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Electronic Design

Lithium-Ion Battery System Upgrades Require Power-Management Analysis

As portable electronic systems become more sophisticated, selecting the optimum battery requires an intensive analysis of the entire power-management system. Many manufacturers now consider lithium-ion (Li-ion) batteries because they hold advantages over older battery technologies.

However, upgrading from the older technologies to Li-ion isn’t 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 (Fig. 1), including:

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

Besides 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 under load and runtimes. 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.

To understand battery power management, we must look at the main components of a typical portable-system battery pack (Fig. 2). Battery cells provide the primary energy source, and electronics within the battery pack supply the intelligence for specific functions, such as:

  • “Fuel gauge” determination on remaining cell capacity
  • Protection circuit (for Li-ion batteries)
  • Thermal sensors to monitor internal battery-pack temperature
  • LEDs that indicate pack or cell status
  • Serial data communications bus that talks to the associated host system

In addition, system packaging should be considered:

  • Custom plastic enclosure with external contacts that provide a physical and electrical interface with the host system
  • Insulation to absorb external shock, as well as retain or dissipate heat generated by the pack

Why Li-ion?
Li-ion batteries offer advantages over other rechargeable chemistries, including higher energy density, lighter weight, longer cycle life, superior capacity retention, broader ambient-temperature endurance, and higher current tolerance. A combination of series and parallel cell strings can deliver the voltage and capacity to support multiple application requirements that were previously impractical and/or cost prohibitive. Energy density is increased primarily because of the higher voltage per cell of Li-ion (Fig. 3).

Also, Li-ion is more environmentally friendly than the other chemistries. Modern Li-ion battery pack designs are safe if they employ reliable batteries along with well-designed protection and chargers.

The cost of Li-ion batteries has dropped due to the economies of scale driven by consumer products, such as laptops and cell phones. Often, a Li-ion solution is at cost parity with a nickel-metal-hydride (NiMH) solution thanks to Li-ion’s higher operating voltage, 3.6 versus 1.2 V, which allows for fewer cells at the same voltage.

Because Li-ion cells go beyond traditional sizes and form factors, they offer more design flexibility. Cells come in cylindrical (traditional nickel cadmium (NiCd) and NiMH shapes), prismatic (flat, thinner box-like profiles), and polymer (very flat, softer foil packaging). In addition, there are no heavy metals in the Li-ion chemical compound, so they’re more environmentally “friendly” than sealed lead-acid, NiCd, or NiMH rechargeable technologies.

Battery life is typically measured in numbers of charge-discharge cycles. When properly used, batteries should last between 300 and 500 cycles. While this is a broad range, it’s appropriate due to the variability in the battery’s utility and specific design. Ultimately, the end of life occurs when a battery can no longer reach 80% of its initial or rated capacity.

With Li-ion batteries, the battery needn’t be completely discharged (called shallow discharge) prior to recharging to prolong the battery’s life. In addition, with a well-designed charger and battery control circuit, shallow discharging and repeated recharging won’t damage the Li-ion battery or cause it to require premature replacement—much unlike other chemistries.

Upgrading to Li-ion
Li-ion battery packs are significantly more complex electronically than other technologies. Many “smart” Li-ion battery packs provide fuel gauging, cell balancing, and protection. Li-ion batteries use a different way of charging and charge cutoff control than other chemistries. Consequently, not all host systems are designed with the flexibility to upgrade from sealed lead-acid or nickel-based batteries to Li-ion.

Overcharging can be a problem with any battery chemistry. Therefore, a well-designed Li-ion battery system will include a power-management circuit that can accurately detect a complete battery-charge cycle as well as terminate the charge. With Li-ion batteries, the constant-current/constant-voltage (CCCV) shut-off method helps ensure users don’t risk overcharge concerns. Systems upgraded to Li-ion batteries from NiCd or NiMH must ensure that they prevent acceptance of their older technology chargers.

Li-ion cell manufacturers recognize that certain operating conditions demand a variety of extreme uses. Applications involving high and prolonged power (power tools), as well as hot or cold temperature (desert or near-arctic operations), are becoming more common. NiCd and sealed lead-acid batteries had served these niche markets, and NiCd batteries perform much better at the lower temperature ranges than either NiMH or Li-ion.

NiCd batteries will achieve approximately 40% of rated capacity at -30°C, while standard application Li-ion batteries aren’t rated to work below -20°C. In fact, Li-ion electrolyte begins to freeze at -30°C. Some Li-ion cells and battery packs are now available for these needs, while others are in development. These needs should be identified prior to or during system development so battery packs can be designed accordingly.

Maximizing Li-ion Battery Life All battery chemistries perform to their greatest potential when employed in the recommended environment, designed correctly for the application, stored correctly, and used with a charger designed to meet the specific battery?s operational requirements. For most Li-ion, this means operating environments from 0°C to 60°C with good air circulation and in host devices where thermal considerations of the other components don?t create ?hot spots? that could cause premature degradation of the battery?s cells or electronics. The temperatures referenced are for the internal temperature of the battery. Thus, batteries operating in cold or hot temperatures but sheltered by something like insulation (cold) or a fan (hot) aren?t likely to be exposed long enough to be impacted by temperature extremes. Although most batteries are designed for the ruggedness required of the system, physical abuse can damage them and require their replacement. In addition, ?smart? Li-ion battery design frequently uses sophisticated power-management systems to improve or optimize overall functionality of the energy delivery system. The basic functions of the power-management circuits are control of energy flow into and out of the battery and prevention of abusive conditions. Therefore, you can enhance user safety by monitoring critical parameters and communicating that information to the system. Batteries no longer capable of holding 80% of their original capacity should be replaced. This can be difficult to judge, though, because not all users have a measuring device at their disposal. Therefore, users should consider how frequently a battery is recharged. Daily use typically results in 200 to 250 charges per year. Such usage should result in batteries lasting 18 to 24 months. If the batteries are used 24/7 and recharged every other shift, then annual replacement is recommended. If the batteries are used less frequently, stored for periods of time, or are employed in a backup role, then they may last three or more years before being replaced. Disposal of used batteries is subject to local regulations and guidelines. Requirements vary significantly throughout the world. In most locations, facilities and companies are set up to receive old batteries. All customers should comply with these regulations and use those facilities. The distributor can provide guidance on how regulations and services should best be followed.
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