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Batteries for Electric Vehicle Conversions

Ben Parker

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There are many different types of batteries that you can use in an electric vehicle conversion. In the early years of electric vehicle conversions the standard was just 12V lead-acid batteries like what you use to power a 12V system in a normal car. There are also deep-cycle lead acid batteries that are designed to discharge most of their capacity and are built to be more durable. Modern electric cars, and most EV conversions today use lithium-ion batteries. But in the proper arrangement you can power the vehicle with any kind of battery. Theoretically you could band a bunch of Duracell AA batteries together to power your conversion, but at 1.5 volts and 2 amp hours each it would take a lot to power your car. Understanding your power and range needs, and your space limitations will be the biggest determining factors on what type, and how many batteries you’ll need.

How many batteries do you need for an EV conversion?


The amount of batteries needed for an electric vehicle conversion depends on the needed power, range, and the space limitations of the vehicle.

Electric Motor and System Voltage


The motor is a good place to start when determining your battery set up. Electric motors are given ranges of input voltage, and that range will determine your system/battery needs. For instance the coveted Tesla Large Drive Unit has an input voltage range of 240V to 404V this motor makes around 450hp when given that maximum voltage. Let’s compare that with another popular EV conversion motor, the HPEVS AC-50 motor, it has an input voltage range of 48V to 144V it makes up to 72HP. So a conversion using the Tesla Large Drive unit would require a minimum system voltage of 240V, while the AC-50 would only need 48V. The higher the voltage, the higher the horsepower, as well as higher top speed. 


It’s important to note that different size conversions may require different voltages to get to the same speed. A motorcycle will only need around 72V to get above 60mph, while a typical sedan may require 120V. So what speed are you looking to get to with this conversion?


For example, let’s say the motor I’ve got is a Netgain Warp 9 Motor. The Warp 9 has a voltage range of around 48V to 170V peak voltage. Let’s say the car I’m using for the conversion is a 1970 Volkswagen Beetle. This is a typical sized sedan, it’s relatively light, so I should be able to achieve 60mph using around 120V.

How much range can an electric vehicle conversion get?


Electric vehicle conversion range is determined by the efficiency of the vehicle, the system voltage, and the system amp hour rating. 


Let’s try to figure out the range for the 1970 Volkswagen Beetle. So the first step is determining the efficiency of the vehicle. Vehicle efficiency is essentially a measure of how easily your vehicle moves accounting for weight, aerodynamics, and rolling resistance. This is measured in Watt-hours per mile or Wh/mi. It’d be really tough on paper to calculate the hypothetical Wh/mi of our conversion so instead we can find someone else with a similar conversion and use their Wh/mi as an approximation for our own. 


A good place to look is EValbum which is a collection of a bunch of electric vehicle conversions uploaded by people who have converted cars themselves. I did a quick search for “beetle Wh/mi” and found this one. A 1973 Super Beetle converted by Jon Glauser. He’s added his Wh/mi of 262Wh/mi, his build is a bit different from what ours will end up being, but this is a good starting point. 


Next we need to get our system voltage. With our Warp 9 motor, we know we can use up to an 170V system, but we shouldn’t max the motor out as Netgain says that 170V continuous can cause issues. We’ve already approximated that we should be able to reach 60mph at around 120V, but let’s say 144V to be safe. So we’ll set that as our system voltage for now. 


So what do we want our range to be? Well let’s say this is a short commuter car we’d like to drive it roughly 60 miles. Using our approximated efficiency rating, and our desired range we can figure out how much power in kilowatts we’ll need to achieve that range. 


Here’s the equation: 


Efficiency x desired range = energy required to reach that range


So plugging in our numbers:


262Wh/mi x 60 mi = 15,720 Watts or 15.7kWh (kilowatt hours)


So 15.7kWh is the amount of energy we’ll need over time to get the range we want. We already know our system power will be 144V so, now we need to know how much battery we need. This is measured in Amp hours or Ah, to get the needed Ah we’ll divide the energy needed by our system voltage. 


Here’s the equation: 

    

Energy required to reach desired range / system voltage = How much battery we need in Ah


So plugging in our numbers:


15,730 Watts / 144V = 109.23 Ah (let’s round it up to 110 Ah)


So, for our motor to get up to the speed that we want we’ll be using an 144V system. To get a range of 60 miles our system will need to be roughly 110 Ah. But there are some things to consider depending on the type of battery you’ll be using. Most batteries should not be fully discharged so we should account for some added energy need so we never actually fully discharge our batteries. For this example let’s assume we’re using lithium-ion batteries because they’re the best. To account for this we’ll multiply our Ah by 1.32.


110 Ah * 1.32 = 145.2 Ah (or just 145 Ah) 


So there we have it. These two numbers are the two most important when figuring out your battery setup. With these two numbers we can begin to test different configurations of batteries to achieve the power, and range we require. 


We need 144Ah and 144V.

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Let’s say for instance we’d like to use the super expensive Tesla battery modules in this conversion. They’re incredibly power dense. One module is 24V and 250 Ah. Well we need at least 144V, so how can we make that happen? Can you just slap a few of them together to achieve that? Yes and no. 


The way you connect and arrange batteries has different effects on the final voltage and amp hours. There are two ways to connect batteries, in parallel, and in series. 


How do you connect batteries in parallel?


To connect a battery in parallel, connect positive terminals together and negative terminals together. Connecting batteries in parallel will increase the system amp hours. 


To connect a battery in series, connect positive terminals to negative and negative terminals to positive. Connecting batteries in series will increase the system voltage. 


So with our Tesla Module we don't need to increase the Ah because it’s got 250 Ah, but we do need to increase the voltage. So with each module being 24V we’ll need 6 modules connected in series (144V / 24V = 6)  to create a total voltage of 144V. So our module arrangement will be 6 in series 1 in parallel written as 6s1p. 


So how much will that cost? Well each module is roughly $1,200 (at the time of writing) so altogether that’s $7,200. Which is about the cost of a whole 1970 Beetle. For most of us Tesla Modules are out of the price range.


So what batteries can you use in an electric vehicle conversion? 


Chevy Volt Battery Modules 


Well another popular option is the Chevy Volt battery. They come in a variety of arrangements, but let’s see if we can arrange some to meet our needs. One module has 50V and 45Ah and costs roughly $350, so we’ll need to put three in series to achieve 144V and to get the right Ah we’ll need at least 3 to even get close to our desired range. That puts us at 3 in series and 3 in parallel. That makes 9 batteries and costs $3,150. This is where it gets tricky. With 3 in parallel we’ll only have 135Ah. So if we want to get to at least 145Ah we’ll need to add one more battery in each parallel for 3 in series and 4 in parallel. That makes 12 batteries and costs $4,200. Maybe let’s try another set up. 


Nissan Leaf Battery Modules


These are another wildly popular battery for electric vehicle conversions as these modules are slim, light and easily maneuverable. Each module is about 8V and 40Ah, and costs roughly $52. So we’ll need 18 in series and 4 parallel. That makes 144V with 160Ah from 72 modules costing $3,744.


Fiat 500E Battery Modules


Another good option, these are about the size of a standard 12V battery. These are 18V and 64Ah and cost around $180 each. So we’ll need 8 in series and 3 in parallel. That makes 144V and 192Ah for about $4,320. This is tough because it’s more Ah than we need or planned for, and we’re paying for it.  


This is what makes it hard to standardize EV conversions. It’s also the reason that more adventurous converters build their own battery configurations from scratch. 


It seems like from all these options off the shelf the Nissan Leaf is the cheapest option that matches our needs, but not by much. You can see how it may be different depending on your own specific needs. We can also adjust our desired range, or our top speed numbers to change the final battery needs.

Accounting for Size and Weight


The Tesla battery modules are incredibly power dense, and that makes them a great candidate for conversions because their design makes them easy to stick in the trunk of a car, or the engine bay. Whereas the Chevy Volt modules are bigger, bulkier, and are harder to hide in a trunk or engine bay. Size is something else that should be accounted for when considering the batteries that you choose. Will your car be able to fit the batteries that you’re choosing? Can you safely secure these somewhere in the car? 



What batteries should I get for an EV conversion?


There is not one correct answer. The battery you choose will be dependent on your budget, power, range, and size needs. 


The batteries are often (if not always) the most expensive part of a conversion. So a lot of considerations and planning should be done before deciding on one. 


More considerations


When dealing with the voltage level required by electric vehicles, batteries need to be cooled, monitored, and kept safe. The batteries can be monitored using a Battery Monitoring System or BMS. BMS’ can monitor and limit charging input, energy output, temperature and more. Some popular brands are Orion, or Dilithium. Getting a BMS that works in tandem with your charger can be a huge headache reliever as well. 


Heating and cooling the batteries can be done using air or liquid. EVWest often uses liquid cooling done through chill plates in the battery box. A radiator can be connected to the 12V system and controlled by the BMS to regulate temperature, using sensors called thermistors. 


Conclusions


There is no silver bullet to determine the battery pack for your conversion. Luckily as more cars are being made electric, more battery options are becoming available for hobbyists and enthusiasts to choose from. There are more options than I’ve listed and many places to look for batteries. I use ebay, facebook marketplace, diyelectriccarforum, and all the EV conversion retailers I can find on google. Deals are few and far between. Hopefully more batteries become available and the prices go down. For now we can just fight amongst ourselves over the few available for reasonable prices.