Replace SLA Batteries with Li-Ion Technology

Mar 1, 2010 12:00 PM
Robin Tichy Technical Marketing Mgr. Micro Power Electronics Beaverton, Ore.

Recent innovations in Li-ion chemistry have made the technology extremely competitive in markets that are weight-sensitive and inconvenienced by sealed lead acid's need for frequent maintenance.

Applications with high-voltage, high-capacity requirements are adopting lithium-ion (Li-ion) technology because of its high energy density, small size, and low weight. Using Li-ion for portable equipment offers many advantages over older rechargeable technologies.

Li-ion battery characteristics include a nominal voltage of 3.6 V, thousands of duty cycles per lifetime, charge times of less than three hours, and a typical discharge rate of approximately 10% per month when in storage. Fig. 1 illustrates that Li-ion technology offers a pronounced energy density advantage with respect to both volume and weight.

It's also important to note the size of the Li-ion bubble; it represents the many flavors of Li-ion available on the market. The specific characteristics of each Li-ion cell's chemistry — in terms of voltage, cycles, load current, energy density, charge time, and discharge rates — must be understood in order to specify a cell that is appropriate for an application.

Historically, sealed lead acid (SLA) batteries have had a few superior technical traits, in addition to their extremely low cost, that have kept them in the lead of the overall battery market. Li-ion and SLA battery markets are expected to grow over the next several years, but Li-ion is expected to overtake SLA in some areas.

Li-ion battery systems are a good option when requirements specify lower weight, higher energy density or aggregate voltage, or a greater number of duty cycles. Conventional Li-ion chemistry, designed for portable applications like laptops and cell phones, is designed to offer the highest energy density by size and weight.

Typically, these applications do not have high current requirements and are relatively price sensitive, so conventional Co-based Li-ion cells are appropriate for applications that need to be smaller and lighter. Newer Li-ion chemistries are optimized around the power tool and electric vehicle markets.

These Fe-phosphate-based cells have remarkable cycle life and current-delivery capability, but their volumetric energy density is less and upfront cost is greater. They are more amenable to direct use of an SLA charger and are appropriate for replacement of SLA technology when total cost of ownership and weight reduction are the primary objectives.

Batteries use a chemical reaction to operate and produce a voltage between their output terminals. The reaction of lead and lead oxide with the sulfuric acid electrolyte produces a voltage in a lead acid battery.

SLA CONSTRUCTION

An SLA cell has one plate of lead and another of lead dioxide, with a strong sulfuric acid electrolyte in which the plates are immersed. The characteristic voltage of the creation of lead sulfate is about 2 V per cell, so by combining six cells you get a typical 12-V battery.

Fig. 2 is a discharge curve for SLA batteries; note the almost linear downward slope. The relationship between the discharge times (in amperes drawn) is reasonably linear on low loads. As the load increases, the discharge time suffers because some battery energy is lost due to internal losses. This results in the battery heating up.

The efficiency of a battery is expressed in the Peukert number, which, in essence reflects the internal resistance of the battery. A value close to 1 indicates a well-performing battery with little loss. A higher number reflects a less efficient battery.

SLA batteries are most stressed if discharged at a steady load to the end-of-discharge point. An intermittent load allows a level of recovery of the very chemical reaction that produces the electrical energy. Because of the rather sluggish behavior, the quiescent rest period is especially important for lead acid. There is an advantage. The advantage of this curve is a simple voltage measurement that can be used for fuel gauging.

Li-ion CONSTRUCTION

The three primary functional components of a Li-ion battery are the anode, cathode, and electrolyte. Lithium ions move from the negative electrode (cathode) to the positive electrode (anode) during discharge, and from the cathode to the anode when charged. A variety of materials may be used for each internal cell component; the most popular material for the anode is graphite, but some manufacturers use coke.

Depending on the choice of material for the anode, cathode, and electrolyte, the voltage, capacity, life, and safety of a Li-ion battery can change dramatically. The electrochemical reaction produces about 3.5 V depending on the chemistry and brand, so four cells in series can produce a range of nominal voltages from 12.8 to 14.8 V.

The electrolyte is a non-aqueous solution of a lithium salt. The cathode is generally one of three materials: a layered oxide (such as cobalt oxide), one based on a polyanion (such as iron phosphate), or a spinel (such as manganese). Battery packs made with Li-ion are not a simple configuration of cells. They are carefully engineered products with many safety features. The main components of a battery pack include the cells, which are the primary energy source; the PC board, which provides system intelligence; the plastic enclosure; external contacts; and insulation. The internal features of a battery pack are shown in Fig. 3.

DIRECT REPLACEMENT OF SLA WITH Li-ion

The Table compares Li-ion battery packs made with traditional Co-oxide chemistry cells and SLA batteries. The first column features six SLA batteries in series and two in parallel. The following two columns are two Li-ion configurations of Li-ion 18650 cells: 4S 2P and 3S 6P, designed to give similar performance and run-times to the SLA.

The series configuration determines the voltage, and paralleled cells determine the capacity. Runtimes are similar, but Li-ion batteries occupy about one-fifth the volume and about one-seventh the weight. Unfortunately, packs made from conventional chemistries are not compatible with SLA chargers.

COMPARING CHEMISTRIES

Making a direct comparison of lead acid batteries and Li-ion batteries is difficult. The operation of the cells is so fundamentally different that direct replacement and comparison is hard.

SLA run-time is determined not only by capacity, but is also highly dependent on rate, as seen in Fig. 2. In addition, SLA batteries cannot be fully discharged. The voltages are not well matched for the two chemistries.

The benefits of new high-rate Fe-phosphate cell chemistries include increased safety, low impedance and high discharge rates, and a voltage that matches well with SLA technology at 12- and 24-V increments. These design features allow the use of a conventional SLA charger. In Fig. 4, one can see the performance of the A123 cells, an example of the high-rate cells. The cells deliver virtually full capacity at a high rate of 30 A. Here we can see how flat the discharge voltage of a Li-ion cell is, which represents a challenge for fuel gauging.

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