EV Battery Selection for DIY EV Conversions: The Architect Phase of SPARK, Part 2

The heart of any EV conversion is its battery pack. It dictates range, power, weight distribution, and even the overall feasibility of a project. Without a well-matched battery pack, the best motor and controller in the world won’t get you very far—literally. If you’re following the SPARK methodology, you already know that each phase builds upon the last. We’ve gone from defining vehicle goals (Select Phase) to understanding energy needs (Plan Phase), and now, we arrive at the Architect Phase, where selecting the right battery pack is one of the most critical steps.

Today we’ll guide you through the EV battery selection process, building up from your Energy Blueprint and your EV Motor selection, until you understand what kind of battery pack you need. Whether you’re considering repurposing an OEM battery pack from a wrecked Tesla or piecing together a custom pack from individual cells, this guide will help you grasp the key considerations to ensure your battery pack meets your performance, efficiency, and safety goals.

Understanding Battery Pack Characteristics

A battery pack isn’t just a box of cells. It is a carefully engineered system that requires a balance between voltage, capacity, current delivery, and longevity. Let’s break down the most important characteristics:

• Capacity (kWh) – Determines the total amount of energy the pack can store. More capacity means longer range, but also more weight and cost.
• Cells in Series vs. Parallel – More cells connected in series increase pack voltage, while more cells connected in parallel increase total energy capacity and current delivery. Your motor dictates voltage, your desired range (from your Engineering Decisions) dictates total capacity.
• Max Voltage & Nominal Voltage – Voltage dictates compatibility with the motor controller and inverter.
• C Rating – Defines how fast a cell can safely discharge energy without overheating. 
• Max Current – Determines whether the pack can supply the peak power demands of the motor without damage. Current is directly related to torque in the motor.
• Cell Chemistry – Different chemistries impact cost, safety, energy density, and cycle life. 

How These Factors Work Together

In simple terms, the battery pack stores certain amount of energy (measured in kWh: kiloWatt-hours), and can “feed it” to the motor at a certain rate (voltage, current and C rating). This is were you need to use your Engineering Decisions and be realistic: do you really need 200 miles of range for a weekend toy that accelerates to 60 MPH in 4 seconds? Or do you really need to accelerate to 60 MPH in 4 seconds in your daily driver when you have a 200 mile commute? Tackle projects that are doable and enjoyable!

Matching Battery Output to Motor Power Requirements

Your motor needs power, and that power comes from the battery. However, there’s a catch—energy transfer isn’t 100% efficient.

1. Motor Efficiency: Not all the energy sent to the motor gets converted into motion; some is lost as heat.
2. Motor Controller Efficiency: Called inverters for AC motors, and controllers for DC motors, these devices control the voltage and current that goes into the motor at any given time to control their output, and they have their own energy losses (energy efficiency typically ~97%).

With that being said, the battery’s voltage should match that of the motor controller. If your controller’s maximum voltage is 400V, that’s what you need to aim for in your battery pack. Never exceed the controller’s maximum voltage in order to avoid damages, and consider that when the battery cells are fully charged, their voltage increases. We would even recommend sizing your battery pack slightly lower than the inverter’s maximum voltage, but always follow manufacturer’s recommendations for your motor, controller and battery pack.

Selecting Battery Cells: Chemistry, Capacity, and Performance

Battery chemistry affects everything from energy density to charging speed. Here is a very quick and dirty overview of the main types used in EV conversions:

• Lithium-Ion (NMC, NCA) – High energy density, used in most modern EVs.
• Lithium Iron Phosphate (LFP) – Safer and longer-lasting but heavier and less energy-dense.

Each cell chemistry has an approximate cell voltage, but you should always refer to the specification sheet of the cell’s manufacturer to understand its specific characteristics. 

This is by no means a comprehensive list nor is it a detailed explanation of cell chemistries. Cell chemistry is a very complex and interesting topic, but it is outside of this article’s scope. Bottom line is: there are certain differences in cell chemistries that you need to consider, but we believe that the availability and cost of the batteries, whether recycled or new from a supplier, will be the main driver for their selection.

Determining the Number of Cells for Voltage and Energy Needs

Since the motor controller’s maximum voltage should dictate the maximum battery pack’s voltage, and your desired range should dictate the energy capacity (from the Plan phase) you now need to figure out how to get to those specifications.

Your battery pack’s voltage and capacity depend on how many cells are connected in series and parallel:

• Series (S) Configuration: Increases voltage.
• Parallel (P) Configuration: Increases capacity and current capability.

If the cell chemistry that you selected has a maximum cell voltage of 4.2V, then how many cells in series do you need to get to the 400V of the previous example? 400V total divided by 4.2V per cell is around 95 cells in series! How much range those 95 cells in series will give you depends on the capacity of the cells, but it shouldn’t be much. 

That’s where cells in parallel come in. The more cells in parallel, the more capacity your battery pack will have! This is were you really need to be realistic! Not only do more battery cells add weight, they are also hard to package in a vehicle that was not designed from the ground up to carry heavy batteries. Weight distribution, among other things, needs to be maintained in order for the vehicle to be drivable. 

Determining maximum current 

In order to determine the maximum current your conversion will need from its battery pack, you need to understand that Power = Voltage x Current.

For example, if your motor has a peak power demand (from the Energy Blueprint) of 200kW, an efficiency of 97%, and a controller efficiency of 97%, this means that the battery should be capable of an output of around 212.5kW (that is 200kW x 0.97 x 0.97 = 212.5kW, or 212,500 W). If the motor is rated at 400V peak, then: 212,500 Watts = 400V x Current. Solving for current you get a value of around 531 Amperes (that is 212,500 W divided by 400V = 531 A). This was a very simple calculation, but it is enough to give you an idea.

Make sure all your components can handle this maximum current to avoid malfunctions or even safety concerns!

Determining required cell capacity (in Ah)

Cell capacity is the amount of energy, measured in Amp-hours (Ah), that can be stored in the cell, and is a very important specification that drives your cell selection.

If you already know your maximum current requirement and the number of cells in series and parallel, determining the required capacity of each cell is straightforward. Since cells connected in parallel share the load, the total battery pack capacity is divided among them. This means that the more parallel cells you have, the less strain is placed on each individual cell, allowing for lower discharge rates and better efficiency.

For example, if your total battery pack requires 200Ah to meet your range and power needs and you have 5 parallel strings, each cell must have at least 40Ah of capacity (200Ah / 5 parallel strings).

Since all cells in series share the same capacity, if you have 95 cells in series, and each cell has a capacity of 40Ah, then the capacity of that one string of cells in series is only 40Ah. Selecting a cell with slightly higher capacity provides a safety margin and ensures consistent performance over time.

Determining the C-rating of the cells

C-rating is, in essence, the “power rating” of the cell. The higher power, or higher discharge per cell that you want, the higher the C-rating you should look for. Battery packs with less cells, but higher C-ratings can feed higher powered motors, so if power is what you’re after, but not range, look for higher C-ratings.

From the previous examples you now know that the battery pack has a capacity of 200Ah, it has 95 cells in series and 5 parallel strings, each cell must have a capacity of 40Ah and the peak current expected is 531Amps. 

It is now time to calculate the C-rating, or how quickly the battery cells can discharge safely.

Since we have 5 strings in parallel, the peak current of 531Amps will be split among them, so each parallel group should be able to provide a current of 531A/5 parallel strings= 106.2Amps. 

Now, the C-rating is the maximum cell current divided by the cell capacity, which in this case is 106.2A/40Ah = 2.65C.

For this example, it’s better to use a C-rating slightly above 2.65C as a margin for safety.

To summarize:

Low C-rating = Good for steady energy use, not for high bursts.

High C-rating = Can handle power surges but may have lower overall capacity.

State of Charge (SOC) Limits

Certain cell chemistries shouldn’t be charged to 100% or drained to 0% regularly, as this reduces their useful life. Most systems operate within a 10-90% SOC window to prolong battery health, though in the context of major automakers, bottom and top SOC constraints depend on a lot of different things. As a rule of thumb, for Li-ion batteries, which happen to be the most common in the EV conversion world, use 10% as the absolute bottom SOC and 90% as the absolute maximum SOC. This will keep them healthy for longer.

Think of it like a smartphone—if you always charge to 100% and drain it completely, the battery degrades faster. Keeping within a safe range ensures longevity.

Key Takeaways

• The battery pack is the backbone of an EV conversion, impacting range, power, and longevity.
• Voltage must align with the motor and inverter’s requirements.
• Series connections raise voltage, while parallel connections increase energy storage.
• Different cell chemistries offer different trade-offs in weight, performance, and safety.
• Safety considerations like SOC limits and C-rating ensure longevity and reliable performance.

Final Thoughts

Now that you understand how a battery pack is selected, we hope the entire idea of an EV conversion is coming together in your mind. Like we mentioned in our first article: this is not a simple process, and should not be taken lightly. These calculations and estimations are laid out in such a way that the process can be planned ahead before spending resources: measure twice, cut once. We highly recommend that you do start off with a project that is simpler: you can always upgrade, and it’s easier to upgrade than to deal with a project that you no longer want to finish. 

Stay tuned for our next and last article of the Architect phase: Charging Systems for EV conversions, before we move on to the Route phase of SPARK.

Join the conversation: share your thoughts, ask questions, or tell us about your EV conversion journey or plans in the comments below. We’d love to answer any questions we can and/or learn from your experience.