Power Electronics

Replace SLA Batteries with Li-Ion Technology

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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Traditional fuel gauges for Li-ion have monitored either the voltage or the capacity, and accuracy has been quite limited due to that flat discharge curve we saw earlier. New gas gauges monitor the number of coulombs being transferred and opportunistically calibrate with the open circuit voltage of the Li-ion pack, allowing the end user to intelligently manage device use and avoid unexpected failures or shutdowns.

CHARGING ISSUES

For Li-ion batteries, constant current/constant voltage (CC/CV) is the only universally accepted Li-ion charging method. A constant current equal to or lower than the maximum charge rate is applied to the battery until the maximum charge voltage is reached. At that point, the operating mode turns to constant voltage output, which is maintained until the charge termination criterion is satisfied.

A Li-ion battery is fully charged when the maximum charging voltage has been reached and the falling value of the charge current is below a certain fraction — usually 1/30 to 1/10 — of the battery's maximum charge rate. There is a very good chance the SLA charge regime already embedded into the device or charging platform will not charge a Li-ion battery while maximizing safety, the capacity of the battery, or the cycle life of the battery.

CHARGING METHODOLOGIES

There are several charging methodologies for SLA batteries. Due to the versatility of SLA battery chemistry, charging electronics are simple and cheap, and numerous options exist, such as:

  • Trickle: charging at a rate equivalent to its self-discharge rate
  • Float: the battery and the load are permanently connected in parallel across the dc charging source and held at a constant voltage
  • Taper: either constant voltage or constant current is applied to the battery through a combination of transformers, diodes, and resistance. Current diminishes as the cell voltage increases
  • Constant Voltage Charging With Current Limit: constant voltage charging method, with current limiting, applies the maximum allowable charge voltage, but has a current limit to control the initial absorption current.
  • Three-Phase Charging: the most advanced charging method for SLA batteries. The first phase is bulk charging. When the preset voltage has been reached, the charger switches into the constant-voltage phase and the current drawn by the battery will gradually drop. The final phase is the float phase.

When a device manufacturer considers migrating from SLA to Li-ion battery technology, it has several options for placement of charge control electronics. If the customer base for the device will allow a wholesale change with the charger, the simplest way is to totally replace the SLA charge electronics with Li-ion charge electronics within the charging bay.

The new Li-ion charger is backward-compatible with the SLA batteries, but the old SLA charger is not forward-compatible with the new Li-ion battery technology. Lithium-iron phosphate provides a nice compromise for migration from SLA, as it will operate with most SLA charging methods. The notable exception with lithium-iron phosphate is that unchecked trickle charge will overcharge the cells.

FINAL CONSIDERATIONS

It's important to choose a battery-pack supplier with experience in the design, development, and manufacture of batteries for your industry. Most Li-ion battery packs are custom-made for each application.

Off-the-shelf battery packs can be purchased, but they are specifically made for high-volume laptops, cell phones, cameras, and other consumer electronics. Industrial, medical, and military equipment almost always use custom Li-ion products.

SLA batteries, in contrast, are almost always off-the-shelf products which have several common voltages and capacities. These can be purchased from many brands at many retail locations.

Li-ion is also more environmentally friendly than SLA. Europe is leading the way in enforcing environmental regulation, with the most relevant piece of legislation being the EU Battery Directive, which bans or sets maximum quantities of chemicals and metals in batteries. It requires proper waste management of these batteries, including recycling, collections, “take-back” programs, and disposal. Additionally, it establishes the financial responsibility for programs.

There is no overarching law regarding the recycling of batteries in North America, but Federal law requires that used Ni-Cd and lead acid batteries be managed as Universal Waste and 38 states have bans on the disposal of lead batteries, whereas Li-ion can be disposed of normally in most areas and can easily be recycled.

Fig. 5 conveys the economic tradeoffs between SLA and two Li-ion chemistry solutions. Of course, there are many details that go into this economic model, such as shipping costs and simplicity of battery replacement, so it is important to vet out the specifics before making a decision.

Over the product lifetime of about 10 years, the SLA would need to be replaced five times. A cobalt oxide pack with similar capacity would cost roughly twice as much but its cycle life is almost double that of the SLA.

The upfront cost would be more, but over the lifetime of the product the total cost may be lower. An iron phosphate Li-ion pack would likely be about three to four times the upfront cost, but the cycle life is so long that the solution will almost certainly have a lower cost over the life of the product.

PARAMETER SLA Li-ion Li-ion
Configuration 6S-2P 4S-4P 3S-6P
Volume (l) 1.85 0.34 0.38
Weight (kg) 4.94 0.67 0.76
Thickness (cm) 6.5 3.4 3.4
Voltage Range (V) 10.5 to 13.7 11.0 to 16.8 8.25 to 12.6
Run Time at 12W (hrs:min) 7:12 6:30 7:18
Run Time at 14W (hrs:min) 6:06 5:34 6:15
Run Time at 16W (hrs:min) 5:15 4:52 5:28
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