Power Electronics

Portable Power Management Centers on Battery Characteristics

Battery-based power management designs depend on the associated battery, so design starts by picking the specific battery type. A battery consists of a cell with two electrodes in contact with an electrolyte that produces a voltage determined by the chemistry used. One category of batteries is the primary type that is not rechargeable, because it employs a nonreversible chemical reaction to produce electricity. The second type is a secondary battery that is rechargeable. This article will only cover the rechargeable batteries for portable systems.

The first step in ensuring a proper battery choice is to fully describe the “real-world usage profile” of the device. The usage profile includes temperature ranges, discharge profiles, charging regimens, expected shelf life and transportation requirements. It also should account for foreseeable misuse as well as intended use. For example, temperature extremes can cause similarly rated cells from different manufacturers to demonstrate widely varying performance results, such as voltage output and run times. Furthermore, shelf life plays a critical role in the selection of the appropriate cell chemistries, so self-discharge rate of the cell chemistry may be the determining factor in selection the optimal chemistry.

The charge and discharge capacity of a secondary battery is in terms of “C,” given as ampere-hours (Ah). The actual battery capacity depends on the C-rate and temperature, with most portable batteries rated at 1 C. A discharge of 1 C draws a current equal to the rated capacity. For example, a battery rated at 1000 mAh provides 1000 mA for 1 hr if discharged at 1-C rate. The same battery discharged at 0.5 C provides 500 mA for 2 hr.

Nickel cadmium (NiCd) batteries were the first widely used battery for portable equipment. Introduction of batteries with higher energy densities and less toxic metals is moving use away from NiCd to newer technologies. A NiCd disadvantage is its memory effect that requires periodic discharge to prevent charging problems. Advantages include its simple charge characteristics and its ability to accept a high number of charge/discharge cycles.

The first successor to the NiCd was the nickel-metal hydride (NiMH) battery, which provides high energy density and the use of environmentally friendly metals. NiMH offers about 30% to 40% higher energy density than NiCd. Also, NiMH batteries are less prone to memory effects, so they have replaced NiCd batteries in many applications. When charging, NiMH batteries employ a more complex charge algorithm and generate some heat. In addition, they require a longer charge time than the NiCd.

Now, lithium batteries have taken center stage. The lightest of all metals, lithium has the greatest electrochemical potential and provides the largest energy density per weight. Rechargeable batteries using lithium metal anodes (negative electrodes) can provide both high voltage and excellent capacity. However, potential safety problems exist with this type of lithium battery.

Although slightly lower in energy density than lithium metal, the lithium-ion (li-ion) is safe, provided certain precautions are met when charging and discharging. Li-ion energy density is about twice that of the standard NiCd. In addition to high capacity, the load characteristics are reasonably good and behave similarly to the NiCd in terms of discharge characteristics. Its relatively high cell voltage allows li-ion battery packs consisting of only one cell. Its life expectancy is 300 charge/discharge cycles, with 50% capacity at 500 cycles.

An added requirement for li-ion batteries is a protection circuit that limits each cell's peak voltage during charge and prevents its voltage from dropping too low on discharge. The protection circuit limits the maximum charge and discharge current and monitors the cell temperature.

Exercise caution when handling and testing li-ion batteries. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse polarity, expose to high temperature or disassemble them. Only use an li-ion battery with the designated protection circuit.

The li-ion polymer battery differs from li-ion types in terms of fabrication, ruggedness, safety and thin-profile geometry. Unlike the li-ion, there is no danger of flammability, because it does not use a liquid or gelled electrolyte. However, the li-ion polymer is still a work in process, with operational improvements expected. Compared to conventional li-ion batteries, an advantage of the li-ion polymer is simpler packaging and lower profile because its electrodes can be stacked easily.

Although some portable systems can use a single battery, other systems require multiple batteries packaged as battery packs. NiCd and NiMH cells provide 1.25 V (nominal) and li-ion cells are 3.6 V to 4.2 V. So, series-connected battery packs with fewer cells usually perform better than those with 12 cells or more. Parallel connections provide higher Ah ratings. Often, a parallel connection is the only option to increase the battery rating. Among the different battery chemistries, li-ion lends itself best to parallel connection.

As portable electronic systems become even more sophisticated, selecting the optimum battery requires an intensive analysis of the entire power management system. For example, upgrading to li-ion batteries to replace older nickel-based battery technologies is not possible as a one-to-one replacement. The first step in determining the feasibility of an upgrade is to fully describe the portable system's battery usage profile, including:

  • Temperature ranges
  • Discharge profiles
  • Charging regimens
  • Expected shelf life
  • Transportation requirements.

In addition to intended use, these profiles should also cover other areas. For example, temperature extremes can cause similarly rated cells from different manufacturers to produce widely varying performance results, such as voltage output and run times. Shelf life plays a critical role in the selection of the appropriate cell chemistries, so its self-discharge rate may be a determining factor in selecting the optimum chemistry.

BATTERY EVOLUTION

Two MIT researchers devised a way to cut the time required to charge an li-ion battery by creating a new surface structure that allows the lithium ions to move quickly around the outside of the material, much like a beltway around a city. When an ion traveling along this beltway reaches a tunnel, it is instantly diverted into it.

Using their new processing technique, the two made a small battery that could be fully charged or discharged in 10 sec to 20 sec (it takes 6 min. to fully charge or discharge a cell made from the unprocessed material).

Researcher Gerbrand Ceder notes that further tests showed that, unlike other battery materials, the new material does not degrade as much when repeatedly charged and recharged. This could lead to smaller, lighter batteries because less material is needed for the same result.

Although it is relatively new and not widely used, the ZPower silver-zinc rechargeable battery (Fig. 1) seems to offer performance comparable with li-ion and li-polymer. It has an very high ratio of energy-to-volume (Wh/l) in applications such as notebook computers, cell phones and consumer electronics. In fact, ZPower is the only rechargeable battery that beats li-ion on energy density for consumer applications.

ZPower batteries have an intrinsically safe water-based chemistry that contains no lithium or flammable solvents. Unlike li-ion and li-polymer batteries, these batteries are free from the problems of thermal runaway, fire and danger of explosion. They are also free from the regulations that limit the size of lithium-containing batteries on airplanes.

In addition to offering high-performance and safety, ZPower batteries use an environmentally friendly chemistry that allows battery cells to be recycled and the contents reused. Unlike other traditional batteries, ZPower batteries have no heavy metals or toxic chemicals.

Micro fuel cells are another candidate for portable, battery-based systems. UltraCell Corp. has announced the availability of the XX55 reformed methanol fuel cell (Fig. 2), the industry's first ultracompact, rugged portable fuel cell equipped with a hybrid battery system.

XX55's technology offers fully optimized battery integration, allowing complete functionality using an industry-first clip-on and clip-off battery module system. The associated rechargeable battery performs two tasks. Initially, it activates the fuel cell and then supplies peak power when required to do so because the fuel cell cannot react to a load spike as quickly as the battery. Then, the fuel cell recharges the battery when its output falls below a certain voltage.

This fuel cell delivers 55 W average and 80 W peak, with a typical capacity of 8 hr of continuous operation with the smallest fuel cartridge. Its disposable methanol cartridge can be replaced easily when the fuel is exhausted. Fuel cartridges are also available with up to a 500-hr capacity.

The system is the size of a typical hardbound book and four times lighter and six times smaller than the nearest production portable fuel cell system in its power class.

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