There is a need to advance the development of high energy density batteries, along with other efficient alternative energy sources. The need for batteries having a higher energy capacity, versus a lower weight, is simple to understand when stated in a battery’s Watt hour per kilogram rating. The focus of this work is on secondary or rechargeable batteries.
To help establish a point of reference, gasoline has an energy density rating of 13.200 Wh/kg to 13,700 Wh/kg, depending on the calculations used. The average efficiency of a typical internal combustion engine ranges from 28 to 32%. In comparison, the overall drive efficiency of the Tesla Roadster is 88%, almost three times more efficient than an internal combustion-powered vehicle, according to the Tesla Motor Company. Tesla is presently using 6,800 individual 18650 form-factor lithium-ion battery cells in its roadster, rated at 240 Wh/kg each. The weight of the entire battery pack is 993 pounds. The Tesla Roadster is capable of delivering over a 200-mile driving range. Depending on utility rates, the Tesla Roadster costs as little as $5 to charge, which is equivalent to $0.02 per mile.
The lead-acid battery is one of the oldest rechargeable battery technologies. Even with the advent of higher-capacity, low-weight batteries such as lithium-ion, the lead-acid battery, in one form or another, is one of the most commonly used battery types. It is found inside most uninterruptible power systems (UPSs), automobiles, utilities, telephones, and countless other applications. It is available in various designs and form factors such as in flooded stationary, wet cell, Valve-Regulated Sealed Lead-Acid (VRLA or SLA), Absorbed Glass Mat (AGM), and Gel Cell. The latter two battery types are differing designs of the VRLA battery. Lead-acid batteries typically have a reasonably wide operational temperature range. The typical energy density of lead-acid batteries is 35-41 Wh/kg. If used in the Tesla Roadster, the battery pack alone would weigh 5,813 pounds, significantly reducing driving range and stopping distance.
The first non-rechargeable lithium battery was commercially available in the 1970s. In the past 20 years, the largest amount of research and development has been focused on lithium-ion battery technology, and it has yielded the most results. It has also propagated a growing number of differing lithium-ion battery design topologies. The differences in lithium-ion battery designs are mainly associated with the materials used to construct the positive and negative electrodes inside the battery cells. For instance, LiFePO4 designates that the positive electrode is constructed from lithium, iron, and phosphate. This is done all in the quest to pack as many electrons into the negative cathode electrode of the battery as theoretically possible.
As lithium-ion battery energy densities increase, while the insulating materials separating the positive anode from the negative cathode become thinner, safety becomes a major concern. With new technology, there often comes the need for new regulations, and lithium-ion battery technology has required a good number of them. A few of the standards and mandates associated with lithium batteries are UL 1642, the 2009/2010 Edition of the ICAO Technical Instruction for the Safe Transport of Dangerous Goods by Air, and the 51st Edition (2010) of the IATA Dangerous Goods Regulations (DGR), UN 3090, UN3480, UN3091, UN3481. Primary lithium metal batteries and lithium-ion batteries with a rating greater than 20Wh/cell must be shipped as Class 9 Hazardous Materials, properly packed and identified.
There is an added cost to implementing lithium-ion batteries, as they require the addition of an advanced Battery Management System (BMS). The BMS (see figure) is usually microprocessor-based and performs several functions. First, it senses the state of charge of the individual cells and when charger power is present, performs a fast recharge of the cells, resulting in a sophisticated cell equalization balancing function. The BMS is the primary safety system for the cells, providing over-current and short-circuit protection. As the BMS is continuously monitoring the amount of power flow in and out of the connected batteries, it provides a very accurate gas-gauge like function informing the user on the amount of energy left in the batteries. In some cases, the BMS can predict when a battery is about to fail and give notification. The larger the battery pack, the higher the cost of the associated BMS. Therefore, this must be included in the overall cost of the lithium-ion battery system with few exceptions.
With the development of nanotechnologies and graphene, newer lithium-ion technologies with more energy density are on the horizon. Through the United States’ funding of Argonne National Labs, these technologies are helping companies develop lithium-ion batteries with energy densities of 400Wh/kg. They also increase the battery’s safety and reduce its cost dramatically when compared with the price of present lithium-ion battery types. As lithium- ion battery research continues, so does the research on the fuel cell. When cost-effective, the fuel cell will combine the efficiency of an all-electric vehicle with the convenience of a gas station.
This article was written by Michael A. Stout, Vice President of Engineering for Falcon Electric, Inc. 49745-124.