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Old 03-06-2012, 07:50 PM   #11 (permalink)
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Note: edges will be rounded, this is not the most accurate model of a car ever.
This is what I think would be the most aerodynamic choice, but unfortunately it would severely limit space for batteries to go.





I know it isn't the ideal nose, as it isn't blunt at the front. But my understanding is that a blunted nose works by increasing the pressure so that the flow adheres to the tail better. In this case there isn't a tail behind it, but rather the windshield, so I don't think that blunting it would help aerodynamically, but it would certainly give more room.

Next set of pictures will be of a nose design with more room, but still fairly aerodynamic.

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Old 03-06-2012, 08:05 PM   #12 (permalink)
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Here are a few nose designs:

Squared




Rounded




And a hybrid style




I think that the hybrid design should offer a significant drag reduction, but still allow room for batteries.
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Old 03-07-2012, 03:26 AM   #13 (permalink)
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I just realized I haven't posted anything about a charger. I don't need anything super quick and I wouldn't want to charge the batteries too quickly or overcharge them. Does anybody have any recommendations? Something that I could plug into a normal outlet would be great. That should be about 1500 watts or so, which is more than quick enough for what I'd be using it for.

Also I've updated my hybrid nose design. This one goes up slightly over the windshield wipers, and is a little bit more blunt at the front.


Note: those sharp edges will be rounded, it's just very difficult to model this using sketch up.

Now looking at this an idea occurred to me. It's kinda along the lines of an aerodynamic top for it(I know this idea has been talked about previously and sort of dismissed, but hear me out) but in this case it may actually lead to a smaller wake area. The template tapers slowly at first, but then later speeds up. If it's just a few degrees for the first part I could build an aerodynamic top that is just slightly taller than the existing roof, but then taper it down, and the accelerating taper coming of the back end would leave a smaller wake than the current Kammback design. I'm not sure how this could affect CdA and I guess I could test it eventually, but for now it's just a though experiment and I'd like to hear some theories as to what the effect would be.

This is a picture of the current Kammback with the template overlayed. I think it matches fairly well.


This is a picture of the current Kammback with the template overlayed as if the windshield were slightly higher and the taper started closer to the front of the car. As you can see, the template in this case would go below my current Kammback.


This last image is what the car would look like with a streamlined top according to the template.


Now to me this seems like it would at least reduce Cd, but I'm not sure about CdA. Also I'm not sure how I would get the hatch to work properly in this configuration if there even is any aerodynamic benefit. I just thought it was an interesting thought experiment, and I would like to share it with everybody.

Note: The aerodynamic top adds 1.75-2inches to the height of the car, but the tail comes down to 3.75-4 inches below it's position at the other Kammback. This means the top would add about 2% to the frontal area which means if drag is the only concern it would have to have a Cd that is an additional 2% less than that of the original Kammback. You could possibly get some more headroom if you re-did the interior a bit but you would also have slightly less glass out the back to see through, so there are trade offs either way.

Now that 2% less isn't just two percent, if the normal Kammback nets a 6% gain, then 2% is 1/3, meaning the new Kammback must be at least 1/3 more effective. The original Kammback cuts about 2in off the height of the wake area, where as the new one cuts 6inches off the height of the wake area. If it's all about the amount of wake area then the new Kammback should give three times the Cd reduction of the original one. Now there are other factors, but that's promising. If the frontal area actually increases by about 2% and the Cd is reduced by about 18% (6%*3) that would give a total new CdA which is 84% of no Kammback (1.02*0.82=0.8364) and about 89-90% of the other Kammback.
The front part coming off the windshield may help alleviate any small separation that occurs at the top of the roof, along with providing smoother airflow from which to streamline which could possibly net a small drag reduction.
*warning all of this information is deduced through logic based off of numbers which may or may not apply to this or your situation. Your mileage may vary*

Please let me know what you think!

Last edited by yugomodder; 03-07-2012 at 03:29 AM.. Reason: Correct pictures this time.
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Old 03-07-2012, 08:02 AM   #14 (permalink)
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I'd completely forget aeromods for now and work with what you got to get it running.

You say you would like 25-30 miles of range. I'm going to tell you that is going to be really hard to do with lead batteries. You've been calculating your power draw at cruising speed. Unfortunately, thats not a very realistic way of measuring power usage. If you look at the other Yugo's page, it says he is using 440Wh/mile. I'd use this, or even 500 Wh/mile (which is what I used for calculating range on a possible conversion I was thinking about) to add some safety buffer. This means for you to go 30 miles, you're going to need 15 kWh of usable battery capacity. With lead acid, to keep DOD to 50% this means you'll need a 30 kWh pack. 30kWh of lead is going to be incredibly heavy. With lithium @ 70% DOD you'd need 21.5 kWh. With lithium @ 80% DOD you would need 18.8 kWh.

I also think you should consider something higher than 72V, especially if you're talking about going 45 mph. I've driven in a 72V EV before and getting to 45 mph is going to take quite a while, and pull a lot of amps out of the batteries. More amps means larger wires which costs more. More amps also means more losses to the peukert effect which means you get less capacity from your batteries (which means even more batteries). If you doubled your voltage to 144V, you would only have to pull half the amps to accelerate just as fast. This is easier on your batteries (lower C ratings) and allows thinner cable to be used.
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Old 03-07-2012, 08:50 AM   #15 (permalink)
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500 watt hours per mile is in the same range as a pickup truck uses, so while your car is pretty brick shaped I don't see that poor of range, but do you have any idea what kind of gas mileage it got as a gasoline car?
Most EV's that are based off small cars get around 300 watt hours per mile, but I do agree that sizing the battery bank size and the voltage as if you are going to need more is always a good idea.
I have driven a 48v electric car that could go 45mph but it drew over 600 amps and on a flat road took a while, 96v would be about as low of voltage as I would bother going with and the smallest, highest voltage lead acid traction batteries that I've seen that I would trust in an EV are 8v golf cart batteries, they tend to be the same size as the 6v golf cart battery that are also a good, common choice.
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Old 03-07-2012, 09:32 AM   #16 (permalink)
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I like to use 500Wh/mile as a worst case to ensure I have enough capacity. Situations like cold days and needing electric heat, or are driving through rain and using wipers and headlights, or driving through bad road conditions like slush and snow. It all increases power used. Perhaps in Mississippi its not as big of a deal since they don't have the harshness of Wisconsin winters.
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Old 03-07-2012, 11:32 AM   #17 (permalink)
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First, thanks for the replies!

Just to clarify the plan at the moment is:
1. Obtain and install new controller and batteries
2. Test to see if motor is up to the job, add cooling or replace with larger motor if it isn't
3. Find some sort of instrumentation to measure the draw on the battery pack at any given point.
4. Do A-B-A testing on modifications to try to lighten the load on the battery pack

The only reason for bringing up the aeromods at this point is just that I was thinking about them and figured I'd share. Maybe I should have posted that in the aerodynamics section.

I'll make sure to build the system to the max voltage the motor can take, if it's still too weak I'll rewire the batteries to the voltage of the new motor.


Tracking down mpg information was harder than expected. I've seen numbers from 30-45mpg. Probably a lot of this is due to variation in driving technique.

Looking at Greg Coleman's page, the wh/mi numbers seem off. He reports having "6 Deka MK Power M24SLDGFT 12 Volt GEL 73.4AH, 12.00 Volt, Lead-Acid, Gel" 6*12V*73.4Ah is 5,284wh. So even if he used 100% of the capacity to get 15 miles that's still just 350wh/mi not the 440wh/mi there or 500wh/mi. Several of the wh/mi figures seem to be off on that site. I saw a Honda Crx that said it used 18,000wh/mi (Michael Brooks' 1986 Honda CRX). This was most likely the impulse reading for watts at speed considering he has a 33mi range. At the same time there are other crx's that report 240wh/mi (Allen Grvoer 's 1986 Honda CRX) and 70miles range using lead acid (Victor Tikhonov's 1991 Honda CRX). Now the Yugo isn't as aerodynamic as a Honda crx, but it sure is more than a pick up truck.

The calculations I did before included cruising speed, as well as energy to get up to speed as if it were completely wasted at a stop (no cruising), which seems overly harsh to me. That would be as if somebody accelerated full out to speed then maintained that until just before each stop and slammed on friction brakes, wasting all the kinetic energy. On the other hand the 99mile range would be as if you could start out cruising with no slopes, crosswind, wipers, lights, etc. I did try to also take a margin of error on all my figures for size, weight, Cd, motor efficiency, etc.

Is there something else that would cause draw on the battery that I'm forgetting?


To clarify some of what I said before. I'm trying to build the battery pack so that the rate of draw is less than what the batteries are rated at. For example if they have a 115 min capacity at 75amps then I want to make sure that the draw is less that 0.5 C on the battery pack. That means it needs to have at least 2 hours of driving time. Now this doesn't have to be two useable hours of driving time, as I'm only looking at the rate. So if I have at least two hours of driving range, in order to prolong battery life I'd limit myself to just one hour of driving per charge. So if the full charge would give me something like 80miles of driving range, I'd limit my own range to 40mi, unless I ran out unexpectedly, in which case the extra buffer would be there even if it damaged the batteries to do so. Now my understanding of lead acid is that the more slowly you draw current from it the more charge you can use, and the longer they last. This is why my first priority would be to limit the rate of draw from the batteries, and then to limit the depth of discharge. I would actually normally drive less than 20miles in a day, but I figure if I could achieve 80 miles of range, and limit the current draw then the battery pack should last much much longer than one which depletes it self down in a matter of a few minutes of driving.

I'll try to find the specs on the golf cart motor to see what voltage it is. If it's less than 72 volts should I just abandon it and look for something that can take a higher voltage?

Also, I would be interested in making a lithium pack. I just don't know where to get lithium batteries with a BMS for anywhere near the price of lead acid. Now on the one hand I shouldn't need as many, and they should last much longer, but on the other hand the price is so high on everything I've seen. If you could point me to a good source I'd be glad to check it out.

I just saw your new comment Daox. Yeah it rarely gets cold here at all. It does rain for a few minutes at a time here and there though. Headlights shouldn't be too big of a draw because I'd be doing mostly daytime driving, but even so I'd gladly look into making some LED ones. A lot of places have to worry about the batteries being too cold and losing capacity because of this, but with temps over 100 in the summer I'd be more worried about keeping the motor and batteries sufficiently cooled.

P.S. found this here on Ecomodder about a guy with an electric Yugo. It's not extremely detailed as to what batteries, range, etc but I thought it was cool. http://ecomodder.com/forum/showthrea...ence-2738.html
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Old 03-07-2012, 12:42 PM   #18 (permalink)
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I don't know the best place to get lithium batteries. If you google for places selling Thundersky or CALIB batteries, those are the ones people are using for DIY EV conversions. Last I compared, the price is about 2.5X more than lead acid. But, they weigh less than half as much, and should last almost 4 times longer than lead if taken care of. In the end, they end up costing less. They're also happier to give you at higher C ratings. Speaking of taking care of the batteries you'll definitely want a BMS. You can buy one or several people here are working on open source BMS systems for lithium (myself included for my PHEV Prius). So, if you DIY you can save a ton of money.

The best way to avoid high C ratings (and thus high peukert effect losses) is to increase the voltage. Many motors are capable of handling much higher voltage than what they are rated for. I think most of the guys are running forklift motors that are rated for 24-36V and they're running between 72V and 144V through them. I believe only permanent magnet motors shouldn't be run over their voltage rating (and amperage for that matter). For instance my mower motor was a 24V that I was running at 48V. It had absolutely no issues with bumping the voltage up.

For the Wh/mile that the Yugo has posted, its probably "from the wall". That takes into account his charger's efficiency.
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Old 03-07-2012, 03:16 PM   #19 (permalink)
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Quote:
Originally Posted by yugomodder View Post
I'll make sure to build the system to the max voltage the motor can take, if it's still too weak I'll rewire the batteries to the voltage of the new motor.

I'll try to find the specs on the golf cart motor to see what voltage it is. If it's less than 72 volts should I just abandon it and look for something that can take a higher voltage?
The higher the voltage the faster you will go, and some speed controllers can be programmed to limit the voltage to the motor as well, so you can take a 144v battery pack and have your speed controller limited to only give the motor 48v or 72v or whatever you want to set it at, or you can just tell your self not to go over say 45mph because at 45mph the speed controller will be limiting the voltage to the motor, the only time that the motor would ever see full pack voltage would be at full throttle.
I bought a new set of motor brushes for my motors and I was told by the engineer who designed them that they should be good for 160v in my 36v and 48v motor, I've also been told that my 36v golf cart motor should be fine running at 72v but that much higher then that and I should install this new set of brushes.
Any chance of taking some photos of your current setup? if your motor to transmission adapter is solid and well built you should have a handful of motor upgrade options too, depending on the motor that was used because there are only a few bolt patterns and shaft sizes that have been used for golf carts and there are two that are extremely common with a wide range of aftermarket motors that bolt right on.


My current battery gauge is a PakTrakr, the only people that I've talked to who don't like them seem to think that they should also act as a BMS instead of a battery gauge, but with the amp meter pick up it can give you amps and watts, along with pack voltage, state of charge, voltage of each battery and a whole pile of other info.
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Old 03-07-2012, 03:33 PM   #20 (permalink)
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Thanks for the replies!
I'll look up those brands Daox. Yeah it seems like several people have said now that they could pump more volts through a motor than it's rated for. I guess it's like overclocking a computer, as long as you don't go overboard and you have the cooling you should be ok, right? For the wh/mi, that would make a lot more sense if it were from the wall stats.

That's exactly the sort of thing I was thinking Ryland. I can build everything so that it has the potential to do more, but just limit myself in order to prolong the life of the system. That PakTrakr looks great! That's all the sorts of info I'd like to be able to get. Unfortunately I cannot take pictures of the system at the moment, as it is still in Michigan. I do have a camera though, and pictures will be put up once it gets here. Yeah if the current motor fits, then it would make sense that being standardized other motors may fit too. I'll check what voltage the motor is rated at and then post here to see how many volts I should try to put through it.

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