PLAN – Designing an EV conversion from scratch: a SPARK walk through

Now that we’ve defined what we want this conversion to accomplish, we can start working toward it in a structured way. The Plan phase is all about translating our Engineering Decisions into real-world numbers — specifically, how much power the system needs to deliver during demanding situations, and how much energy the battery needs to store for the vehicle to be usable in day-to-day driving. We call this our Energy Blueprint: a simplified but realistic way to estimate performance and range without needing a wind tunnel or a physics degree. The idea is to confirm that our plans make sense before spending a single dime.

We can boil the Plan Phase down into 3 main steps:

1. Calculate all forces acting on the vehicle (worst-case scenario)
2. Estimate Peak Power requirements
3. Estimate Energy requirements for range

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1.   Calculate forces acting on a moving vehicle

Worst-case scenarios are those in which the resistance forces are the highest. Most of the time this means full power accelerations. We will estimate all forces acting on the vehicle using first principles and common sense.

In this particular case, I estimate that my worst case given how I plan to use this vehicle would be a 0-60 MPH acceleration in around 10 seconds on a 1% incline with 2 adults, a large dog, and some light cargo.

In order to do this calculation, we need to understand the basic principles of how a force is calculated:

$F = m \cdot a$

Or force is equal to mass times acceleration.

So how much mass are we talking about here? Vehicle + occupants + cargo.

So, let’s estimate:

The stock vehicle weighs around 3500 lbs, but we’re removing the ICE components and replacing them with EV components, so let’s call it a net-neutral swap when considering EV motor, inverters, OBCM, auxiliary components. Batteries are heavy, so let’s add 300lbs for this thought exercise.

Therefore:

Vehicle= 3800 lbs

2 adults = 350 lbs

1 large dog = 80 lbs

Light cargo = 50 lbs

Total mass to move = 4280 lbs (or 1941 kg)

We defined our expected acceleration in this case to be 0–60 MPH in 10 seconds, which would be 0 to 26.8 m/s in 10 seconds. Acceleration is change in velocity divided by change in time, so:

Going back to our Force equation we now know that

We will call this the Tractive Force $(F_{tr})$.

Cool, that’s the force we need to push the vehicle’s mass, but there’s more forms of resistance: rolling, aerodynamic and grade resistance forces also exist.

Rolling Resistance Force

Rolling resistance is a force that opposes motion of a tire. It comes from the internal friction of the rubber molecules when deformed under stress. At highway speeds, the majority of our EV conversion’s losses come from aerodynamic drag. At lower speeds, however, like stop and go traffic, rolling resistance is much more important.

Rolling resistance mainly depends on the vehicle’s weight, and the tire’s coefficient of rolling resistance, the latter we will be estimating based on standard values.

Where:

• $m$ is the mass of the vehicle in $kg$
• $g$ is the acceleration caused by gravity in $\frac{m}{s^2}$
• $C_{rr}$ is the coefficient of rolling resistance

Let’s assume a Coefficient of rolling resistance of 0.0115 on touring tires. (This coefficient comes from published SAE data mostly, but we will not get into specifics in this article.)

Aerodynamic Resistance Force

Aerodynamic Resistance force depends on more factors. Vehicle speed, frontal area, and overall drag coefficient affect it, as well as air density. We can summarize this force in the following equation:

Where:

• $\rho$ = air density in $\frac{kg}{m^3}$
• $C_d$= Coefficient of drag
• $A$= frontal area of vehicle in $m^2$
• $v$ = vehicle speed in $\frac{m}{s}$

In my worst-case scenario, Aerodynamic Resistance will be highest at 60 MPH.

NOTE: Coefficient of drag, frontal area and air density were estimated. Vehicle speed of 60mph was converted to meters per second.

Grade Resistance

When the vehicle is going up an incline, or grade, there is a component that is pulling it back. Think of a car rolling backwards when it’s parked on a hill: that’s the force that the motor has to overcome.

Where:

 $m$i s the mass of the vehicle in $kg$

$g$ is the acceleration caused by gravity in $\frac{m}{s^2}$

$\theta$ is the angle of road incline in radians or degrees

For small inclines we can assume $\sin(\theta) \approx 0.01$.

Therefore:

Total Force

Well, now we have all the forces at the defined worst-case scenario, so what now?

We know the forces that oppose motion: rolling resistance, aerodynamic resistance, grade resistance. We also know the mass of the vehicle and the acceleration we want it to achieve, so that gives us a tractive force.

This means that the motor needs to be able to make enough power to: 1) move the vehicle’s mass at a given acceleration and 2) overcome resistance forces.

This translates into the following summation:

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2.   Estimate peak power requirements

Now that we’ve done the hard part and calculated all the forces acting on the vehicle in our worst‑case scenario, the next step is to translate those forces into power. This is where things usually start sounding intimidating, but the logic is actually very simple.

Power is just force applied at a certain speed. If we already know the total force the vehicle needs to overcome, all we need to do is look at how fast the car is moving when that force is highest.

In our case, the worst‑case scenario we defined was accelerating from 0 to 60 mph in 10 seconds on a 1% grade. At the end of that acceleration, the vehicle is traveling at 60 mph, which is 26.8 m/s. That’s the speed we care about for peak power, because aerodynamic drag is highest there and we’re still demanding acceleration.

We already calculated the total force required at that point:

So we can calculate the mechanical power required at the wheels using:

Where:

$P$ = Power in W

$F_{Total}$ = Total Force in N

$v$ = vehicle speed in $\frac{m}{s^2}$

Substituting our values:

That means we need roughly 158 kW of power at the wheels to meet our worst‑case performance target.

But that’s not the whole story. The motor doesn’t send power directly to the ground with zero losses. Between the battery and the wheels, we have losses in the inverter, the motor, the wiring, and whatever drivetrain components we’re using. A reasonable assumption for an EV conversion is around 85% overall efficiency.

When we account for those losses:

This tells us that, in our worst‑case scenario, the battery needs to be capable of delivering around 186 kW of peak power.

I like to add a bit of margin here — not because the math is wrong, but because reality always finds a way to be less clean than the spreadsheet. Temperature, component aging, voltage sag, and real‑world inefficiencies all stack up. Adding a 10% buffer is reasonable and keeps us out of trouble.

That puts our target peak power at roughly:

This number now becomes a key input for the rest of the build:

            •          The motor must be capable of delivering this peak power (even if only for short bursts)

            •          The inverter must be rated to handle it

            •          And the battery pack must be able to supply that power without exceeding current limits (Remember that Power = Voltage * Current)

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3.   Estimate Energy requirements

In order for this conversion to be usable in my specific scenario, it has to be able to cover a certain range with a fully charged battery. The amount of energy available for propelling the vehicle is stored in the main high voltage battery, or the propulsion battery. How “big” should this battery be? And what does “big” mean?

Battery capacity is mostly measured in Watt-hours, or Wh. This unit is used because it tells us how much power (Watts) the battery can deliver over time (hour). So, then, how much power do I need this battery to deliver over time?

As you might recall from the original SPARK series, for our Energy Blueprint we can use an estimate of energy consumption per mile for different vehicle types.

Vehicle Type	Examples	Consumption (Wh/mile)
Small Hatchbacks	Ford Fiesta, Honda Fit, Volkswagen Golf	200–250
Small Convertibles	Mazda Miata, BMW Z3, Fiat 124 Spider	220–280
Sports Cars	Ford Mustang, Chevy Camaro, Mazda RX-7	300–350
Large Family Sedans	Chrysler 300, Toyota Avalon, Honda Accord	350–400
Off-Road Vehicles	Jeep Wrangler, Toyota FJ Cruiser, Ford Bronco	400–500
Small SUVs	Toyota RAV4, Honda CR-V, Ford Escape	300–400
Large SUVs	Chevy Suburban, Cadillac Escalade, GMC Yukon	450–600

Based on this table, I would say this is not quite a Small Convertible, and it might not be quite the Sports Car, but the latter is the category that I believe suits it best for a few reasons. First, it is a coupe, so not quite as light as a Small Convertible, nor quite as heavy as a Large Family Sedan. Due to its older, boxier design when compared to modern Sports Cars, I am going to estimate energy consumption at the higher end of this category around 350 Wh/mile.

But since this is a walkthrough, and I wouldn’t just blindly trust this number, let’s do a slightly more accurate calculation without needing to come up with OEM level simulations.

This vehicle will be used mostly in the highway at a steady state, with some city driving. I estimate longer highway stretches to be my conversion’s most vulnerable use-case since I wouldn’t want to drive 30 miles to run errands and finding out the hard way that I needed more highway range.

City driving with constant stop and start is not my main concern because regenerative braking will help us get some of the energy back into the battery and, in my particular use case, will not constitute the majority of my drive cycles.

How do I get to an energy consumption estimate per mile in highway driving? First, I need to understand the forces acting on the vehicle in this particular condition. For one, there’s no acceleration: highway driving is estimating constant speed, on flat ground. Constant speed means no acceleration. Flat ground means no grade. This means the only forces we need to overcome are resistance forces: Aerodynamic, and Rolling, both of which we calculated in the first section, but with one caveat: I want to cruise at 75 MPH, not 60 MPH. Using the same equations from section 1 we get the following forces:

Force times velocity is power. We know the force acting on the vehicle, and we know the speed:

This represents power at the wheels. What about power from the battery? Divide this by 0.85 to account for losses (efficiency).

Considering that Power is Energy over Time, and that at 75MPH we cover 75 miles in 1 hour, along with some other first principles magic that I won’t detail here, we get that:

I was close enough in estimating around 350 Wh per mile, but now we know with a bit more certainty.

In the Select walkthrough article I defined my range expectation to be 60 miles of combined driving. I will now estimate the energy required for 60 miles of constant highway driving, and will add a 20% buffer for city driving, passing, grades, extra electrical loads like lights, audio, cabin conditioning. I will also add 10% for lithium battery protection since these batteries shouldn’t be charged nor discharged fully.

Now the total 30% buffer:

Rounding, I would be happy with a 30 kWh battery pack for this application.

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What now?

The energy estimation above is just the starting point for selecting a battery pack. We now know the energy we need to meet our goals with this conversion, but size may vary due to other factors. For example, we might find that the discharge rate of the available battery packs in the market (used or new) is not enough to meet our peak power requirement with the size we calculated. For example, a battery pack available in the market might not meet the peak current discharge to achieve our 204.6 kW power. In this hypothetical scenario, we have the option to look for another battery pack with a higher C rate, or we might add more modules to the readily available ones to increase the peak current capacity, even if the pack ends up being bigger than 30 kWh.

I will get into more details about motor and battery pack selection for this conversion in future articles of this walkthrough series.