ELECTRIC VEHICLES

The Technology: How it works

BATTERY SYSTEM

Today, the battery of an electric vehicle is the most restricting component on the vehicles performance.  The battery controls the range, acceleration ability, and recharge time for the vehicle.  Because the battery contains the energy to fuel an electric vehicle, and because today's batteries don't provide EVs with the same range and power as ICE vehicles, batteries are the most heavily studied areas in electric vehicle technology.

How a battery cell works

A battery mainly consists of a positive and negative electrode, and electrolyte, and a separator.  The electrodes allow the charges to flow, the electrolyte allows charges to flow between the electrodes, and the separator isolates the electrodes. 

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When the battery becomes part of a circuit allowing charge to flow, the flow is a result of oxidation and reduction reactions occurring at each electrode.  Electrons flow from the negative electrode to the positive electrode, and the voltage of a cell is fixed the chemistry of the oxidation/reduction reactions.  For example, a lead-acid cell has a fixed voltage of 2.04.  While voltage is fixed, cell capacity depends on the quantity of active material it contains. 

Because EV's require a tremendous amount of electric energy, batteries are wired in various different ways to meet the needs of the vehicle.  When batteries are wired in series, the voltage is the product of the number of batteries and the voltage one of the batteries.  When batteries are wired in parallel, the voltage stays the same. 

example of multiple batteries wired together

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Different types of batteries.      

There are many different types of batteries in use today in electric vehicles.  Some of the common ones include flooded lead-acid, sealed gel lead-acid, Nickel Cadmium, and Nickel Metal Hydride (NiMH).  Each type of battery is very different, and has characteristics that make it more or less desirable in certain applications.  When designing and EV, battery types, sizes, and configurations allow for a wide variety of options.  Space, weight, range, cost, rechargability, and power are some of the main factors manufacturers must choose from when choosing a battery for a car. 

Cost is usually the most important factor, with flooded lead-acid batteries being the cheapest, while NiMH batteries are the most expensive available today.  Despite their cost, NiMH batteries have nearly twice the performance (range and power) or regular lead-acid batteries.  While the range of a lead/acid battery is 70-90 miles, mickel/metal batteries range from 140-180 miles per charge.  Still interesting to note, a tank of gasoline contains nearly 100 times more energy then an equal mass of lead/acid batteries.  Energy density and power density are two important characteristics of a battery because they define the range and acceleration capabilities of the vehicle.  Along with energy considerations, cycle life is also important to know because, in general, it determines how long a battery will last.  The cycle life is the number of charge-discharge cycles the battery can have before the capacity diminishes from the original 100% to 80%.  Another factor for consideration is recharge time.  Lead-acid batteries usually take 6+ hours to charge and can not be charged any faster because it will damage the battery, while other newer batteries can recharge in nearly 30 minutes.  Below is a table of the performance for various vehicle battery systems. 

Performance of Vehicle Battery Systems

Table courtesy of http://www.etvi.org/High_Tech/New_Electric_HiTech_right.html

Check out this link for the most current battery technology being explored that could turn EV's into the vehicle of the future.  http://www.sdge.com/EV/Benefits/batteries.html

Charging

There are two main methods in which electric vehicles are recharged.  Inductive charging has no direct electrical connection to the vehicle but instead uses a weatherproof paddle that transfers power to the car with induction from the magnetic field created.   Conductive charging on the other hand uses metal-to-metal contact to supply the power link to the car just like any electrical plug. 

Example of inductive charging

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Source: http://www.rquiley.com/ev-tech.html
Source: http://www.etvi.org/High_Tech/New_Electric_HiTech_right.html