[yulek]

Hello,
I have always wanted to build an electric car! I built a new motor for my hotrod and the state patrol will not allow it because it is too tall. I have another can to put it in so im not to upset.

M background is pretty exstensive. I am a machinist and I can make just about anything with a bridgeport and a lathe. I also used to race R/C cars so I have some experience with electronic motors, controlers, and batteries.

I have a few questions for you guys,
My car is 1700lbs with the motor and tranny in it. I would like to travel longer distances more than high speeds. I would like to go 70 mph once in a while but not that often.

What size motor? I have been looking at warp motors and theres a dealer near me. 7,8,9,11?
What controler would work for me, and would a transmission be needed? I have an automatic now.
Any help would be great!
Thanks

I will be making a hood so the batteries can sit up front, and replacing the gas tank with a large square tank that will hold extra batteries.

 

[toddshotrods]

Welcome fellow bucketeer! (see my signature for the reason behind my cheeseyness)

The choice of motor and transmission depends largely on your budget. More specifically, it depends on how much you invest in your battery pack. The battery pack is your engine, so to speak, and everything else should be based on it. If you have a really lightweight, energy dense, lithium pack you could save more weight with a 9" motor and manual transmission or Powerglide. The fact that you're not planning to race it makes it easier, IMO.

More details please.

 

[yulek]

Bump for some more help please!

 

[Yabert]

Like Todd said we need more details.

Kind of acceleration do you need?
Kind of range do you need?
Kind of budget do you have?
Top speed of 75 mph it is correct?

The battery is the most important part for your convertion. Especially because you don't have than much place.

For exemple, I will put a 12.3 Kwh Lithium battery pack on my 1750 lbs Smart Fortwo. This one take around 3.4 cubic feets of space, weight more than 300 lbs and cost 7000$ (excluding charger, BMS, battery box, ...).
And 12.3 kwh give to me than I will can traveling during one hour when my drivetrain take 12.3 kw (16.5 hp) from the battery.

 

[Woodsmith]

You can start from the battery pack and see how much of it you can fit in to the space you have, you can then work out what you can do with the energy in it.

Have a look at some of the site sponsors to get an idea of the batteries available. Some will have dimensions for them and that is what you can use to start with.

For example you can work on the starting point of getting, say, 144V using 45 3.2V LiFeP04 batteries.
Then look at what capacity batteries you can fit in, 60Ah, 100Ah, 160Ah or 200Ah.
Although they may not be the ones you ultimately use it will give you a starting point of of being able to have, say, a 23kWh pack made up of 45 x 160Ah batteries.
You will be able to use 80% of that capacity so that is 18.4kWh available to you.
on't forget to check the weight of the batteries and weight distribution on the vehicle.

Keeping the voltage higher with more lower capacity batteries is better then a low voltage with fewer large capacity batteries.

At that stage you can then look at what sort of motor you can fit in and what performance you are likely to expect.

 

[notailpipe]

Keeping the voltage higher with more lower capacity batteries is better then a low voltage with fewer large capacity batteries.

Hi Woodsmith, that is a quite profound statement and I'm wondering if you can shed some light on why you think that, or what situations that is true. The reason I ask is I have been starting to plan my car budget and coming to the reverse conclusion than that...

BUT, my assumptions are that range is relatively low (~25mi) and high speeds (where the high-voltage helps) are rarer. Main performance metric is acceleration, so high-amps. Thus a high-capacity battery can put our more amps for the same C rating. I also assume, from reading lots of controller posts, that the controller acts more like a current source than a voltage source. So it will dump as much current until it's limited by throttle input or max setting..

Please clarify, thanks!

 

[Woodsmith]

Hi Woodsmith, that is a quite profound statement and I'm wondering if you can shed some light on why you think that, or what situations that is true. The reason I ask is I have been starting to plan my car budget and coming to the reverse conclusion than that...

BUT, my assumptions are that range is relatively low (~25mi) and high speeds (where the high-voltage helps) are rarer. Main performance metric is acceleration, so high-amps. Thus a high-capacity battery can put our more amps for the same C rating. I also assume, from reading lots of controller posts, that the controller acts more like a current source than a voltage source. So it will dump as much current until it's limited by throttle input or max setting..

Please clarify, thanks!

My statement was a bit generic really.

The basic premise I am working to is that, for the same overall battery capacity and power drawn by the controller, at relatively higher voltages the respective current will be lower and so the losses will be less.

There are a lot of variables though and not a lot of information provided by the OP.
I just figured that an alternative to starting at 'performance required' that it might be useful in a very small vehicle that one starts with how much battery can be installed and then working back from there. And on that basis using a higher voltage pack with, say, 40Ah cells could be more effective then a low voltage pack using, say, 200Ah cells.

With more information and input from the OP my statement could well be proved wrong in practice.

 

[notailpipe]

Thanks for the explanation.

I think both conclusions are right based on the design targets. My question is growing increasingly irrelevant since this morning I discovered these amazing Headway batteries with such high current output... I think I'm back to shooting for high voltage.

Sorry OP for hijacking the thread!

 

[Duncan]

Hi Notailpipe
BUT, my assumptions are that range is relatively low (~25mi) and high speeds (where the high-voltage helps) are rarer. Main performance metric is acceleration, so high-amps. Thus a high-capacity battery can put our more amps for the same C rating. I also assume, from reading lots of controller posts, that the controller acts more like a current source than a voltage source. So it will dump as much current until it's limited by throttle input or max setting..

The DC series motor controller acts like a power in power out device

200v and 100amps in = 20v and 1000amps out at low motor rpm (when it need low voltage)

200v and 200amps in = 40v and 1000amps

200v and 400amps in = 80v and 1000amps

200v and 800amps in = 160v and 1000amps

So by going for a higher voltage you reduce the BATTERY current even though the controller controls the MOTOR current

 

[notailpipe]

The DC series motor controller acts like a power in power out device

200v and 100amps in = 20v and 1000amps out at low motor rpm (when it need low voltage)

200v and 200amps in = 40v and 1000amps

200v and 400amps in = 80v and 1000amps

200v and 800amps in = 160v and 1000amps

So by going for a higher voltage you reduce the BATTERY current even though the controller controls the MOTOR current

Duncan,
Took me a while to understand what you were showing, but I love this way of thinking about it. Similarly, for the same amperage-limited controller, increasing the pack voltage will roughly inversely-proportionately decrease the battery current necessary.

So, say, a motor was rated to 144V max but the controller was limited to say 200V, could you build a 192V pack and not damage the motor? Somehow tell it peak voltage output?

Thanks again for the way to think about this. Neither voltage-source or current-source were satisfactory models of understanding.

 

[Duncan]

Hi Notailpipe

The way I understand it
It only takes 10-20v across the motor to get 1000amps BUT there is the back EMF caused by the motors rotation to consider
So

Zero revs you only need 20v (to get 1000 amps)

1000rpm will give 30v back EMF (example) so you need 20v + 30v =50v

2000rpm will give 60v back EMF so 20 + 60 = 80v

4000rpm - 120v so 120 + 20 = 140v

The result is that the maximum speed and the maximum voltage are linked,
If you can only use 150v then you are limited to ~ 4500rpm (depends on the motor)

and also if your motor can only do 4500rpm (before it explodes) then you only need 150v

So you can use 1 300v battery and use your controller to limit your motor voltage to 150v