[Qer]

Originally Posted by valerun

Re the motor - you're probably right that's W11 is overkill - I am just trying to compensate for lower max amp capability of my battery pack with higher torque constant. I know the tradeoff between that and max RPM for a given power level but on the other hand I care a lot about the starting / low RPM torque (see my 0-60 requirements).

Hm. You're almost right, but you're missing the whole picture. I'll try to explain how I mean.

At 72 Volt and 300 Ampere the Warp 9" gives ~60 lbs/ft while the Warp 11" gives ~75 lbs/ft, but at the same time the 9" will rotate at ~2200 rpm while the 11" will only reach ~1800. Now, the power you get out is rpm * torque (although you have to compensate for the units you use) and those numbers gives roughly the same power out so by getting a bigger motor you just trade rpm for torque, pretty much like when you shift gears in the gear box.

Now; motor amps doesn't equal battery amps. This has been covered several times in the forum so I'll just give you the answer directly since I'm in a bit of a hurry. A controller converts power to power, pretty much like a motor or gear box does. This means that when you convert stored battery power to horse powers that propel the car, you can choose if you want more torque or rpm, but power stays the same.

The amount of power you get out will be the maximum current you can draw from your pack multiplied with the pack voltage, if you can get max 300 Amps from the pack at 150 Volt it means you get 45kW. You can have them at 1000 motor Amps at 45 Volt, 600 motor Amps at 75 Volt or 300 motor Amps at 150 Volt, but it's still just 45 kW. Same in the drive train, you can select different motors, change gears etc and thus get more or less torque at less or more rpm but in the end you get 45 kW.

The Soliton (or any other controller) can always give you maximum motor current no matter what pack you connect, but when rpm goes up the voltage over the motor will follow and at a certain point battery current will reach maximum current. At that time a smart controller (that's set up correctly) will start to limit motor current to protect the batteries while a dumb controller will just gladly increase the battery current until you damage your pack.

And now I have to go. Have to pick up missus at the train. I'll gladly answer questions when I'm back!

[dimitri]

Originally Posted by valerun

I was definitely looking at getting batteries separately and adding miniBMS onto them, dimitry. I am just new to this so ready-made packs represent certain attractiveness to me :-)

This is the easiest part of the project for anyone who can tell positive terminal from negative Plenty of help in Battery forum section. No such thing as ready made pack anyway, you still have to assemble everything.
If you plan to increase the pack later, the best way is adding same size cells in series and get more volts than adding strings of smaller cells in parallel, this is the worst thing you can do. However, 2 things to consider up front. Charger with adjustable voltage and high upper limit and controller with high voltage limit. Controller is easy, Soliton is the answer. Charger is not so easy, only Manzanita can do it, but its expensive and not isolated, although still one of the most popular chargers on the market. Choices, choices....

Re the motor - you're probably right that's W11 is overkill - I am just trying to compensate for lower max amp capability of my battery pack with higher torque constant. I know the tradeoff between that and max RPM for a given power level but on the other hand I care a lot about the starting / low RPM torque (see my 0-60 requirements).

Controller will compensate current for voltage and vice versa. At low RPM controller will dish out high motor current while keeping battery current low since motor voltage is low. At high RPM Warp11 won't help either since its RPM limit is lower. I'm not a motor expert but I doubt W11 will help you since battery is your bottleneck no matter what. Might as well get $1000 more and 100lb more battery, better bang for your buck.

[EV-propulsion.com]

Hey Val,
Head spinning yet? But in agreement with Dimitri and others, really the best recommendation (which we use for a minimum) is something along the lines of 160 ah batteries if using prismatic cells. This will keep your C draw down to reasonable levels, eliminate many extra batteries (as in series parallel arangements), and you could conceievable start with as low as 120 volt system to keep the initial cost down and add more in series later to raise the voltage/performance/range(through lower current draw all things being equal). The advantages to this are you won't be creating high C draws from your pack extending their life (and 500a battery current(a little over 3C) is not uncommon, especially if you like a little performance) and you get extra range!. yes it can be done with 100AH cells,(EVfun is proving that with his 60ah cells) and many have used them (some with and some without sucess), but constantly worrying about amp draw while driving is not enjoyable.
It hurts upfront to buy the larger cells, but you won't regret it after you are rolling. Do some more research here before you pull the trigger on buying batteries.
Also a good point by Dimitri, consider what charger you can use if you are planning to change things later.
Mike
www.EV-propulsion.com

[valerun]

ok great. This was awesome help, guys! I think I am going to go with 100-160Ah, starting with 120-144v then. Will also likely step down to Warp9. Does any one of you have experience with Kostov motors though and can give some perspective on how they compare with Warp? As I mentioned, Kostov 11" is 20kg lighter than Warp 11 with similar (at least on paper) performance...

Lastly, any recommendation on what the best sources for batteries are? I have been looking at Elitepower and EV Source so far...

Thanks!

[peggus]

Awesome choice of car if I may say so, I may be a bit biased though.

If you do go with an 11" motor, just skip the transmission and go direct drive, you'll make up for the added weight and you'll still have awesome performance. My 1977 fiat 124 has a 9" ADC with a 500A Curtis controller and I have no problem driving around in 4th (1:1 gear), heck I can even take off in 5th gear going uphill. If you're in socal you're welcome to come over for a test drive.

I also agree that you should go with at least a 100Ah battery pack if you use thundersky or similar batteries.

[dimitri]

If you want to get freeway speeds, start with at least 40 cells ( 128V nominal ) ideally plan to go to 48 cells or more if you can fit them and afford them. The more cells, the more voltage and overall energy capacity, the better.

This is number one lesson I learned with my conversion. If I was to do everything again, I would squeeze few more cells in. 40 was the minimum for my car to get decent top speed. I can still get to 80mph, but acceleration drops at high RPM due to back EMF rising.

Cell size is most critical decision you must make. You are constrained by available room, weight and cost. More cells is better for voltage reasons, but keep an eye on C ratings. Since your car is reasonably small, you might be OK with 100AH cells, but try to compensate by higher cell number/voltage.

You can also gauge the range by using 350Wh/mile rule of thumb. Say the pack of 40 cells 100AH would be ( 40*3.2V*100AH = 12800Wh ) / 350Wh/mile = 36 miles to 100DOD, or 29 miles to 80% DOD. This is another reason to get more cells.

[valerun]

Originally Posted by peggus

Awesome choice of car if I may say so, I may be a bit biased though.

If you do go with an 11" motor, just skip the transmission and go direct drive, you'll make up for the added weight and you'll still have awesome performance. My 1977 fiat 124 has a 9" ADC with a 500A Curtis controller and I have no problem driving around in 4th (1:1 gear), heck I can even take off in 5th gear going uphill. If you're in socal you're welcome to come over for a test drive.

I also agree that you should go with at least a 100Ah battery pack if you use thundersky or similar batteries.

Thanks peggus! The question is: will that get me to 0-60 in under 6 seconds... Here are some calcs I just did to check that for direct drive:
1. Assume final drive of 3.5
2. Tire radius of 10 inches, or 0.8 feet
3. Assume motor torque of 300ft*lbs throughout 0-60 (with this final drive and tire size, will be around 3500-4000RPM at 60mph; this is Warp11 spec at 1000A)

This gets me traction force at the wheels of 300*3.5/0.8 = 1260lbs, which means 0.5g acceleration force. If sustained over 0-60, that gets me to 27m/s (60mph) in 5.5s. Kinda checks out...

But, a couple of issues:
1. getting 1000A. Not sure what the voltage at the motor would be at this rating (this is outside of the performance curves published by manufacturer...). My pack might not be able to deliver...
2. Motor efficiency is lower at low RPM, isn't it? So I will be wasting more energy when going anywhere below 50-60mph...

Comments?

Thanks!

[Qer]

Here's a performance graph from when we were testing the tachometer input on the Soliton 1.



Just ignore that RPM is stuck at 1000 RPM in the graph, it was a pretty early version of the code and we were testing the accuracy, which was spot on once the software started to register pulses at about 1018 on the X-axis in the graph.

Anyway, I don't remember if we used a WarP 9" or a Kostov 9" at the time, but I think it's a WarP and, well, it shouldn't differ very much anyway. So for a 9" to run at 3500 RPM you need about 100 Volt, you will need more when current goes up since the internal resistance in the motors are a few tens of milliOhm (I think we measured about 20 mOhm for a WarP 9") so I think Dimitri's pretty spot on when he claims that 40 cells won't quite cut it. 40 cells will definitely be too few if you go with a WarP 11" since you get less RPM per Volt with a bigger motor.

Oh, and almost no pack can handle 1000 Amps. If you really want high current over the whole RPM-range your pack probably won't fit in your car anyway, unless you go for A123 or some other serious stuff with 2 digit C-rating. However, they're pricey, if you have to ask for the price you probably can't afford it anyway.

The Soliton, Zilla and probably also Warp-Drive can limit battery current, it's the only economical way to do it. They will provide full motor current until the battery current starts to get dangerously high (as a result of motor voltage rising) and then they will dial down motor current to protect the batteries. So what you will get is a performance graph with flat torque curve and increasing power up to a certain RPM when motor voltage has risen so high that battery current has reached it's limit, from there on the power curve will turn flat and torque will start to drop instead.

If you have a pack with too low pack voltage the power curve will start to drop again when motor voltage reach pack voltage, but if your pack voltage is high enough (typically 200 Volt or above for single motors) your power curve will stay flat until the motor overrevs and turns into scrap metal. That is, if you haven't added a tachometer so the controller can cut off the power before you scrap the motor.

[Duncan]

Hi Valerun
I am going with an 11 inch and direct drive - but my car is a bit lighter (650Kg)
You will need 1000 amps to spin your wheels
I think that is a wee bit high,

with a gearbox you will be able to spin tires at a lot less amps - this will help your battery choice

I think you have a nice fiat 5 speed, I would use that

The other reason I went with direct drive is that I put the motor where the gearbox would have gone - this left the whole motor bay for batteries

[peggus]

Well, my 1977 fiat has a 4.3:1 final drive but your math is about right.

I've got a 144V nominal pack and my torque seems to start to fall off above 3000rpm somewhere, so you're definitely going to need a higher voltage pack to keep the motor fed with full current all the way to 60, as Qer and Dimitry has pointed out.

Oh and the fiat transmission can't take much more than 160 ftlbs of torque IIRC, I can't find the exact number right now but it definitely wasn't 300ftlbs. You can swap in a 131/brava transmission but that seems like a lot of work: http://www.mirafiori.com/faq/content...1transswap.htm
Skip the tranny, you won't be disappointed.

The efficiency suffers a bit at lower RPM but it is pretty flat from 2000 to 4000 rpm where you'll be most of the time. The efficiency loss of the transmission should not be underestimated either.

Originally Posted by valerun

Thanks peggus! The question is: will that get me to 0-60 in under 6 seconds... Here are some calcs I just did to check that for direct drive:
1. Assume final drive of 3.5
2. Tire radius of 10 inches, or 0.8 feet
3. Assume motor torque of 300ft*lbs throughout 0-60 (with this final drive and tire size, will be around 3500-4000RPM at 60mph; this is Warp11 spec at 1000A)

This gets me traction force at the wheels of 300*3.5/0.8 = 1260lbs, which means 0.5g acceleration force. If sustained over 0-60, that gets me to 27m/s (60mph) in 5.5s. Kinda checks out...

But, a couple of issues:
1. getting 1000A. Not sure what the voltage at the motor would be at this rating (this is outside of the performance curves published by manufacturer...). My pack might not be able to deliver...
2. Motor efficiency is lower at low RPM, isn't it? So I will be wasting more energy when going anywhere below 50-60mph...

Comments?

Thanks!