Drive Train - Battery Measures

How to measure (and compare) the batteries used in the drive train of various electric vehicles? With the various types of electric vehicles around and the various specs they have for their battery packs, one would prefer to have a uniform measure to compare those batteries with.  

 

The first obvious parameter is the amount of power, or rather, charge they can contain. The unit to describe this is the kWh, or kilo Watt hour. This figure is a measure that describes how much power the battery can supply over a given time. If for example a battery is 24 kWh, like the Nissan Leaf, this implies that it can deliver 24 kW for one hour before it is empty. It can also supply 12 kW for 2 hours, of 6 kW for 4 hours. You get the idea. This is by the way not taking into account any physical/chemical constraints of the battery. If you discharge a battery at a too high power rate, it might overheat and get (permanently) damaged, or even melt!

 

The second obvious measure in the electric vehicle, is weight. For an electric vehicle, less weight equals more range. This is the main reason companies like Ford and BMW are investing a lot of their effort and funding into carbon fibre components to create really light vehicles. For comparison, the Tesla Roadster is also mostly carbon fibre. Less weight equals more range, so a lighter battery with the same charge is getting your more range for your charge. If you would divide the charge by the weight, you end up with a unit which is referred to as the power density. It is a unit in kWh / kg. The more kWh per kg you can achieve, the better.

Source 1: All about batteries Source 2: Wikipedia - Lead Acid Battery

Note: From one of my appreciated reader's comments, I've been corrected on the energy density of the lead acid battery. For now, I've put both values in and keep both sources, but I'm inclined to believe Wiki's value a bit more. In the end, the Lithium types of batteries are far superios when it comes to EV applications, but it is always nice to have the right values to compare with. Thanks Wolfgang!

The third thing to measure batteries by is the volume they use in a car, not counting the battery temperature management system. A good example in this case is a supercapacitor, or supercap. These devices have a very good ratio of kWh/kg, but they take up a large volume in the car before you have an amount of power that is comparable to a decent battery. In this ratio one looks at the most available power, at the least amount of weight and volume. In practice, the volume of the battery in an electric vehicle is not the primary focus, weight has a far bigger impact on the vehicle (performance) than the volume has.

The last, and perhaps currently most important, measure is the financial cost. Some battery types are more expensive than others. You can get the best battery, the most power, the least weight and the smalles volume, but pay the highest price. It is always a matter of finding the right balance, the table below triest to be a quick reference in this comparison. Unfortunately I have not been able to find an up to date source for pricing information for electric vehicle batteries. Most prices that are currently used on the internet are a few years old and vary from 1000 to 450 USD per kWh for an electric vehicle battery pack. If anybody can help me out and supply some of the prices of the various battery types it would be much appreciated by me.

Electric Vehicle - How to build an electric car 03

Selecting the electric motor and battery when building your own electric car can be quite a challenge. There are a few approaches here that can be followed in this post I’ll just touch on the easiest of all, a good look at the alternatives and some educated guessing. This approach is to see what the others are doing and check what power the engines are that are used in similar vehicles. If you plan to convert a sedan like vehicle that is roughly the same size/weight as a Nissan Leaf, check the power that those engines are specified for. (the Nissan Leaf has a motor which is rated at 80kW btw, also check Wikipedia).

 

The Nissan Leaf, you can learn a lot from the current EVs already out there.

 

The idea behind it is simple, they did the math, and it works good (they are selling the product). If you want something that can go quicker or accelerate quicker, you’ll want to look for a little bigger engine, but you get a feel of the industry at least and know what you should be aiming for.

Always make sure that the vehicle you are working on and the vehicle you are looking at for the engine specification are reasonably the same. In other words, don’t look at the Tesla Roadster if you want to convert a heavy SUV, the weights are far off, but also the aerodynamics of the two are not comparable.

 

For the battery a similar approach is viable, check what comparable vehicles have for the battery capacity (the kWh figure!) and ensure that the battery/motor are compatible. An electric motor that runs on 48V will be no good using a single 12V battery. You can check again what the size of for example the Nissan Leaf is (24kWh) and use it as a guideline. The Nissan Leaf can travel around 100 miles / 160 km. If you have a comparable vehicle as the Leaf and put in a 12 kWh battery pack, you will likely get around half the range of the Leaf. This is not entirely a linear scale, but ok for some educated guessing as long as you don’t use a factor 10 bigger or smaller batteries.

 

Another nice guideline that might help is that the average passenger cars use between 20-30 kWh per 100 miles / 160 km. If you have a relatively light vehicle, you’re bound to be on the low end, if you have a heavier, you’re on the high end of this range (or even higher if it is a really heavy vehicle).

 

You can save a lot of fuel by converting this one to electric, just don't put in a Leaf-like drive train.

 

Assume you have a light vehicle, and want to be able to drive 300 miles with it. Assuming a 20 kWh consumption per 100 miles / 160 km, you would need a battery that supplies 60 kWh. Batteries are preferably not fully discharged, and are operated generally in a safe range, so it is more likely you will need 70-80 kWh pack to ensure you have 60 kWh available to drive. The problem that arises is that with battery packs of this size, the weight plays a big factor. If you have to add a battery pack that weighs half that of the original light weight vehicle, your consumption will go up more. This makes for an iterative process where you have to balance the motor and the battery pack versus what you want and what vehicle you have.

 

Not an easy process and one that can be quite time consuming as well. Luckily there are various companies around, mostly US or Australia based, that offer so called electric car conversion kits. As the name implies, it allows you to convert your petrol vehicle to an electric one. There are even some that are tailored to specific models. The biggest advantage of this: you get a fully matching system that is suitable for your vehicle. Why do the guesswork if they found out and likely the hard way. They know what works and what not.

Electric Vehicle - How to build an electric car - 02

This guide is part two on how to build an electric car, continuing on the previous post. The drive train of a pure electric vehicle is the easiest configuration, the main components are the electric motor and the battery. True enough there are some other (vital) components that are required, such as a convertor between the electric motor and the battery.  

The rather simple configuration of an electric vehicle drive train.

 

The electric motor is there to move the vehicle forward, the battery is there to store the power. The convertor in the middle is used to control the amount of power that moves from the battery to the motor and ensure the voltages on each side remain within a specified range. For a more easier understanding of what the converter does, compare it with a valve in a water line; where the valve is turned more open when the a consumer wants to increase the flow of water. Similar, when a driver wants to increase speed, he/she pushes down the accelerator pedal which controls the converter. The converter then knows how much power to allow from the battery to the motor.

The electric motor is also used during the process that is called regenerative braking. This process allows to recoup the energy from braking or decelerating the car. Basically what happens is that the vehicle slows down on the friction of the motor, which acts as a generator. Power then flows from the motor through the converter and into the battery to be used at a later time. During this process the converter’s job is to ensure that the voltages on both end remain within the specifications and try to balance out between what amount of power is available and allowed at that time into the battery. The goal in this case for the converter is to recoup as much energy as possible, but protect the battery and ensure it stays in good condition. In case the power that is generated becomes too much for the converter or the battery to handle, mechanical brakes can be used to take out the excess energy.

With regards to what the specified conditions the converter has to keep the voltages; that really depends on the battery and the motor that are chosen in the configuration. Better is to first check what motor and battery are required/desired for the application and then check the specifications of the available models before looking at the converter.

To determine which type of electric motor to pick the two most important factors to consider are the total mass of the vehicle and the driving characteristic that is required or desired. The heavier a vehicle, the bigger the motor will have to be. Also include the added mass for the batteries and the optional large cargo loads that you want to take along with it. The method to determine the size of your motor/battery will be discussed in a next post.

 

 

Drive Train - Keeping the battery comfortable

The battery plays a significantly bigger role in the drive train of a hybrid or electric vehicle, than compared to an internal combustion engine based drive train. It is therefore that a lot more care and attention goes out to the battery; it is usually kept in a well insulated compartment and has a special control system to maintain a good temperature. The reason that this temperature needs to be controlled is to preserve battery life and keep the specifications as much as possible the same in all weather conditions. The battery degrades faster in temperatures that are too warm, but the chemical reactions slow down at too low temperatures. If the battery gets too warm this results in damaged cells, which is noticed as a loss in cell capacity, or simply put: it shortens your range. At cooler temperatures the electro-chemical reactions slow down. It can slow down to such a level that the battery can not hold up with the power demand, which you notice at first that acceleration is becoming slower. The better known effect is of an internal combustion car not being able to start in winter.

Basicly there are two types of control systems at the moment; one using air to control the battery temperature, the other uses a liquid system.

 

A battery pack from Tesla

 

With the air-based system, outside air is used to cool the battery. A benefit of this system is the low cost of the system and the low weight of its components. A downside is that air can not hold as much heat as for example water. It requires much larger volumes of air to get a similar amount of heat to be transferred out of a system, compared to a water based system.

With a liquis based system a (closed) circuit of (for example) water is used to transport the heat out of the system. The components required for such a system cost and weigh more, but you need a smaller flow to achieve the same; resulting in a more compact system.

 

Thermal images of batteries: some areas produce more heat than others

 

Another aspect that is often overlooked is the spread of the temperature inside the battery. Ideally the battery has the same temperature throughout the battery, not that one side is 10 degrees warmer than the other. The more uniformly the temperature can be kept, the better this is for the battery life. With an air based system it requires a substantial amount of air to ensure that every corner in the system is cooled at the same temperature. Compared to the liquid system, which more easily accesses every nook and cranny and thus can cool very much the same in every corner.

All in all I think from a technical point of view, the liquid based system is the better. This is probably also the reason it is used in many high-end/critical cooling systems. From a cost perspective the air-based clearly has its advantages and it remains to be seen how critical maintaining a battery temperature and an even spread really is. With the current batteries around and in particular their prices, the quality of this system is allowed to cost a bit more. However, if batteries get cheaper as the technology develops and more are being manufactured, using the cheaper air-based system might be a wiser solution.

 

Source: Earth2Tech

Source: NREL

 

Drive Train – (Material) Efficiency

 

When talking about efficiency for (hybrid) electric vehicles, one often refers to the mileage of the car or the conversion of electric power to mechanical power. The efficiency I’d like to discuss here is material efficiency, as two nice (potential) breakthroughs popped into my reader this week.

 

The first is regarding the platinum use in fuel cells, where with a special mechanism it is possible to use less of this precious material to construct fuel cells. With the current share of car manufacturers promising us to deliver fuel cell powered vehicles in the near future, that will sure have an impact on those plans (and the price of such vehicles ofcourse!). The second is regarding a material called grapheme, a material of one atom-thick which can come in sheets and is expected to have a huge impact in the electronics industry. The problem with this material is that the discovery of it is quite recent (Nobel Prize in Physics was awarded this year to Andre Geim and Konstantin Novoselov for their groundbreaking experiments with this material. At the moment everybody is trying to find a commercially viable method to produce it and it seems good old sugar might do the trick.

The core of this technology, palladium with a platinum coating

For the construction of a fuel cell platinum is required to play the role of the catalyst in the reaction. The early models used quite a substantial amount of this precious material, making them hideously expensive. With the current breakthrough of the US Department of Energy (DoE) Brookhaven National Laboratory it is expected that ony about 10 grams will be needed for a fuel cell in a vehicle. Conveniently that is the same amount that is currently used in the catalytic convertor that is on ICE cars now, to treat the exhaust fumes. Effectively this means that a fuel cell vehicle and a conventional ICE vehicle requite the same amount of platinum to manufacture.

 

The key in this breakthrough is the use of palladium-gold cores which are then coated with the platinum. In a normal fuel cell over time the platinum dissolves, taking out the catalyst of the fuel cell which needs to be replaced. With the palladium-gold cores, the palladium gets dissolved first, while maintaining the catalytic ability of the platinum and ensuring a longer lifespan for the fuel cell.

Common sugar, the key to advanced electronics?

Graphene hit the news earlier this year when Andre Geim and Konstantin Novoselov were awarded the Nobel Price for Physics for their experiments on the material. Their famous method to construct the material by using scotch tape and applying it to graphite to scrape of small layers has popped up in many news articles. This method is not something that can be implemented on a commercial scale though, for that the researchers of Rice Research have found a way to produce graphene using common sugar. This is a super cheap material to produce such potentially high tech products with. It is expected that this discovery of graphene will lead to lighter electronics altogether and since modern vehicles tend to contain more electronics with every new model, this will surely help losing weight (and gain some miles in range).

Additional info:

Fuel Cell Breakthrough article

Graphene on article Cleantech

Graphene on Wikipedia

Nobel Price for Physics 2010