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CVRX8 (My First Build - Need Input)

1291 Views 4 Replies 4 Participants Last post by  brian_
Hi All,

I’m finally getting going on my build after reading through these forums and learning a ton from the amazing individuals here. I’ve got the outline of a plan and have only purchased a few of the components. I was hoping to post to lay out my plan and let you all at it to tell me if I’m making a world of hurt for myself. Obviously, there will be challenges with any DIY build, but I’m still learning the basics here and there is a difference between “difficult” and “stupid hard/expensive”.

The first thing I should probably mention is that I want this to be a budget build. Money is a bit tight right now (as is time for that matter) but I want to have a complex, long term project to work on. I’m a mechanical engineer with some experience in high-voltage systems so at least I’m not starting from square one.

My goal is to create a budget EV that is comfortable to drive, modern-ish and can be used as a daily driver when (if) I get it driving reliably enough. My round-trip commute is about 40 miles.

My plan, as of right now, is to use the battery pack out of a 2017 Chevy Volt (purchased) with a DC\DC converter and charger (also from a Volt, not purchased yet) for the power system. I got the battery for a great price. It has a liquid cooling system built in along with a BMS and HV control block I’m intending to utilize. Reusing these HV control components should be a large cost savings as long as they can handle the current I put through them. At least their replaceable in case I push them too far.

For the vehicle I was originally planning on a Mazda Miata for the weight. Unfortunately, those hold their value remarkably well. So now I’m thinking of a 2004 Mazda RX-8 which I keep seeing amazing deals on. The Wankel engines on these go out quickly if you don’t take care of them and I don’t need the engine anyways.

For the electric motor I was thinking a used fork-lift motor that I find and refurbish. I’ve read through most of the forum post on choosing a DC motor for this purpose but admit I am still a bit uncertain how much you can realistically over-voltage these motors. I would try to find the high voltage rated one I could in a good size, but even at 48V a 360V (7.5X rating) battery seems like it might be too much. I know it’s generally current that smokes a motor so hoping this will be okay.

To drive the motor, I was planning to follow Damien’s approach from EVBMW’s $1000 build series. I’ve already categories all the scrap and salvage yards in the area and will be hunting for enclosure and heat sink components once I get to that point.

Choosing a newer car and components means I’ll have to deal with CAN bus messaging to convince the car it still has an engine and the Volt components that they’re still in a Volt. The biggest problem I see right now is that, while some of the CAN messaging has been deciphered for order Volt components, I’m not sure if these will work on the newer ones. If these aren’t out there currently, the signals needed might be hard to come by short of figuring out how to rent one.

I’ve given this a great deal of thought and am eager to keep going. I’d really appreciate any input you all have about this plan along with any resources you think might be helpful. I know there are one or two other people on this site also looking to use these Volt components and I would love to collaborate with you to figure these parts out. Thanks for taking the time to have a look and share your thoughts.
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· Administrator
1,761 Posts
Your pack voltage doesn't have to be your motor voltage.

Your controller has to be able to handle your pack voltage, so you can build a 340v pack, have a controller than can handle 340v, but limit your motor to 170v if you want to.

At least, that's what I've been told.

I don't quite grasp that, since most controllers chop the DC via PWM, so, seeing 340v spikes 50% of the time means the average voltage to the motor is only 170v, but, insulation-wise and, I dunno, brush-wise, is that more like 340v or does the inductance of the motor smooth that out so it looks like 170v? I'm not quite sure.

· Registered
5 Posts
Discussion Starter · #3 ·
Yeah, I was thinking the same way about this. But it was also my understanding that insulation doesn't really care about average voltage. That's why I was worried I might need an inverter that steps down the voltage to a lower level but I'm not even sure any of them do that and I certainly have no idea how to make a DIY that does that.

· Administrator
6,613 Posts
Hi Brian - welcome to the site!

A PWM controller works by varying the on and off times - from zero ON - 100% OFF to 100% ON and zero OFF

Because the motor is a big inductor that results in the motor voltage changing

So you have the Battery Voltage - and the Motor Voltage

Some controllers permit you to limit both

The controller is going to try and match your "demanded current" with "actual current"

You can't "overvolt" a DC motor without either overcurrenting it or overspeeding it!

The voltage/current/rpm are all linked

I'm overcooking my motor by using 1200 amps - 340 v and hitting over 4000 rpm

But if I was limited to say 600 amps then I could reach 4000 rpm and I would only be applying about 200 v to the motor

· Registered
8,516 Posts
Have you seen the RX-8 projects that have been reported in this forum? While there is no single "right" way to convert any particular model of car, other projects are a good source of ideas (and sometimes lessons about what not to do).

The DIY Electric Car Garage feature is not currently functional, but there are some build threads... just put "RX-8" in a search. The ones that I remember show up early in the search results list.
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