How to choose the right battery for your electric vehicle conversion?

Choosing the right battery for an electric vehicle (EV) conversion is a particularly important step in the EV conversion process. If the battery pack does not match the drivetrain, the desired performance and range cannot be realized and there is also an additional risk of damaging the drivetrain components or the batteries themselves.

Keywords in choosing the right battery are the required power and range for your electric vehicle. The required power and range determine the design of the battery pack. Also, the space available for a battery pack is important. In this article we'll help you mapping out the important battery requirements for your EV conversion.

Required driving range

The range that you want for your EV determines the final size of the battery pack. Therefore you need to decide on the theoretical range you will need in order to convert this to the capacity in kWh. This is a key requirement to keep in mind when designing the battery pack.

The range you want to have between charging, determines the battery capacity you need. For example: a Tesla car uses 0.2 kWh per km. So, for a car you can estimate the capacity you need for your desired range by multiplying the amount of km you want, with a factor 0.2. That will give you a rough estimate of the minimum capacity of your battery pack.

Battery power calculation

The amount of power you want for the EV determines the kind of batteries that you’ll need to use in the battery pack. The peak power the motor demands from the battery pack determines the maximum discharge current of the batteries. The continuous power the motor uses whilst using the EV decides the continuous discharge current of the batteries. Before you proceed, you need to ask yourself the following:

1. What is the peak power output of the motor? And what is the peak current the motor will use? These two questions decide the maximum discharge output of the battery pack.

2. How much power will you continuous be using? This will determine the continuous current of the batteries.

If you are planning to connect all the batteries as one serial string with one battery parallel, the values mentioned above are the maximum discharge values for each battery (module). If you decide to connect two or more batteries parallel to each other, the discharge currents need to be multiplied by the number of batteries connected parallel, to calculate the maximum discharge current of the battery pack.

To clarify the kind of calculations involved, we have two examples prepared for you.

Battery power calculation - Example 1

20 batteries serial, 1 battery parallel
Peak discharge: 30A per battery
Continuous discharge: 15A per battery.
Maximum discharge power of the battery pack is 1 battery parallel x 30A = 30A
Continuous discharge power of the battery pack is 1 battery parallel x 15A = 15A

Battery power calculation - Example 2

20 batteries serial, 4 batteries parallel
Peak discharge: 30A per battery
Continuous discharge: 15A per battery.
Maximum discharge power of the battery pack is 4 batteries parallel x 30A = 120A
Continuous discharge power of the battery pack is 4 batteries parallel x 15A = 60A

For further calculations, use our Power Battery calculator to quickly find out the amount of modules you need to fit in your battery pack for your required power output.

Operating temperature and battery chemistry

In which environmental temperature range are you planning to use the EV? If the battery pack needs to deal with temperatures below zero degrees Celsius, it will affect the chemistry of the batteries.

There are two different kinds of battery chemistries that are suitable for EV’s, the lithium-ion (Li-Ion) batteries and the lithium-iron-phosphate (LiFePO) batteries. The Li-Ion batteries have an operating range between 10 – 60 degrees Celsius. The LiFePO batteries have an operating 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 an issue or low power is discharged from the battery pack (causes raise of temperature), Li-Ion batteries are the most favorable.

Lithium-ion is the battery with the highest energy density 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 then when you use LiFePO batteries. This means that in most cases Li-Ion batteries are favorite.

For both chemistries it is important to always keep the batteries within their operating temperature range. If the batteries are used outside of this range, it will affect the lifespan and capacity of the battery pack. It’s hazardous is the temperature of the batteries rises above the 60 degrees Celsius, because that’s when fires and explosions can happen.

Spatial constraints for the battery pack

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

The available space for the battery pack is an important factor when designing your battery pack. The design of the box wherein the battery pack is placed is often quite a puzzle. When the required power is known, you can choose the battery/module you are going to use. Then you also know how much batteries/modules need to be in parallel to achieve the desired power. The rows of batteries/modules need to be a plural of the number of batteries that are in parallel to each other. 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, like the motor and controller, then you know the voltage that you will need. Each battery that is connected in serial adds up to the total voltage.

Here are a few formulas to calculate the capacity and power of the battery pack:

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

With these formulas you can calculate what each layout of batteries will mean for your battery pack’s dimensions.

When you have all the information together, you should be able to design and build the right battery pack for your EV. If you run into some problems or dilemma’s along the way, you can always get advice from us.

Custom advice for your EV project

In short, choosing the right battery for your electric vehicle can be quite a challenge. At Power Battery we develop, test and produce battery packs and modules.

Please contact us if you have any questions regarding your project, or request consultation.