Choosing the right battery for an electric vehicle (EV) conversion is a crucial step in the conversion process. If the battery pack is not properly paired with the drivetrain, you will not be able to achieve the desired performance and range. You also run the risk of damaging the drivetrain components or the batteries themselves.

Important factors when choosing the right battery

The key words when choosing the right battery are the required power and the range for your electric vehicle. These factors determine the design of the battery pack. Also the available space for a battery pack is important. In this article, we will help you map out the most important battery requirements for your EV conversion.

Required range

The range you want for your EV will determine the final size of the battery pack. Therefore, you need to decide on the theoretical range you need to convert this to the capacity in kWh. This is an important requirement to keep in mind when designing the battery pack.

The range you want to have between charges determines the battery capacity you need. For example, a Tesla car uses 0,2 kWh per km. For a car, you can estimate the capacity you need for your desired range by multiplying the number of kilometers you want to drive by a factor of 0,2. This will give you a rough estimate of the minimum capacity of your battery pack.

Calculation of battery power

The power you want for the EV will determine the type of batteries you should use in the battery pack. peak power that the engine demands from the battery pack determines the maximum discharge current of the batteries. The continuous power that the engine uses while driving determines the continuous discharge current of the batteries. Before you go any further, you should ask yourself the following questions:

1. What is the peak power of the motor? And what is the peak current that the motor will use? These two questions determine the maximum discharge current of the battery pack.
2. How much power do you use continuously? This determines the continuous flow of the batteries.

If you plan to connect all batteries as one serial string with one battery in parallel, the values ​​mentioned above are the maximum discharge values ​​for each battery (module).

If you decide to connect two or more batteries in parallel, the discharge currents must be multiplied by the number of batteries connected in parallel to calculate the maximum discharge current of the battery pack.

Examples of battery power calculations

Example 1:

• 20 batteries in series, 1 battery in parallel
• Peak discharge: 30A per battery
• Continuous discharge: 15A per battery
• Maximum discharge capacity of the battery pack is 1 battery in parallel x 30A = 30A
• Continuous discharge capacity of the battery pack is 1 battery in parallel x 15A = 15A

Example 2:

• 20 batteries in series, 4 batteries in parallel
• Peak discharge: 30A per battery
• Continuous discharge: 15A per battery
• Maximum discharge capacity of the battery pack is 4 batteries in parallel x 30A = 120A
• Continuous discharge capacity of the battery pack is 4 batteries in parallel x 15A = 60A

For further calculations you can use our Power Battery calculator use to quickly find the number of modules you need for your required power.

Operating temperature and battery chemistry

What temperature range do you plan to use the EV in? If the battery pack is exposed to temperatures below zero degrees Celsius, this will affect the chemistry of the batteries.

There are two different types of battery chemistries that are suitable for EVs: lithium-ion (Li-Ion) batteries en lithium iron phosphate (LiFePO) batteries. The Li-Ion batteries have an operating temperature range between 10 – 60 degrees Celsius. The LiFePO batteries have an operating temperature range between -10 – 60 degrees Celsius.

For applications where operating temperatures below zero degrees are common and there is no room for additional systems such as a heating element, LiFePO batteries are the most suitable.

For applications where temperature is not a problem or where low power is discharged from the battery pack (causing temperature increases), Li-Ion batteries are most beneficial.

Lithium-ion is the battery with the highest energy density that is currently available. Even when external systems are added to keep the batteries within their temperature range, the energy density, including the weight and volume of these systems, is still higher than when using LiFePO batteries. This means that in most cases Li-Ion batteries are the preferred choice.

For both chemistries it is important to always keep the batteries within their operating temperature range. If the batteries are used outside this range it will affect the life and capacity of the battery pack. It is dangerous if the temperature of the batteries rises above 60 degrees Celsius, as fires and explosions can occur.

Battery pack spatial limitations

The space available for the battery pack will influence your choice of battery chemistry. Li-Ion battery chemistry has a higher energy density than LiFePO. This means that LiFePO batteries must be larger to achieve the same output as their Li-Ion counterpart.

De available space for the battery pack is an important factor when designing your battery pack. The design of the box in which the battery pack is placed is often a puzzle. When the required power is known, you can choose the battery/module you are going to use. Then you also know how many batteries/modules must be connected in parallel to achieve the desired power. The rows of batteries/modules must be a multiple of the number of batteries connected in parallel. Otherwise it will be difficult to connect all the batteries.

The voltage determines the total capacity and power of your battery pack. If you have already chosen the other drivetrain components, such as the motor and controller, you will know the voltage you need. Each battery connected in series contributes to the total voltage.

Here are some formulas to calculate battery pack capacity and power:

• Capacity = capacity per battery x number of batteries connected in parallel x nominal voltage
• peak power = peak current per battery x number of batteries connected in parallel x nominal voltage
• Continuous power = continuous current per battery x number of batteries connected in parallel x nominal voltage

These formulas will help you calculate what each battery layout means for the dimensions of your battery pack.

Once you have gathered all the information, you should be able to design and build the right battery pack for your EV. If you run into any problems or dilemmas along the way, you can always seek advice from us.

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Tailor-made advice for your EV project

Choosing the right battery for your electric vehicle can be quite a challenge. At Power Battery we develop, test and manufacture battery packs and modules.

Please feel free to contact us if you have any questions about your project or request a consultation.

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