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Power Efficient Battery Formation/Testing System with Energy Recycling

EM news bureau ,  Wednesday, September 23, 2015, 14:06 Hrs  [IST]

Lithium-Ion batteries are widely used in portable equipment such as laptop computers and cellphones, but due to their low storage capacity of less than 5 Amp-Hour (Ah), concern over manufacturing efficiency has taken a back seat to manufacturing cost. Meanwhile, batteries used in vehicles have much higher total capacity, typically in the hundreds of Amps. This is achieved with thousands of small cells or a few high-capacity batteries. In this case, power efficiency and cost have to be taken into serious account during the manufacturing process.

The rechargeable battery industry currently faces three main challenges. First, they must ensure batteries meet their performance and reliability benchmarks. This is especially true in batteries used for home energy storage and electric vehicles. Power outages and catastrophic failures are to be avoided at all costs. The second challenge is reducing the cost of energy storage as much as possible to compete with other alternatives such as fossil fuels. Finally, the manufacturing, test and operation of batteries must result in a net positive environmental impact. The energy footprint of the entire battery lifecycle must be lower than that of alternative sources of energy.

In order to face the challenges confidently, it is helpful to take a look of the battery industry eco-system. At a high level, systems are very similar for different types of batteries, so we will use hybrid and electric vehicles as an example. The batteries in the vehicles are one of the most important components, as they make up a high percentage of the total system cost, and are the focus of most safety concerns. Figure 1 shows a simplified diagram of this example ecosystem.
fig1_bater.png
One of the most effective ways to address the cost and environmental impact of rechargeable batteries is by making the manufacturing process as efficient as possible. Figure 2 shows an overview of a typical Li-Ion battery manufacturing process. The battery starts out as chemical compounds that are shaped into the desired form factor and assembled into cells. The cells then go through a formation process that allows them to store charge. This process requires at least one full charge and discharge cycle at a rate of 0.1C (where C is the cell capacity), taking up to 20 hours.

After the formation step, the batteries go through an electrical testing and grading step, which involves additional charging at 1C and discharging at 0.5C. There may be several cycles in the process.

fig2_bater.pngFormation and electrical testing have tight accuracy specifications, especially when manufacturing cells which must be matched into larger battery packs for electric vehicles. The accuracy requirements can be as tight as ±0.05%.

Reducing the cost of storage through lower cost test systems

Reducing costs starts with the manufacturing equipment used in formation, testing and grading. Because of the long time periods required for the formation process, manufacturers must have a very large number of test channels in their production facility. Even small reductions in the cost of test equipment translate into large savings.

Traditional battery test systems are based on discrete designs using precision components such as amplifiers, resistors, resistor networks, etc. Although the cost of the semiconductor devices can be relatively low, the precision, low drift resistors or matched resistor networks required to achieve the high accuracy called for in these application can quickly increase system cost. In addition, the large component count required increases engineering design effort and manufacturing costs.

Using an integrated solution, where most precision components are contained within a single integrated circuit, not only greatly simplifies the system design, it also reduces cost and can increase system reliability due to reduced parts count.

Reducing the cost of production and its environmental impact

Many battery testers are based on linear regulators because it is much easier to meet the accuracy requirements of formation and testing. This is mostly suitable for smaller batteries given the very low efficiency of linear regulator-based testes. The efficiency of these systems can be 40% or lower depending on the specific operating conditions.

In addition, many systems use simple active or passive resistive loads to perform the discharge cycles, further decreasing the efficiency of the production process.

Analog Devices’ new battery test chipset, made up of the AD8450 and ADP1974, addresses these issues by allowing system designers to build switching regulator-based, energy recycling battery test systems. Such systems enable lower cost battery formation and test systems without sacrificing performance. The improved accuracy allows shorter and fewer calibration cycles resulting in longer uptime. In addition, simpler design and smaller power electronics components as a result of the higher switching frequencies also contribute to lower system cost. Channels can also be combined to output higher currents with minimal effort. The very low drift performance allows using only air as the cooling medium, rather than very expensive, complex and potentially environmentally polluting oil cooling systems. Software development costs are kept low by performing all the control in the analog domain, eliminating the need for complicated algorithms. Finally, energy recycling, coupled with high system efficiency reduces on-going operation costs.

Energy Recycling

Although using resistive loads is the simplest method of battery discharging, the costs quickly add up when large numbers of batteries must be put through charge/discharge cycles. A system built around the AD8450 and ADP1972 can achieve efficiency of over 90%, but its real value is in its ability to recycle energy from the discharging batteries with minimal additional complexity.

Rather than discharging batteries into resistive loads, a system built around the AD8450 and ADP1972 can control the battery voltage and current while ‘pushing’ this energy back into a common bus where other banks of batteries can use it for their charge cycle.

Each battery channel can be in charge mode, drawing energy from the DC bus, or in discharge mode, pushing energy back into the DC bus. The simplest systems include a unidirectional AC/DC power supply, which can only source current from the AC mains into the DC bus, such as the system in Figure 3. This means the system must be carefully balanced to ensure the net current from the AC/DC power supply is always positive. Pushing more energy into the DC bus than what is being consumed by the charging channels would result in an increase in bus voltage, possibly damaging some components.

fig3_bater.pngA bi-directional AC/DC converter addresses this challenge by pushing energy back into the AC grid as shown in Figure 4. In this case, all of the channels can be set first to charge mode, followed by discharge mode, returning the current back to the grid. This requires a more complex AC/DC converter, but provides additional flexibility for system configuration and there is no need to carefully balance the charge and discharge currents to ensure a net positive current from the power supply.

fig4_bater.pngEnergy Recycling Efficiency

To further illustrate the benefit of energy recycling, consider a comparison between a linear system with efficiency of 40% using resistive loads, and a switching regulator-based system with 90% efficiency and energy recycling. For this example, we assume a process with 100 cells rated for 3.2V and 15Ahr.

In order to deliver the 4,800W required to charge the cells, the linear system will consume approximately 12,000W due to its low efficiency. This power must be either generated on site or purchased from a utility company. When the batteries are discharged, the energy will be converted into heat.

When using the switching regulator based recycling system, we can break up the batteries into two lots of 50 cells. The system will first charge 50 cells, using approximately 2,667W to deliver 2,400W to the cells. When the cells are fully charged, the system can be reconfigured into discharge mode, taking the stored energy and pushing it back into a common bus so the energy can be used to charge the other lot of 50 cells. The discharging efficiency is also 90%, so the system will push back 2,160W into the common bus. Since the power required to charge the other lot is also 2,667W, the system power supply will only need to deliver an additional 507W from the utility.

We can reuse the energy stored on the second lot of batteries to charge a third lot, and so on. If we simply look at the original 100 batteries, the switching system will have consumed 3,174W, which represents over 75% savings in energy consumption over a linear, non-recycling system. Although the savings are not as dramatic when comparing against a switching regulator system with no recycling, the savings are still high enough (saving about 40%) to warrant considering the energy recycling approach.

Conclusion

A switching power supply approach provides a high-performance, cost-effective solution for modern rechargeable battery manufacturing. The AD8450 and ADP1972 simplify the system design with better than 0.01% system accuracy, higher than 90% power efficiency and power recycling capable, helping solve the rechargeable battery manufacturing bottleneck problem. It makes hybrid and electric vehicles environmental friendly for the whole eco-system.

 
                 
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