# Rx-7 Electric Conversion



## neljoshua (Oct 21, 2008)

Having finished all I can do with my diesel VW (converted to WVO and now for sale), I would like to do an electric conversion. My wife has graciously consented, so away we go.

The donor car is a 1984 Mazda Rx-7 GSL-SE (all wheel disc brakes, manual moon roof, limited slip diff.). I have done some preliminary calculations and would like a little checking and advice.

Power calcs:
P=m*g*v*Rr+0.6465*Cd*A*v^3
m=1000 kg (est. with engine, etc removed--a little low)
Rr=0.020 (from one of my Profs at Purdue)
Cd=0.36 (from data sheet)
v=28 m/s (ambitious--I am willing to settle for less, maybe 22)
P=15706.2 W (about 16 kW)

I have seen some discussions on voltage, but as a mechanical guy, I am not quite sure how to figure this, other than a higher voltage gives a higher top speed. As far as range, I would like in the 40 mile area, but am willing to settle for less.

Because of cost, I think I am going with lead-acid batteries. It seems that the Trojan T-1275 is a good battery (according to what I have read here).

I also plan to incorporate regenerative braking. I was planning to use a brushless dc motor, and was in the process of talking with a representative from a manufacturer, but they have not gotten back with me for a few weeks. And they said I could have an educational discount!

Anyways, thoughts on voltages and battery selection are greatly appreciated, as well as any general tips, especially if anyone else has worked on an Rx-7.

Have not bought anything yet--going to sell the VW first (hopefully tonight!).


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## neanderthal (Jul 24, 2008)

I am about 95% done with my 1985 rx-7 conversion, so if you have any questions feel free to ask.

You are right about the voltage If you search on the forums and do other forms of research you will find a lot of info on voltage vs top speed

The t-1275 is a good battery, but it is huge. It is large and heavy. Before deciding you should probably look up the pros and cons of flooded lead acid vs sealed types.

I used a 144v pack. 60ah batteries. 39lbs a piece. The car is just a bit heavier than stock. A pack of t-1275 would put the car WAY over its weight limit, which means a beefier suspension. I think you can hit your goal of a 40 mile range with a lighter battery pack.

http://www.evconvert.com/tools/evcalc/

this site will help you get a feel for range vs weight vs batteries vs voltage etc.


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## madderscience (Jun 28, 2008)

Your math seems reasonable. 

If you make sure to get an alignment job and run low rolling resistance tires after the conversion, and do anything you can to keep brake drag down, you can probably use a smaller number for your rolling resistance. 0.015 to 0.010 is achievable. 

28m/s (62mph) will be no problem at all with 120v and up. How fast you get there will depend on the car's weight and your motor and controller choices.

T-1275s are heavy (82lbs each, I think) because they are 150AH instead of 60AH for Neanderthal's car. 

With lead acid, weight is range. assuming 200Wh/mile (a very good number) for an RX-7, Neaderthal's battery pack should be good for at the absolute maximum about 30 to 35 miles, and that is a full discharge. (he has about 8KwH of total capacity, but maybe 5 to 6 of that will be usable)

If you went with a bigger (and heavier) battery pack like the 1275's, you should easily be able to crack 40 miles. 150AH at 144v is 21Kwh, but probably about 10KwH will be usable. (higher peukert) If that is a sane number, the range would be about 50 miles to a full discharge.

There are other tradeoffs of course. The AGMs (assuming they are, anyway) that neanderthal is using will allow much better performance due to the lower weight and higher power density vs. flooded batteries like the 1275s. The 1275s will sag more under load due to higher internal resistance. More voltage sag and more weight means poorer performance.

Good Luck.


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## neljoshua (Oct 21, 2008)

I was a little concerned about the weight. Thanks for the tips. Did not sell the VW last night =(. I'll keep trying...

Neanderthal, what kind of AGMs are you using? Who is the mfgr? Are you using 6v or 12v and how many? Also, were you able to put all your batteries in the rear, or did you have to place some under the hood?

I was thinking of using lead-acid because I read that AGMs have a shorter life. Is this correct? How big is the difference?

Would I get more distance out of 150Ah than 60Ah?


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## judebert (Apr 16, 2008)

neljoshua said:


> I was thinking of using lead-acid because I read that AGMs have a shorter life. Is this correct? How big is the difference?


AGM is a kind of lead-acid. The traditional lead-acid batteries are flooded with sulfuric acid; hence their name, "flooded lead-acid", or FLA. Sometimes they come in sealed models, with special valves allowing them to discharge the gas that builds up during charging; these are SLA or VRLA.

AGM stands for "Adsorbed Glass Mat". The acid is contained in a fiberglass mat between the lead plates. There's just enough acid to wet the fiberglass, so there's no way for it to leak. AGM batteries can have the plates squished very close together, so they often have lower resistance, and therefore better power.

As you've noticed, that can come at a price. AGMs are much fussier than their FLA cousins. Part of this is because their acid can never be replaced: lose any, and the battery has been damaged. Therefore AGMs should never be overcharged.

FLAs can also stir up their acid, which helps remove or erode sulfur crystals that build up during deep discharges. AGMs can't, so they should never be discharged and left for long periods.

If you can treat AGMs nicely within those parameters, they should last for almost as long as FLAs. 



neljoshua said:


> Would I get more distance out of 150Ah than 60Ah?


Short answer: yes, about 2.5 times as much.

Long answer: your range is a function of how much *total energy* is in your battery pack, your weight, and your wind and rolling resistance. We'll ignore wind and rolling resistance, since most people don't change them much. That makes your range dependent on your weight and energy.

Long experimentation has shown that the best tradeoff between weight, range, and performance is about 1/3 of your total vehicle weight. Rip all the gas stuff out of the car and weigh the shell. Add on the weight of your motor and controller. You'll probably be happiest if you put half that weight of batteries in the car.

You asked with numbers, though. Let's cover "total energy", quickly.

Electrical power is measured in Watt-Hours (Wh). One watt (W) is one volt (V) x one amp (A). One Wh is one watt for one hour, or 1W x 1h. One Ah is one amp for one hour, or 1A x 1h. 

You're choosing between 150Ah and 60Ah batteries. If you're planning on using the same voltage for them -- and I assume you're expecting 144V in both configurations -- then you're choosing between (150Ah x 144V) or (60Ah x 144V). For the mathematically confused, Ah x V is the same as (A x h) x V, which is the same as (A x V) x h (because you can do that with multiplication), which is W x h, which is Wh.

So, we generally say that your controller's max Amps determines your acceleration; your battery pack's Ah determines your range; and your system Voltage determines your top speed.

Two subtleties remain. Both are based on "Peukert's Exponent". It's mathematically complicated, but it means: the harder you push the batteries, the less you get out of them. When you trickle electricity out of a 60Ah battery, you get all 60Ah. When you try to yank out a lot of Amps at once, you only get 40Ah. Bigger batteries usually have a little less trouble with this, but not much. In fact, manufacturers measure them at 1/20 of the Amps they're rated for (so a 40Ah battery is measured by taking 2A out for 20 hours).

The first subtlety is that AGMs have less trouble than FLAs. In fact, if you're going to be driving for less than an hour, you'll probably want AGMs, because you won't be able to outrun Peukert in that time. If you drive for more than an hour, you want FLAs, because you'll catch up and surpass the effect.

The second subtlety is the relation between volts and amps. When you accelerate, you're requiring *power*. Power is a little different than energy; power is instantaneous, while energy is power over time. Power is measured in Watts (or Kilowatts when we've got a lot of it).

If you intend to accelerate a lot, you'll need a lot of power. AGMs provide power better, and they suffer less from Peukert, as I mentioned earlier. However, keep in mind that Peukert depends on Amps. For the sake of simple math, let's assume you intended to drive so you regularly used 1000W of power. If you had a low-voltage pack of only 100V (once again, simple numbers!), those batteries would need to provide 10A. If you increased the voltage to 200V, the batteries would only need to provide 5A, and they wouldn't suffer so much from Peukert effects.

My, this is a lot longer than I intended. I hope I've answered your questions. An RX-7 is a neat conversion; I assume you've seen the "Maniac Mazda" pictures, where the electric RX-7 is doing a wheelstand?


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## neanderthal (Jul 24, 2008)

the other guys said it beautifully but to answer your question, i am using kinetik brand batteries. model hc1400. 12v 60ah 1500amp output for a short peak (supposedly) They are very expensive agm/s but I used to work at a car audio shop, so I got the at cost, making them cost a bit less than optima yellow tops

I was able to put 9 in back and 3 up front. If I were to do it again, i would have MAYBE put more weight up front, and I would have used slightly larger, heavier batteries for more range. Battery management for the agm is a pain, upping cost, but the power from the batts is great

I dont think that my batteries would suit your needs, not enough range. The car is scary fast so far, with all the settings on the controller very low. The car is really light. But I think for your goal you will need bigger batteries than mine.


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## neljoshua (Oct 21, 2008)

Thank you all very much for the input. I have until summer to buy all the stuff, so I have some time to plot. I sold the Beetle this weekend (praise the Lord!), so I hope to get the donor car this week.
I am really more concerned with making a "daily driver" at this point, but I will not complain if I get a little speed as well.
I have not seen Maniac Mazda...


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## neljoshua (Oct 21, 2008)

Ok. Car sold like 1 hour ago. Now I can start buying parts.
Looking at a Curtis 1231C controller, perhaps a WarP9 motor, and AGM batteries.
I hope to get the car this weekend.
Cheers!


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## neljoshua (Oct 21, 2008)

Actually, last night a friend told me he may be able to get a brushless DC motor for me--15 kW rms, 30 kW peak. Is anyone else using brushless DC?


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## neljoshua (Oct 21, 2008)

Also looking at the Sky Energy batteries. Just a thought...


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## neljoshua (Oct 21, 2008)

Just got done with spring break. No parts yet, but getting close. Still need to get the donor.
A friend asked me about intertia matching the motor and vehicle. I have the moment of inertia for the motor from the data sheet, but I am unsure of how to calculate my car's moment of inertia, especially with the differential and the tranny. Does anyone have any thoughts?


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## neljoshua (Oct 21, 2008)

Ok. So torque was really what I wanted, and I'm fine.

Totally different question: is there any advantage to getting a larger number of smaller batteries? For example, what is the difference b/w 10 12v vs. 20 6v (assuming the ah rating is constant b/w the two). I realize that the smaller batteries would probably weigh a little more, but is that it?

Ok. Looking around at some other conversions, I see that not everyone is using the same voltage. What would be the effect of using, say, 48v as opposed to the 120v I was originally thinking about. I see that it will reduce my top speed, but I am not that concerned with a high top speed, seeing as I drive almost all in town. Even a 50 mph speed would be fine.

Looking at this mainly to reduce weight...


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## neljoshua (Oct 21, 2008)

Ok...I am a dork.

Since I plan to use a bldc motor, speed is related to frequency of the drive. I can pick my voltage.

Anyone have any experience with Kelly BLDC controllers?


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## neljoshua (Oct 21, 2008)

I hate to keep posting without pictures, but I am still waiting on a title for the donor vehicle. It is probably better this way, as if I actually had the car, I probably would not be doing ANY homework .

In any event, I bought the motor yesterday and the controller this morning. The motor is a 15 kW bldc from MC Electric Power (see motors post for more info); the controller is a Kelly KBL12401H.

I am looking at batteries--probably going to buy them next week. I have an Excel file comparing some of the batteries I have been looking at...but the forum does not accept Excel files...I guess I'll put it in a Word file.


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## neljoshua (Oct 21, 2008)

Just ordered batteries. I decided on the Trojan T-1275. They are rather heavy (the whole pack weights 370 kg), but not much more than the glass mat batteries I was looking at. They were also much cheaper (about $170, shipped each). They each provide 12V and 120 Ah.
I am now trying to find a charger and a simple battery balancer. I recall seeing some information about a relatively inexpensive lead-acid balancer that simply placed resistance across each battery when its voltage reached a certain level, but I though that these were discontinued. Any ideas?


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## neljoshua (Oct 21, 2008)

Hey, quick question. Does anyone have a suggestion for a charger? I would like to be able to charge the batteries in 8 hours or less. I would need 120 volts, and I think we have a 15 amp outlet in our garage.


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## neljoshua (Oct 21, 2008)

Just looking at chargers. My battery pack will be 10 12V 120Ah batteries, so the pack is 120V 120Ah (this is the five hour rating). They are the Trojan T1275. How do I fit a charger to this pack? Does it need to be able to provide 120V and 120Ah? If I go with something smaller, how much longer will it take to charge?
Thanks.


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## neljoshua (Oct 21, 2008)

If you happen to be keeping up, I just updated my car in the garage. As of Saturday, I actually have my donor! I started working on it that night, but did not get far--there is a little body work, etc that I will have to do.
In any event, I have been pondering going without the tranny--just hooking the motor up to the differential. Any thoughts on this? It would save quite a bit of weight...


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## DavidDymaxion (Dec 1, 2008)

Keeping the tranny is cheaper and generally lighter. Freeway capable conversions that don't use a tranny generally use 2 motors, and generally a 2nd motor is heavier than a tranny.


neljoshua said:


> ... In any event, I have been pondering going without the tranny--just hooking the motor up to the differential. Any thoughts on this? It would save quite a bit of weight...


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## neljoshua (Oct 21, 2008)

DavidDymaxion, are you saying that using just the differential and a motor would lead to speeds too low for highway use? What about torque?
If this is true, couldn't this be corrected by using a different gear ratio in the differential?


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## bblocher (Jul 30, 2008)

neljoshua said:


> DavidDymaxion, are you saying that using just the differential and a motor would lead to speeds too low for highway use? What about torque?
> If this is true, couldn't this be corrected by using a different gear ratio in the differential?


Yes but then you'll loose your bottom end power. You'll need a LOT of torque to make up for this and have a vehicle which accelerates fast, maintain highway speeds and can do it with one gear.


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## DavidDymaxion (Dec 1, 2008)

I'm saying the opposite, that direct driving the diff tends to gear you for too high a top speed (a speed you probably don't have enough power to reach), while making your takeoff sluggish unless you have alot of power.

It is hard to gear down a motor enough with just a differential -- even a 6:1 differential ratio (very few diffs offer this low a ratio) will let your car do freeway speeds, if you have enough power. A typical diff of 3:1 or 4:1 gears the car too high, and makes takeoff and hill climbing a struggle, unless you have dual motors with lots of current.

A transmission will give you about 4 times the drive wheel torque in 1st gear as a direct drive to the diff. Unless you have gobs of power (Like a Zilla 2k and 288+ Volt battery pack) it's going to be tough to take off faster in the direct drive car.


neljoshua said:


> DavidDymaxion, are you saying that using just the differential and a motor would lead to speeds too low for highway use? What about torque?
> If this is true, couldn't this be corrected by using a different gear ratio in the differential?


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## neljoshua (Oct 21, 2008)

Ok. Thanks!


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## dtbaker (Jan 5, 2008)

neljoshua said:


> Hey, quick question. Does anyone have a suggestion for a charger? I would like to be able to charge the batteries in 8 hours or less. I would need 120 volts, and I think we have a 15 amp outlet in our garage.


take a look at the zivan chargers. a little pricey, but you can ask for them to be set for pack voltage and they have correct charge curves built in to balance and treat batteries nicely.


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## dtbaker (Jan 5, 2008)

neljoshua said:


> Totally different question: is there any advantage to getting a larger number of smaller batteries? For example, what is the difference b/w 10 12v vs. 20 6v (assuming the ah rating is constant b/w the two).


the aH will not be the same.... a 12v battery has more thinner plates than 8v or 6v at the other end of the spectrum. higher volts essentially give more zip, more Ah give more range. If you stray too far in either direction, results are not very drivable.

for highway speeds you will want a MINIMUM of 96v system, and better with 120v. If you go over 144v, then components get harder to find and more expensive. Then, depending on how much room you have you decide between 12v, 8v, 6v, or Lithium depending on range desired and money available....


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## jeffey (Jun 2, 2008)

*neljoshua,*
I did some research a few months back on direct drive. Was looking at an S-10 conversion. (Chevy 8" diff I think) Ratio's are available. (Project is on hold for lack of $$$) You need to decide on a motor. Figure the RPM range and play with gear ratio as well as tire size to get the right combination. There are motors available with couplers that connect directly to U-joints. 
Bottom line, it is possible. Most EV are placed in one gear and rarely shifted. So why not lose the weight and drag of the trans?
Check Richmond Gear for available ratios.
Link to a previous post: http://www.diyelectriccar.com/forums/showthread.php?p=45005#post45005


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## dtbaker (Jan 5, 2008)

jeffey said:


> *neljoshua,*
> Most EV are placed in one gear and rarely shifted. So why not lose the weight and drag of the trans?


not quite true.... without mechanical advantage off the line your acceleration will be painfully slow. You will be way happier retaining tranny and having an option to start in first up hills or if you have traffic stacked up behind you.

d


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## bblocher (Jul 30, 2008)

dtbaker said:


> not quite true.... without mechanical advantage off the line your acceleration will be painfully slow. You will be way happier retaining tranny and having an option to start in first up hills or if you have traffic stacked up behind you.
> 
> d


Yep, I have a six speed and in my daily commute to work I use 1-5. I've only used 6th when doing 75mph+. My car accelerates so much faster by going through the gears, another benefit of keeping my clutch!


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## ftaffy (Mar 13, 2009)

Talking with another guy who converted a lotus 7 replica to EV he has a 5 speed. Does not move from 1:1 very often, using 120V, 1000K Zilla, 9" ADC in a 750kg car and it is said be very fast and capable. 
You can calculate what you need fairly easily. As someone said just mess round diff and tire size.


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## jeffey (Jun 2, 2008)

Best explaination as to why / why not have a transmission:

"Chris Tromley wrote:
> My understanding so far is that a direct drive single motor is
> generally less efficient and will therefore deliver less range
> than using a trans.

Not quite. You'll note that *all* purpose-built EVs are direct drive, even those with series motors. Going with a fixed gear ratio requires a larger, more expensive motor -- but the cost and weight savings from eliminating the multi-speed transmission more than pays for it. And, the larger an electric motor, the more efficient it gets. So the larger motor with no transmission winds up being *more* efficient.

You see transmissions in conversion EVs because it was already there (free), and is integrated into the vehicle in such a way as to make it hard to remove (it includes the differential, or motor mounts). Once you know you have a transmission anyway, then you can downsize the motor a bit to save money and weight.

> Using two motors with direct drive adds a new wrinkle. They are still
> running at low rpm, but at only 1/2 load each. That has to improve
> efficiency. I wonder if two direct drive motors are (generally) as
> efficient as one with a trans?

It's very close. Two half-size motors will be less efficient than a single motor of the same horsepower. But the difference is small.

The main advantage to having two motors comes from possible mechanical simplifications. You may be able to eliminate the differential, or position the motors more effectively in the space available. Again, this applies mainly to purpose-built vehicles, not normal vehicles that already came with a differential and only have room for one motor.

For the special case of DC motors, having two motors allows for simple series/parallel operation to simplify your speed controller.
-- 
Every act of conscious learning requires the willingness to suffer an injury to one's self-esteem. That is why young children, before they are aware of their own self-importance, learn so easily. - Thomas Szasz
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net"

http://www.mail-archive.com/[email protected]/msg05755.html


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## DavidDymaxion (Dec 1, 2008)

Nitpicking a bit: Most purpose-built EVs go direct drive, but not all. The Buckeye Bullet (315 mph!) was high budget and as purpose built as you can get, and had a 5 speed tranny. The early Tesla had a 2 speed tranny. The Commutavan had a 3 speed tranny. The Chrysler TEVan had a 2 speed tranny.

Also, it is not universally true direct drive is more efficient, that depends on the driving cycle. On the freeway, direct drive wins. Crawling up a steep hill the tranny car wins.


EVDL Post said:


> ... Not quite. You'll note that *all* purpose-built EVs are direct drive, even those with series motors. Going with a fixed gear ratio requires a larger, more expensive motor -- but the cost and weight savings from eliminating the multi-speed transmission more than pays for it. And, the larger an electric motor, the more efficient it gets. So the larger motor with no transmission winds up being *more* efficient.
> 
> You see transmissions in conversion EVs because it was already there (free), and is integrated into the vehicle in such a way as to make it hard to remove (it includes the differential, or motor mounts). Once you know you have a transmission anyway, then you can downsize the motor a bit to save money and weight. ...


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