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Planning BMW 750i Conversion

19377 Views 51 Replies 13 Participants Last post by  Spir0
Hello guys and hopefully gals,

The basic plan is to convert a E38 750i to electric, and I was wondering if people could give me some advice and/or guidance.

My background
I am a 19 year old architectural assistant from the UK, I have been thinking about converting a car to electric for over a year. And i have been watching the forums for a while and I have finally made the decision to post my plans.

I have always been interested in engineering both mechanically and electrically, so hopefully I have the knowledge and the determination to take on a project like this.

The plan
Car: E38 7 series (750i preferably)
Model year: 1995-2001
Motor: 1 or 2 9-13 inch motors (Most likely Netgain)
Range: 50+ min (More the better )
Performance: Mid-high (150Kw+)
Cost: About £10,000 ($16,500)

Final bit
If I am quick I can snatch up a 750i for £1,500 which is in fully working order, but that is if people support my view. I don't need allot of range because I only live 10 miles from work and the journey is made up of a 40mph 70mph and a slow crawl. I would rather have a well performing car than one with range but if it came to it I would settle with a motorway (highway) cruiser with a large range. If a 7 series is a bad idea which I am hoping it isn't but I have the feeling it will be. Then I would use something like an E30, I want a timeless and classy car.

I plan on keeping the original/swapping for a simple to mod dive train. The reason is that I will most likely be the only one working on the car, I can get help from family and friends but not very often.

Peeps if you could leave your ideas and/or comments that would be great, even if it is a bad idea.

Thanks a mill.
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E38 750i has a curb weight of 3800 to 4820 lbs according to wikipedia. That is a huge range of weights and I don't know why that would be. Maybe the V12 is a lot heavier.

If you assume the /10 rule of thumb and the conversion comes out on the high end of the weight range that means 480 wh/mile and to get to 50 miles you would need a battery pack of 24kwh for a drop dead range with no accessories on. Same assumption on the light end of the range gives 380 wh/mile and a battery pack of 19kwh. Even if you strip out a lot of the stuff that makes these great cars to drive they are heavy and weight is one thing that kills EV range in stop and go driving conditions.

You will probably want an 11" motor of some kind. 150kw (output) is going to require a pretty good battery pack. At that power level the motor will probably be around 70% efficient so you will need to feed it over 200kw. You can get there by having a battery that can put out 1000 amps at 200 volts or a higher voltage at lower amps. Say you have a 340 volt pack that sags 15% under full load this would give 289 volts. A motor like the WarP 11HV can take 288 volts so to get your 200kw input you would need 694 amps. To get that voltage you need 98 cells. You could probably do this with 60AH calbs briefly. A 98 cell pack of 60AH CALB would give you an 18.8kwh pack which is close to the 19kwh minimum estimate. Using 100AH cells at 1000 amps and 200 volts would give a pack of about 74 cells for a 23.7 kwh pack size.

Guessing your usable range with this setup will be closer to 40 miles for the smaller pack and 50 miles for the larger one assuming 3800 lb car.

Not saying you shouldn't do this, but it is going to be more expensive than a smaller and lighter car. I don't think you are going to hit your budget numbers as the batteries alone will be in the $9000 range for the small pack after shipping and intercell straps, and $11500 for the large pack. A more realistic budget for this build would be about $20000 when all is said and done.

Best Wishes!
 

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Hi fellow uk converter!

Being blunt.... It's not possible to meet those specs (with any reasonable quality) with your budget in such a heavy car.

You could go for something lighter and it may be possible but even then I think you'll need at least £12k...

Why the 7 series?
 

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Suggest you ask him that! I'd guess its because it is not his first conversion and he wanted a bigger car.

Bottom line with a heavier car is you will be spending more money to reach the same performance and range.

Is £10k a hard limit? Is the 750i a must?
 

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Ah ok. Is that for image or practicality? Just trying to understand your thought process.

Cant really make the recomendation as I dont know the car.

The weight isn't a show stopper but its definately worth considering the options!
 

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I cant understand why the EV community insists on living in its Brushed DC rut.
An AC induction motor would make the 750 a feasible concept because of regen.
Remember kinetic energy is proportionate to vehicle weight.
All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
Get out of the rut people, free energy is out there.
 

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I cant understand why the EV community insists on living in its Brushed DC rut.
An AC induction motor would make the 750 a feasible concept because of regen.
Remember kinetic energy is proportionate to vehicle weight.
All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
Get out of the rut people, free energy is out there.
It is, however AC systems are generally more expensive than dc systems. The budget for this build wont really allow for it.

The hpevs ac75 (biggest ac motor before the prices skyrocket (expect to pay £4-5k for it in the uk with curtis controller) is still half the power of the smallest ice fitted to this vehicle.
 

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AC systems are generally more expensive than dc systems. The budget for this build wont really allow for it.
Short sighted
your not buying a lolypop here.
the extra expense now will be well worth the rewards later.
My friend here who has a Suzuki Carry, now wishes he had upgraded to an AC system instead of the bigger DC motor and controller he put in.
27% extra range in my Mira. Cant be ignored
 

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The hpevs ac75 (biggest ac motor before the prices skyrocket (expect to pay £4-5k for it in the uk with curtis controller) is still half the power of the smallest ice fitted to this vehicle.
With big cars like the 750, its going to be difficult to reproduce the same performance figures as the ICE unless you find a Solectria AC90.
Unlike small cars it actually difficult to NOT improve on performance over the ICE version.
 

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Short sighted
your not buying a lolypop here.
the extra expense now will be well worth the rewards later.
My friend here who has a Suzuki Carry, now wishes he had upgraded to an AC system instead of the bigger DC motor and controller he put in.
27% extra range in my Mira. Cant be ignored
How is that short sighted!? I'm agreeing with you.

The issue is budget, not which is the better technology.

If the OP can find the extra up front then AC is definately the better way to go.

Dont forget that we'll be looking at about £7-8k of batteries to get a solid 50 miles in this!
 

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I cant understand why the EV community insists on living in its Brushed DC rut.
An AC induction motor would make the 750 a feasible concept because of regen.
Remember kinetic energy is proportionate to vehicle weight.
All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
Get out of the rut people, free energy is out there.
All it takes is a reasonably priced 150kw+ inverter and then you have something. There are plenty of motor choices but the limitation is the inverter. Higher voltages than the Curtis can do at 500 amps or more would help a lot

I would prefer to have an AC system myself because the regen energy is as you say free even though it does not extend range much. If I have to spend an additional $7000 for a 150kw inverter I would rather spend it on more batteries.

The dual AC-35 setup for $10000 would be able to power this car but it is only 123kw at the shaft and 176ft-lbs according to the HPEVS dyno tests.
 

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Spir0 , you can convert any car. If you like big heavy beemers as I do then go for it. Don't sit at the keyboard and type yourself out of doing the build. I see this so often its depressing and one of the reasons I don't post on here and other forums much anymore.
 

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Spir0 , you can convert any car. If you like big heavy beemers as I do then go for it. Don't sit at the keyboard and type yourself out of doing the build. I see this so often its depressing and one of the reasons I don't post on here and other forums much anymore.

Can I ask why people are telling me not to do this and you are? I dont want to come off rude.
 

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Can I ask why people are telling me not to do this and you are? I dont want to come off rude.
Just to be clear, I am not telling you not to do it. I am just letting you know it will be more expensive than what you have indicated is your budget. I think it would be a really neat conversion and I would like to see it happen.
 

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I didnt realise weight played such a large part in it?
What sort of weight would you suggest?
Acceleration is inversely proportional to mass: a = F/m, where F is the tractive effort – force the tyres apply to the road. The tractive effort is proportional to motor torque (depends on gearing and wheel size). Increasing mass by 20% decreases acceleration 20%, or you get a motor with 20% higher peak torque to maintain the same acceleration. More $.

Rolling resistance force is proportional to mass: F = C*m, C a constant. Increasing mass by 20% increases the work done per mile to move the car by 20% and decreases vehicle range by 20%, or you buy 20% more kWh worth of batteries to increase pack energy by 20%. More $.

2100 kg is about 2x the mass of my vehicle, so you will need 100% more peak torque and power, and 100% greater pack capacity for similar performance (assuming similar drag force, but yours will likely be higher due to larger vehicle cross section). My vehicle’s acceleration is likely not acceptable to you (0 to 100kph in 16 sec), so you will likely need more like 4-5x the peak torque and power. Many more $.

An AC motor and controller with that peak torque and power will likely cost 2-3x that of a DC motor/controller. Many many more $

My motor’s peak torque and power are about 120 Nm and 48kW, so look for a series DC motor with 500 to 600 Nm peak torque, determine what current it requires to deliver that, and ensure it can deliver it to high enough rpm for 190-240kW peak power (motor shaft power = torque*rpm*2*pi/60, torque in NM), find a controller that that meets those requirements (likely a Zilla or Evnetics), and batteries with high enough C rate to deliver the required current.

My pack is a bit under 21kWh and my range is about 75 miles mixed highway/city with 80% DoD. You want 2/3 of that, so 2/3 of 42 kWh (100% more mass), or 28 kWh minimum. If you want that in colder weather with a heater running, then add 10 to 30% depending on temperature and whether you will do mostly highway or city driving.

Look at components and run some numbers. Zilla is sold by Manzanita Micro and Evnetics has an add on the RHS of your screen. Netgain or Kostov are the most frequently used series DC motors.
 
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