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Discussion Starter · #1 ·
My project is a light reverse trike and I have owned one with a 2008 'busa engine that was around 172HP. For a 700 lb trike that is almost enough power :) A Warp 11 motor make 114 ft/lbs of torque at 1440 rpms (44 hp) drawing 1280 amps. I am mostly interested in extreme power , not so much with range. 30 to 50 miles would be great. I was thinking with a 2:1 direct drive that is 244 ft/ls available any time, and the HP numbers are not really that low if the motor is at 6000 rpms. Would the torque be the same at 6000 rpms? If yes tha is 329 HP but i think a dc motor loses torque as revs increase, to a point,

Is that too much motor, would a Warp 9 be better?

I am totally lost on how to choose the right battery voltage and discharge rate. I am assuming that the battery must be capable of delivering 1280 amps at least. Is that not the real determining factor on performance? How many amp hours would I need for 30 miles and four or five bursts of 10 seconds of full throttle?

I hope I have asked this question intelligently. I would like to know how to calculate the battery requirements for future conversion projects too. Would you buy an electric, tilting trike that looks something like a T-Rex Aero if the price could be kept around 35K? I wanted it at 25K but have no clear idea of the battery costs.
 

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A T-Rex (for instance) uses a P275/40R18 rear tire, which is 678 mm overall and so turns 485 revolutions per kilometre (780 revolutions per mile); that's only 808 rpm @ 100 km/h or 780 rpm at 60 mph. With a 2:1 drive ratio the motor would be turning less than 2,000 rpm at a reasonable highway speed. Yes, that's faster than the spec for 44 horsepower, but that doesn't seem fast enough to get peak power out of any suitable motor. Unfortunately, the torque of a motor like this will drop off radically far before 6000 rpm, because torque depends on current and you won't have the voltage to push 1280 amps through the motor at high speed.

Again comparing to a T-Rex, their electric version weighs 599 kg with 26 kWh of battery. I think you should expect your finished project to be substantially heavier than the 700 pound gasoline trike.
 

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Discussion Starter · #3 ·
I have a good bit of experience with RC brushless motors, can I assume a car DC motor also has a kv rating? I also know it's absolutely critical to choose the right kv motor for efficiency. What I don't know and had not considered was the best rpm range, and 60 mph seems a very good speed for max effciency. I may change that to 50 mph as I would tend to drive on surface streets rather than the highway. Plus highway speeds where I live tend to average 80mph. If max speed is important I have never gone above 150 mph, even in my twin turbo viper (just 800 rwhp). I doubt I would even attempt 150 in a roller skate with a motor like a T-Rex.
I listed the gas trikes specs so I could find out what is necessary to achieve similar performance, and I always expected the weight to increase. What is more important for EV performance, total voltage or cell discharge rating? I expect that delivering the necessary amperage for a given rpm is the limiting factor, and doing it with the lowest weight is the goal.
Am I ontrack here, or should I be more concerned with kwH ? Its easy to convert HP to kwH, but with an electric motor I think you would need to integrate over the torque curve for a real comparison of engine vs electric motor. Are there any links to the math needed for a true compare?
 

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I have a good bit of experience with RC brushless motors, can I assume a car DC motor also has a kv rating?
Not really. The voltage constant (KV) describes a linear relationship between applied voltage and resulting no-load speed, but it assumes that the variable field (the rotor in a brushed DC motor, stator in an AC motor) is reacting with a constant field (such as a permanent magnet stator in a brushed DC motor or permanent magnet rotor in an AC motor)... however, many DIY conversions still used series-field brushed DC motors such as that WarP family, in which the same current flows through the armature and field (rotor and stator windings), so none of the basic relationships are so simple.

Series brushed DC motors have a similar issue with the torque constant (KT): that constant assumes that torque linearly increases with current, but with both more field flux and motor armature flux, the torque actually rises something like with the square of current.

In AC motors used in EVs, KV and KT could be applied, but are typically not used. There are so many factors to consider (maximum current of both controller and motor, maximum power of controller, etc) that these motors are generally described by their maximum torque (at maximum current), speed at which they transition from current-limited to power-limited or voltage-limited, and power limit. Serious motor suppliers publish full motor performance charts, and vehicle designers match a motor to the vehicle requirements.
 

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What I don't know and had not considered was the best rpm range, and 60 mph seems a very good speed for max effciency. I may change that to 50 mph as I would tend to drive on surface streets rather than the highway. Plus highway speeds where I live tend to average 80mph. If max speed is important I have never gone above 150 mph, even in my twin turbo viper (just 800 rwhp). I doubt I would even attempt 150 in a roller skate with a motor like a T-Rex.
Peak motor and controller efficiency is typically found at moderate speed and relatively high load. In practice, production EVs tend to be designed so that the motor reaches maximum usable speed at the vehicle's maximum rated speed, which determines the gearing. To make power available at moderate speed, and to maximize torque to the wheels at low speed, that top speed is kept down as much as the market for the vehicle permits (e.g. much lower for a Nissan Leaf than for Tesla Model S), so the highest possible gear reduction ratio can be used. This assumes a single-ratio transmission, which is normal for a production EV.
 

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What is more important for EV performance, total voltage or cell discharge rating? I expect that delivering the necessary amperage for a given rpm is the limiting factor, and doing it with the lowest weight is the goal.
Neither, by itself. With the same number of the same cells you can build a high-current and low-voltage battery, or a low-current and high-voltage battery, or combinations in-between, all with the same energy capacity and same power capability. The system voltage is selected to work with the motor (and other practical limitations), and is nearly universally about 360 V (nominal) in current production EVs.

Given the same cell design and module construction (cooling, etc) the discharge rate (or "C rate") is about the same, so the energy capacity implies the power capability - a battery with more energy stored can put out more power.

Am I ontrack here, or should I be more concerned with kwH ? Its easy to convert HP to kwH, but with an electric motor I think you would need to integrate over the torque curve for a real comparison of engine vs electric motor. Are there any links to the math needed for a true compare?
Horsepower (HP) is power, and kilowatt-hours (kWh) is energy - one does not convert to the other. Did you mean kW (kilowatts), not kWh?

The nice thing about typical EV motors is that they are limited by the battery, or the controller's power-handling capacity, or motor cooling, so regardless of what the motor could possibly do it is limited to a constant torque up to some speed, then a constant power from there to near the maximum speed. In a high-performance vehicle the low-speed torque limit isn't a big deal because there isn't traction to use more, so for much of the speed range it's just a constant-power situation.

Edit: Just to be clear, my comments above about "typical EV motors" refer to the motors used in real production EVs, and similar motors available aftermarket (which are rarely used because they are so expensive), not brushed DC motors such as those salvaged from forklifts.
 

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Discussion Starter · #7 ·
Thank you so much for the in depth answers. I obviously need to study non permanent magnet brushed dc motors. Power is applied so differently than an ICE I'm finding it hard to choose batteries and a motor, and an AC motor just makes it worse. I'm looking through the builds posts trying to find something similar.
 

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In terms of batteries, this is a little tricky. Nearly everyone on here these days uses junkyard batteries (usually Tesla, Volt or Leaf). They are cheaper than anything else available.

But your application is not that well suited for those. You have a small vehicle, and you don't need a lot of range, so you're looking for a pretty small pack, probably on the order of 10 kWh. But on the other hand you need a lot of power.

There are two challenges with using salvage OEM batteries. First of all, with OEM batteries the capacity (Ah) is what it is, and typically it is a larger number than you would want for a small battery, and if you use just a subset you don't get the voltage you need. Going with a very low system voltage, like you would do if you used a brushed DC motor, helps in this regard a little. But it still limits you somewhat. For example, the Tesla Model S batteries have an Ah capacity of about 260 Ah. So even if you went all the way down to 144V nominal, your pack would be 37 kWh. Way too big for your application. Probably the only OEM batteries that would have a chance would be Volt batteries, as they have a relatively small size while still being at 400V. But they are much lower density so it's not a complete slam dunk. You'd just have to see if you could fit enough in to get to the voltage you need.

The other challenge is power. You want your pack to be small but still pretty powerful. But most OEM packs use low power cells and get their power by making the pack very big. The cells in a Tesla pack actually aren't very powerful, but they put 100 kWh of them in and that's how they can make their car powerful. If you took just 10 kWh of Tesla cells, you would not have much power at all. Again, probably Volt cells would be your best bet. Like before, since they have a small pack they have to use higher power cells. For sure Leaf cells would be out of the question, as they aren't very powerful at all.

If Volt cells wouldn't work for you, you may have to look past the salvage market. But that's going to cost more.
 

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Probably the only OEM batteries that would have a chance would be Volt batteries, as they have a relatively small size while still being at 400V. But they are much lower density so it's not a complete slam dunk. You'd just have to see if you could fit enough in to get to the voltage you need.
It's not just the Volt - any plug-in hybrid is in a similar situation of having modern EV voltage in a smaller package than a typical battery-only vehicle.

Of course the Volt is by far the most common plug-in hybrid, but there's also the Chrysler Pacifica Hybrid, the Mitsubishi Outlander PHEV, and various Volvo models. The Volvo battery is particularly small, made to fit in just the tunnel. I don't know of any comparison of these alternatives, but what appear to be the Pacifica modules are sold by various vendors under the cell and module manufacturer's name (LG Chem) if anyone wants to assemble the numbers.

The other challenge is power. You want your pack to be small but still pretty powerful. But most OEM packs use low power cells and get their power by making the pack very big....
This is the other area where the plug-in hybrids are a somewhat better source - they have a smaller battery (physically, and less energy because less range is required) but they still need enough power output to drive the vehicle.

Just don't try to use a non-plug-in hybrid battery - those are just too small, and typically are not even lithium (NiMH still works well for that application).
 

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Discussion Starter · #10 ·
Making a few assumptions I may be way off base here, but I'm guessing that salvaged batteries cells have a low discharge rating. I know RC pack costs go way up for higher discharge rates, and that you can use a higher C rated pack with a suitable motor for more performance but with reduced range (mh) for a given battery weigh. I also know using new high C cells would be very expensive, so it kinda seems the choice is a cheaper salvage pack or a custom expensive (maybe very) pack. To get the results I want option 2 seems like the way to go, and if necessary add more range later with additional packs as funds allow or we get some new tech with better energy density. I think what I'm looking for are some real world number (like 15kwh) so I can calculate what the costs are.

For a given weight there has to be a way to compare an electric and ICE powertrain performace. I thiunk most people would understand 0-60, 60-100, 0-100, etc much better than HP, torque, etc. comparisons. I would really like to see comparisons from 30-60 and 30 to 100 mph to help eliminate traction issues (wheelspin). Can these be calculated from torque curves and drive ratios, and are there tools to do that?

I am still really lost on what I need. Can someone just suggest a motor / battery / controller setup that would work. Maybe I nned to look at the EV T-rex and try to find one for a test drive so I have an idea of what I'm comparing. I have never run either the trike or my viper at the dragstrip, but seat of the pants feel the trike accelerated harder.
 
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