Amps are usually what destroys components. Amp/hrs are what give you range. Voltage is what gives you speed. You need to decide what you want. A setup with a 200 a/hr pack at 72 volts ( 2 - 72v 100a/hr packs in parallel ) would give you twice the range at half the speed... A 100 a/hr pack at 144v ( 2 - 72v 100a/hr packs in series ) would give you half the range at twice the speed. This is an generalization, but you get the point.

My 144v Ford Contour would do 60-70 mph...

 

I have a manual trans 5th gear is over drive. When I started this procect 144v was my choice then the high voltage and burning things up made me concider lower voltage. If two 72v motors produce the tourqe with 5th gear OD then about 2000 rpm should get me close to 60 mph. I have seen on one web site ( HI-Tourqe electric ??) where they have conected two motors end to end. This gave me the idea of using two lower voltage motors. Now I'm thinking would the controler handle the amps? Though I would put this out and see if anyone has looked into this concept. I need to get a bit more educated on electronics, I'm a Machinist so building the hardware is no problem it's the volts,amps,ohm's ect. Thanks for the info

 

You are not really going to save yourself much trouble unless you are after a lower power car anyway. If you want to hold freeway speeds with this car, 72V will not make that easy.

All of your wiring will have to be heavier to carry the current, and finding a controller for that lower voltage and high current will also be a problem.

Powering two motors and possibly two controllers would also make things more costly. Why are you worried about 144V?

In reality 144v will be easier on a motor like an 8 or 9 inch series wound motor (warp or ADC) than huge current at lower volts. And again, the controller will not be well suited to those conditions.

 

If you run twin motors in my opinion the best option is to connect them with a switch (electrical change in set up). Adds 2 gears as such.
At the start they are running in series - twice the power
and then switch to running in parrallel - top end speed

Gearing down possible, but i suggest you check the torque/hp requirements and then match that to the motor/s. You can then shift your torque/rpm curve around according to what speeds you would like to travel.

How many amps you will be pulling: Calculate the power maintain speed (Watts), devide by voltage and you have amps.

Eg: (these are ball park as there way too many vairables)
In my calcs: To maintain 100km/hr will take 23392Watts
So with a 144V system i will be on around 162 Amps.

To acceleration 0-100kph (62mph) in 5 secounds
will take 64300W. So with a 144V system i will need around 446Amps

To acceleration 0-100kph (62mph) in 5 secounds and maintain
will take 87693W. So with a 144V system i will need around 608.97Amps

Edit: These figures are for a 750kg, cd=0.7, fronal area 1.59m^2 lotus 7 replica.

 

OK, I thank you guys. It's the amps I need to be concerned with not voltage. I will do some more reading and get to work with my calculator and be back with more questions for shure. any more input is welcome.

 

That is an excellent question, one I asked myself when I did my build. My advice, I went with 72volts, I have a top speed of 50MPH. Yes you can gear up to attain higher speeds, but the current draw becomes excessive. I drive in 1-3rd gears in a 5 speed. If I stay in 2nd or 3rd the current stays 250amps or less, but if I go to 4th or 5th the current goes to 500amps, BAD. I can go 48 MPH in 3rd, but in 4th or 5th the car doesn't go any faster it just uses more curent. A higher voltage allows for more motor rpm. An electric motor works best at the top RPM, So gear low and work near the top of the RPM. At 48MPH in 3rd gear the current draw drops to 150amps. You may not need 144volts, but I would at least go with 90volts for a car. The more voltage the less current for the same torque, so there are several advantages to going higher on the voltage, less voltage drop across the controller, (less power lost as heat). less heavy duty wiring, (most wire insulations are rated at 600volt, most of us use far less than this), but current requires square area of copper.

 

I went with 72volts, I have a top speed of 50MPH. Yes you can gear up to attain higher speeds, but the current draw becomes excessive.

I agree with everything you've said, except that I think you have missed one point. 2 packs,,, 2 motors, redundant everything. I don't think that amp draws are going to be excessive. I am building a system similar to the one being discussed,, it's expensive (not cost prohibitive) but the idea is that when something goes 'smoke' (Not if,,, when) I'm hoping to be able to limp home on the other motor/controller. Compared to the cost of towing, trailering, or ??? maybe the price is a wash.

 

Greyballs, with all due respect, you dont have alot of faith in your conversion, do you?

With the additional weight and cost, you could have went with a better controller/batteries...

In a little over a year, I towed my car one time, and that was because I didnt pay attention to the "fuel gauge". I had installed the lugs for a towbar on the car during the conversion, so I called the wife, she brought my truck and the tow bar, and my car was back on the charger in 20 min...

 

Lets say I go with a 96 volt ADC with T105. The weight of car before I started removing every thing I will not need was 2300lb. So I remove 450lb and add 1500 to end up with about 3300lb finnished. engineer Bill said the motors run best at the top of there rpm range. So if I use a 2-1 reduction so the motor runs at say 5000 rpm I would be turning the trans input at 2500 rpm. I have a 35 mile round trip commute that is flat excep for the last 1/2 mile back home and it would be nice to get back up the hill, do you guys think this would do it? Anyone know what rpm is optimal for these motors? If I could run the motor at optimal rpm using the gears up to 5th would this get me about 60 mph? I'm shure there are some math equations to figure this out but I dont know them, anybody know some? I have AAA but I wont need a tow.

 

One thing I'll say here, and I'll say many times again. One of the best aspects of an electric car is it's elegant simplicity.

For this reason (and all of the others stated above) I would vote for the 144v system.

 

Parallel strings do not require any increase in wire size unless you use a higher capacity controller and bigger motor. What you would do then is to parallel the conductors from the controller to the motor and those are usually short. If you keep the motor and controller the same size as a single string 72 volt system then all of the wiring is the same size. One thing that you would want to do is to add is a fusible link and disconnect in the second string so the strings can be isolated. Beauty of this setup is that if you have a battery failure one string can be isolated and you can still get home without having to bypass the failed cell.

One draw back is that you may need 2 chargers if you want a fast recharge.

If the string is only good for 200 amps adding a parallel string does not increase the each strings capacity to 400 amps, you still have 200 amps per string.

I'm not an EV expert by any means, this is my first post. Looking at options to stop the old Subaru from leaking oil. I was actually contemplating this same setup for the Suby. My experience is from working on 500kW UPS's so we use parallel strings a lot to extend discharge time. 500kW will discharge 3 strings of 40 12 volt 370aH cells in 5 minutes. I wonder what the 0-60 time would be for that.

 

A motor running at 72 volts won't run as fast as a motor running at 144 volts, no matter how many 72-volt motors you have.

 

They wont run as fast but will there be any torque gain? If more torque would help them into there efficiency range faster, as engineer Bill said the motors run best at the top of there rpm range. So if I use a something like a 2-1 reduction so the motor runs at 5000 rpm I would be turning the trans input at 2500 rpm then using the gears up to 5th would help keep motors in there optimal rpm range. I think it would increase range and maybe get up to speed without using up battery.

 

They wont run as fast but will there be any torque gain? If more torque would help them into there efficiency range faster, as engineer Bill said the motors run best at the top of there rpm range.

Don't confuse torque with efficiency. A motor running at maximum efficiency is NOT delivering maximum torque. Most motors deliver maximum torque at ZERO RPM, which is, by definition, also the most inefficient state. And the notion of adopting an inefficient design for the sole purpose of getting "into there efficiency range faster" (even if it were a viable theory, which it isn't) is like filling the first nine floors of a ten-story building with concrete just to more firmly support the top tenth floor. As long as you live only on the tenth floor, that's fine, but who drives like that? Accelerating from dead stops, slowing down, and speeding up is part of driving, and you shouldn't disregard that any more than you would disregard the first nine floors of a ten-story building.

And that's even if it were more efficient, which it isn't.

Heed Lordwacky's advice. Given the choice between low-voltage/high-current or high-voltage/low-current, the high-voltage/low-current will always win, both for torque and efficiency, no matter how many motors are in the design.

 

Very interesting conversation. I've been a lurker for months as I have felt that since my conversion is only just beginning a have painfully little experience. I have a degree in physics, so I tend to over simplify, so if I am please let me know.

It seems to me that a higher voltage system is going to be inherently more efficient.

Power at the motor is the product of the Voltage and the Current at the motor. Electrical power losses in the system is going to be Current^2 times R (electrical resistance in the system). To get the same power at the motor a lower voltage system is going to have to push more current. Since you are having to push more current to get the same power, your power losses are actually going to be higher, because losses go as the square of the current. Granted you have to assume the electrical R in both setups is more or less equivalent.

Also not to mention the damned Peukert effect. You'll get more range if your current draw is less on your batteries. Granted in a 72 volt system you'll have twice as many batteries, but in general again you'll have draw more current. So any 2 batteries in the 72 volt system will have to work "harder" then any one battery in the 144 volt system, at the same power.
So my conclusion sans any concrete data is that a 144 volt system will perform better then a parallel 72 volt system.

 

I have no experience and have been checking out suppliers on the web and trying to figure what to do for a while. I am a Machinist so I may over complicate the electrical part due to lack of knowledge. Just got the car, picked the Saturn for a little aerodynamics and they are mostly plastic so a bit lighter. After starting with this question and getting the responses I think 144v single pack and motor is the way to go. I still would like input on the reduction to obtain efficiency as for the motor rpm.

 

re: "...are some math equations..."

The following link is the 3rd generation of a modeling program which tries to predict the range and speeds of an EV given its weight etc. I didn't write it. It even took me a while to figure out how to use it.
http://www.evconvert.com/tools/evcalc/

The parent website is also interesting:
http://www.evconvert.com/eve

I'd strongly recommend you go the 144v approach w/ 1 motor and 1 pack. My 3620 lb ev is 120v and its 0-60 time is measured in days. Actually its 55-60 time is also in days.

And configuring a single pack to contain twice the amperage at half the voltage runs afoul of the Peukert Effect which will really hurt your range:
http://www.39pw.us/car/peukertEffect.html

The above was written to help remind me about the P.E. Sometimes work takes me away from EV stuff for awhile and my poor memory has forgotten something important - hence that web page.

frank, walnut creek, calif

 

wow the things i learn from this site...you guys are amazing.

 

This is a great discussion. I plan to use 144V for actual driving but, due to an ignorant Oklahoma state law regarding EV systems over 80v, I will be using a reversing contactor to switch the pack to 72v at rest. Switching the pack is to keep things legal. My question is weather to use a 72v charger or a 144V charger.

What research I have done says that the higher the voltage the better.

Brian

 

Maybe I could buy it back?

 

Frank Harvey,

thanks for the link about Peukerts equation and it's obvious errors.
I have struggled with the standard equation because it suggests that if you connect two identical batteries in parallel and run them at half the current, then you can draw this current for twice the time. This cannot be correct since the batteries will have more time to "recover" in that longer time so will still have energy left! Your link gives a much better and fuller explanation.

Just testing my fifth trike but first EV at the moment. 72V, Etek RT, Alltrax 7234, 254Kg including 150Kg of LA! Hoping for 40 miles max but only need 27 miles. Batteries are 110Ah C20.

This thread suggests I was lucky to have opted for 72V rather than 48V with bigger Ah batteries, Phew!!!!!!!!!!!