Fast Charge Becomes a Reality For Li-Ion Batteries

Oct 1, 2007 12:00 PM
By Robin Sarah Tichy, Technical Marketing Manager, and Jeff Van Zwol, Marketing Manager, Micro Power

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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