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.