat rather well.

 

[John in CR]

What exactly does the cold start entail? Do you have to leave it totally unplugged overnight to reach ambient temp? Is there any prep time before the start? What about charging afterward. I'd consider making use of an electric blanket...low weight, limited temps, UL approved. If it can't be plugged to the wall, but can recharge after the cold start, then I'd consider just running the electric bike off the pack overnight, maybe with a timer and temp sensor cutoff to make sure it doesn't get too toasty. Starting all of the events with the pack a nice temp will pay big dividends.

 

[Xrain]

What exactly does the cold start entail? Do you have to leave it totally unplugged overnight to reach ambient temp? Is there any prep time before the start? What about charging afterward. I'd consider making use of an electric blanket...low weight, limited temps, UL approved. If it can't be plugged to the wall, but can recharge after the cold start, then I'd consider just running the electric bike off the pack overnight, maybe with a timer and temp sensor cutoff to make sure it doesn't get too toasty. Starting all of the events with the pack a nice temp will pay big dividends.

Basically, they let it sit overnight, your not allowed to plug it in. They pull the cover off, turn it on and drive 150 feet.
This part doesn't really concern me as it only has to drive a very short distance, which is doable with cold cells.

I'm planning on using a few 12V flat silicone pad heaters of some sort, and using a simple thermostat circuit to control it.

A few hours after this cold start event, there are the acceleration and draw bar pull events. So after the cold start, the heaters will be left on to heat up the pack to operating temperature.

That way we wont have to carry a gigantic 12V battery pack to keep the heaters on overnight, just one large enough to last during the events.

 

[mushymelon]

Are you allowed to build a your 12v system with a high ah lithium battery.
lots of extra power could be gained. Could recycle some headways perhaps.

 

[auraslip]

Or some hand warmers

 

[John in CR]

Maybe the silicone pads are the same, but electric blankets use low power. I'd keep the pack at temperature all night, recharge after the test, and keep it warm throughout. With the pack insulated, it will be a piece of cake to keep warm, especially since you have an extra 12V supply that doesn't count to your 8kw. Keeping the pack at temp could easily be the difference between winning and losing. Remember LFP said you can damage Lipo if you try to charge them while cold. Lot's of thermal mass + insulated + makes it's own heat in use = easy to keep warm as long as you don't let it get cold. Once cold then you have issue of uneven warming etc. It seems like some type of low temp heat source is perfect for the job, and electric blanket is the first thing that comes to mind for me, but then I never heard of a silicone heating pad. Are they big enough to completely cover the pack? I'd have a few thermostats buried in that pack.

 

[x88x]

Do the rules allow you to leave the vehicle plugged in overnight to charge? If so, you could just have the heating elements run off the charger power if it's plugged in, and the battery if not. That way you could keep the pack warm all night without using any battery capacity.

 

[Xrain]

Well I though I might give you guys a quick update.

We got the batteries in a little over a week ago, and the wiring and connectors a few days after that.

So now 1000+ solder connections + heat-shrinks later,as well as a large amount of polycarbonate bending. We now have a 177.6V nominal 45Ah 20C lipo pack

It has two 75W 12V heaters sandwiched into the pack, and we are using 16BM6's, (8 on-board for LVC, and 8 exterior for HVC during charging.)

They are connected to each parallel string using 9 56 conductor ribbon cables (one for each Parallel string).


We ended up going with the rule's recommended string then parallel arrangement so we would only need 9 fuses, and just used the ribbon cables to avoid having to buy a 509 channel bms, so we can charge the pack one string at a time, and isolate it by disconnecting the PP120 Anderson connector at the negative end of each string.


We did manage to test it a few times, and the results were spectacular. It drove like a bat out of hell

I'm hoping to cat-walk the 500 feet for the acceleration test :lol:


We didn't get to test it nearly as much I hoped we would but, we basically pulled all-nighters from the day we got all the parts in, finished it last morning, took it out played around in-front of the building in the snow, brought it back in, made sure everything was rule compliant, crated it and shipped it out.


Not exactly Ideal but you do what you have to.


Thank you guys for all the info you provided, so long as nothing breaks, it should do well to shake up the EV section of the competition!


I'll be sure to post the results, and some pictures and videos of what happens for all of you. (as well as of the machine itself)


Wish us luck!