Power Electronics

Fast Charge Becomes a Reality For Li-Ion Batteries

High-drain-rate Li-ion batteries developed for power tools accept high-charge currents, but alter the demands on the battery charger.

Consumers of frequent-use products, such as mobile phones, bar-code scanners and military radios, have been clamoring for fast battery charging since the introduction of rechargeable batteries. The adoption of lithium-ion (Li-ion) batteries in portable systems has decreased charge time significantly compared to nickel-metal-hydride (NiMH)-based systems, but traditional Li-ion cells still can only accept a 0.7-C charge rate.

However, the power-tool industry's demand for high-discharge-rate batteries with lighter weight and smaller and better cycle life than nickel-cadmium (NiCd) batteries has driven a few cell manufacturers to invest in the development of high-drain Li-ion cells. The impending elimination of NiCd — due to the hazardous-waste restriction imposed by the RoHS mandate — has greatly improved the chance of success for high-power Li-ions in the market.

With the emergence of this new variety of cells, there are now two types of Li-ion cells. One type is shaped by the demands for high capacity, while the other is developed to deliver high power for shorter periods of time. The latter variety of Li-ion cells can support the high-discharge currents required for many applications, and as a result, they will also support high-charge currents for fast charging.

There is no standard definition for high-drain-rate cells, but basic design guidelines dictate that standard cobalt-oxide-based cells can support a 2-C or maybe a 3-C rate, continuous current. High-drain cells based on cobalt-oxide support roughly double those currents, but only for seconds. The new high-drain cells support 20 C continuous.

Given that a high-discharge-rate cell can support high-current discharges over a very short period, in theory, a battery charger could fully charge that cell in an equally short amount of time. But to take advantage of this possibility, the conventional battery-charger design must be modified. For the sake of simplicity, these changes can be illustrated with the example of a single-bay charger supporting a single-cell battery pack.

Cell Characteristics

On the surface, fast-charging Li-ion cells seem straightforward. It seems that one could simply increase the current delivered during the constant-current phase of the charge cycle. However, as shown in the table, the overall charge time is not significantly decreased when the current is increased from 1 C to higher rates.

The difference in charge time with a 2-C rate versus a 3-C rate is only about one minute, regardless of the cell vendor. Essentially, the cells will just reach the upper-voltage cutoff faster, but the time in the constant-voltage charge mode will be much longer. Obviously, this increases the potential for damage to the battery due to overvoltage. The resistance of traditional Li-ion cells will cause them to heat up more during faster charges, so the cells will begin to break down. Fast charging significantly reduces the battery life cycle.

Designing a cell that can accommodate high-discharge and high-charge rates is an effort to reduce the path length and resistance for the transport of ions and electrons. Fig. 1 shows a cross section of a typical Li-ion cylindrical cell. Changes start with the battery's active materials. Traditional Li-ion cells are based on a lithium-cobalt-oxide (LiCoO2) cathode compound. In this material, Li-ions, which diffuse in and out of the cathode, can only be inserted through 2-D paths in the crystal structure.

The path length can be shortened by changing the physical morphology of the battery's active material or changing the material's chemical structure, or by doing both. One approach to addressing the problem physically is to decrease the particle size of the materials to as small as nano-scale. New chemistries such as manganese spinel (LiMn2O4) offer 3-D pathways for ion insertion.

In addition to these changes, the resistance of the cells must be lowered by using thin materials, increasing the amount of current collectors, and increasing the electrolyte concentration and reducing its viscosity with solvents. Many of these changes suggest that Li-polymer cells, which can be very thin, lend themselves for use in designing for high rates.

Li-ion cell manufacturers have been experimenting with their formulations in order to implement designs specific to high-rate applications. A few manufacturers have come up with solutions. E-One Moli Energy introduced a high-discharge-rate cell based on a manganese-spinel cathode material for cordless power tools.

As Fig. 2 shows, this cylindrical 26700 (26-mm in diameter × 70-mm in length) cell can support 80-A pulses for more than 10 seconds. This cell has been highly successful in power-tool applications, so Moli very recently introduced a spinel-chemistry cell in the traditional 18650 size.

The drawback of a high-rate cell is the lower capacity. The 26700 and 18650 have capacities of only about 2.9 Ah and 1.4 Ah, respectively. Another cell supplier, A123 has also introduced a cell that supports very-high-drain rates for the power-tool market.

Offered in 26650 and 18650 sizes, the A123 cell exploits nano-scale particles to achieve a performance that is very similar to the Moli cell, as seen in Fig. 3. The fundamental cathode chemistry is also different from the Moli technology, so the voltage is somewhat lower. The A123 has an operating voltage of 3.2 V instead of 3.6 V, because lithium-iron-phosphate (LiFePO4) material is used for the cathode.

Yet another cell vendor, Kokam has brought a polymer option to the market. This Li-polymer battery — which uses a polymer rather than liquid electrolyte — is able to draw up to a 20-C rate discharge continuously, with a peak discharge rate of 40 C. All of these new cells offer similar advantages and disadvantages. The cell impedance is low and the discharge and charge rates can be high, so fast charge, high power and fewer cells per battery pack may be possibilities.

Unfortunately, the energy densities for such cells are relatively low. Because the cells are new and somewhat uncommon, they are more expensive. Moreover, off-the-shelf safety circuits and fuel gauges are not yet available.

Charger Design

One of the first factors to be considered is how fast the user needs the cell recharged. Traditional cobalt-oxide Li-ion cells appreciate a 3-hour charge cycle using a 0.5-C rate constant-current constant-voltage (CC-CV) charge regimen. Typically, a 5-W power supply could power the battery charger. A high-discharge-rate cell with terminal voltage of 3-V and a 3-Ah capacity can easily support an 18-A draw for 10 minutes before being fully depleted.

This battery could be recharged within 15 minutes. So, if the user required such a fast-charge cycle, a 60-W power supply would be required to support a fast-charge scenario. The inclusion of a 60-W power supply within the battery charger affects many other aspects of the charger. Obviously, one of the first considerations is the cost/benefit analysis of including a 60-W power supply. Will the user pay the price premium of a larger power supply for a faster charge cycle?

Thermal management is greatly affected by larger power supplies and high-charge-current electronics. Not only is the heat generated from the power supply detrimental to the electronics of the charger, but this heat can put the batteries out of allowable charging temperature range or deteriorate the cells as they are sitting in the charger and getting charged.

The amount of heat generated by a moderately efficient 60-W power supply is 10 W. Many design principles exist to dissipate heat quickly within a charger. The most obvious is the inclusion of a fan and vents. Chargers for traditional Li-ion cells with cobalt-oxide cathodes do not typically need fans. But fans may be needed to quickly charge high-power cells due to the larger-than-usual 60-W power supply. However, many OEMs object to fans as they add cost, lower the reliability due to their electromechanical nature, provide an ingress point for debris into the charger enclosure and can be a primary source of noise.

With battery-charger designs, there are other techniques to minimize the affect of heating within the charger. Strategic placement of the pc-board assembly in relation to the cells is critical. As presented in Fig. 4 (a cross-sectional view of an older charger design), the pc-board assembly is mounted to the cell cup, sits directly under the cells and incrementally heats them. The heat from the charger is in addition to the self-heating of the cells during the charge cycle.

If you compare this architecture to a newer charger design presented in Fig. 5, one sees that the pc-board assembly is more distant from the cups holding the cells and has a prominent air gap for insulation. In addition, this newer design sports an aluminum enclosure that provides a heatsink for the pc-board assembly and aluminum cooling fins on the enclosure for better radiance of internal heat.

Another factor of thermal management is ensuring the cells do not overheat during charging, as there is a large amount of energy quickly transferred into a relatively small container. The risk of thermal runaway with any cell is much higher in a quick-charge cycle.

Variable current charging includes the active monitoring of the cell temperature during the charge cycle. Microcontrollers, embedded with the battery charger, allow the charger to monitor all electrical and environmental aspects of the cell. These microcontrollers can administer variable charge currents based on available power, cell-temperature conditions and maximum allowable charge current. With this approach, cell temperature can be monitored in real time by the communication bus or thermistor from the cell, and charge current can be maximized until the battery approaches its high-temperature limit. If the cell hits its high-temperature limit, the charger can be designed to reduce or suspend the charge current to the cell.

Interconnects represent another area of charger design that needs to be assessed. This area includes the traces on the pc-board assembly, as well as the contacts between the charger and the battery. A typical design using 2-oz copper may need to support a continuous current of 2 A and a peak current of 3 A. For such designs, traces on the pc-board assembly could be 1-mm to 2-mm wide for proper current density. However, to support a 15-A continuous charge current, traces would need to be expanded to 4-mm to 6-mm wide.

As for interconnects between pc-board assemblies, typical chargers can operate with 1-mm to 2-mm diameter beryllium copper contacts with gold or nickel plating. Increasing the charge current to 15 A dictates that the interconnect material should be upgraded with a larger cross section and more conductive material. Similarly, external contacts between the charger and battery will need to be upgraded.

Several options are available to support high-current charging. One can use off-the-shelf contacts such as pogo pins, but most are limited to 2-A delivery. Off-the-shelf high-current contacts are available, but are more expensive than the lower-current alternatives.

A second approach is to use multiple low-current contacts in parallel to deliver positive and negative voltage to the battery. The use of several positive and negative contacts improves the redundancy of the overall connection between the charger and battery in the event that one individual contact may fail.

The final option is the development of custom contacts, such as a spring-loaded or bent-wire contact, where the gauge of the metal contact is designed to carry the maximum current. The custom contact could be designed with multiple contact points if heat-related contact issues need to be addressed.

EMI

Electromagnetic interference (EMI) also needs to be considered with high-power battery chargers. Even if the EMI may not interfere with the operation of the charger, battery or portable device, the charger must be designed to pass emission standards and regulatory tests imposed by the Federal Communications Commission (FCC) and the Special International Committee on Radio Interference (CISPR).

Obviously, larger power supplies become a larger source of EMI issues. The amount of EMI emissions can be minimized by implementing good design practices for circuit and layout conception and by using shorter cables within the charger, as the cables may act as radiators. Minimizing the possibility of capacitive coupling between wires and between subassemblies is recommended.

Another consideration is the use and placement of EMI filters along the power paths within the charger. The paradox of EMI filters is that they are power dissipaters, so stringent use of EMI filters is recommended. Finally, shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and coating the enclosure interior. If one has to resort to coating the interior of the charger to minimize EMI, a coating of aluminum over copper onto the enclosure interior is recommended.

Tackling First Designs

The use of high-power cells and the affiliated charging schemes for these cells provides a new set of challenges for both electrical and mechanical engineers. We have highlighted some of the primary design challenges associated with high-power cells; however, engineers should consult experts in this field before embarking on their first design of this nature.

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