Batteries

One of the best features of using a laptop computer is that it can run on batteries. Even if you don't normally run your laptop computer on battery power, the fact that the battery is there and can take over instantly whenever necessary is like having an uninterruptible power supply (UPS) built in to the system. A UPS capable of running a full-function PC, including a monitor, normally costs upward of $1,000, and you'll have to spend even more than that if you want one that can run the system for more than just a few minutes. The built-in UPS capability of the modern laptop has saved me from losing my work on several occasions when unexpected power outages have interrupted operation. While desktop users would be down for the count (often losing any unsaved work in the process), laptop users with a fully charged battery can continue working for up to 4 hours or more during a power outage . Although the typical laptop computer costs up to twice as much (or more) as a desktop system of similar speed and power, you should factor in that you are also getting a built-in UPS worth easily $1,000 or more with the deal.

Many people are not aware that many laptop computers have three different batteries:

• The main battery (or batteries)

• The suspend/standby battery

• The CMOS/clock battery

The first two battery types are rechargeable , whereas the CMOS/Clock battery is not. Some systems don't incorporate a suspend/standby battery, instead using only the main battery for power when in Suspend or Standby mode. Most people are aware of the main battery, and many know that like desktop systems, laptops also have a CMOS/Clock battery. The other battery found in some of the more sophisticated laptop models is the suspend/standby battery, which powers the RAM when the system is in Suspend or Standby mode. This battery is rechargeable, just like the main battery, and will power the memory for a limited time when the system is in Suspend or Standby mode. I recommend always saving any work before you use Suspend or Standby mode, because when the main or suspend/standby battery discharges, you will lose everything currently in RAM. If present, the suspend/standby battery normally lasts the life of the system and recharges automatically anytime the system is powered on. Not all laptop systems use a suspend/standby battery, and instead draw a small amount of power from the main battery. Unfortunately, if the main battery fully discharges, the suspend will fail and all unsaved information will be lost.

Many systems have an additional "Hibernate" mode, which copies the RAM to the hard disk and then shuts the system off. When the notebook is in Hibernate mode, no power is used, so the system can be left that way and restarted later without losing data. Still, it is possible for the system to lose track of whether it was in Hibernate mode, so I highly recommend saving your work before using this mode.

CMOS/Clock battery

Portable systems normally incorporate a chip that combines a real-time clock (RTC) with at least 64 bytes (including 14 bytes of clock data) of nonvolatile RAM (NVRAM) memory. This is officially called the RTC /NVRAM chip , but it is often referred to as the CMOS or CMOS RAM chip because the chip is produced using a CMOS (complementary metal-oxide semiconductor) process. CMOS design chips are known for very low power consumption, and the RTC/NVRAM chip is designed to run off a battery for several years . The RTC/NVRAM chip is built in to the motherboard chipset South Bridge or I/O Controller Hub (ICH) component in most modern systems.

The function of the real-time clock should be obvious: The clock enables software to read the date and time and preserves the date and time data even when the system is powered off or unplugged. The NVRAM portion of the chip has another function. It is designed to store the basic system configuration, including the amount of memory installed, types of disk drives installed, Plug and Play device configuration, power-on passwords, and other information. Most chipsets incorporate 256 bytes of NVRAM, of which the clock uses 14 bytes. The system reads this information every time you power it on.

The RTC/NVRAM is powered by a battery while the system is off. This battery preserves the information in the NVRAM and powers the clock. Most systems use a lithium coin cell battery because it has a very long life, especially at the low power draw from the typical RTC/NVRAM chip. Typical batteries include the 20xx series (20mm diameter, varying thicknesses), available in two slightly different battery chemistries. Figure 7.3 shows a cutaway view of a CR20xx lithium coin cell battery.

Figure 7.3. CR20xx lithium coin cell construction.

Table 7.1 lists the specifications of 20xx lithium coin cell battery.

Table 7.1. 20xx Lithium Coin Cell Specifications

Type	Voltage (V)	Capacity (mAh)	Diameter (mm)	Height (mm)
------	-------	--------	--------	------
BR2016	3.00	75	20.00	1.60
BR2020	3.00	100	20.00	2.00
BR2032	3.00	190	20.00	3.20
------	-------	--------	--------	------
CR2012	3.00	55	20.00	1.20
CR2016	3.00	90	20.00	1.60
CR2025	3.00	165	20.00	2.50
CR2032	3.00	220	20.00	3.20
BR = Carbon monoflouride
CR = Manganese dioxide

The BR and CR series are normally interchangeable because they have only slight differences in chemistry and performance. The CR types are more plentiful (easier to get) and offer slightly higher capacity. The BR types are useful for higher temperature operation (above 60 °C or 140 °F).

Battery life can be calculated by dividing the battery capacity by the average current required. For example, a typical CR2032 battery is rated 220 mAh (milli-amp hours), and the RTC/NVRAM circuit in the Intel 855 chipset (for Pentium M mobile processor systems) draws 5 µA (micro-amps) with the power off. In a Pentium M “based laptop using a CR2032 battery and the Intel 855 chipset, battery life can be calculated as follows :

220,000 µAh / 5 µA = 44,000 hours = 5 years

If a smaller capacity battery such as the CR2025 is used, battery life becomes this:

165,000 µAh / 5 µA = 33,000 hours = 3.7 years

Battery life starts when the system is first assembled , which can be several months or more before you purchase the system, even if it is new. Also, it is possible that the battery may be partially discharged before it is installed in the system, and higher temperatures both in storage and in the system can contribute to shorter battery life. All these reasons and more may cause battery life to be less than might be indicated by calculation.

As the battery drains, output voltage will drop somewhat, and lower battery voltage can impair the accuracy of the RTC (real-time clock). Most lithium coin cell batteries are rated at 3V; however, actual readings on a new battery are usually higher. If your system clock seems inaccurate (runs slow, for example), check the voltage on the CMOS battery. The highest accuracy will be obtained if the battery voltage is maintained at 3.0V or higher. Lithium batteries normally maintain a fairly steady voltage until they are nearly fully discharged, whereupon the voltage drops quickly. If you check the battery voltage and find it is below 3.0V, you might consider replacing the battery, even if it is before the intended replacement time.

Main Battery

Relatively limited battery life is one of the biggest complaints users have about portable systems. A great deal has been done to improve the power-management capabilities and efficiency of today's portable computers, and better battery technologies, such as lithium-ion, have also been adopted on many systems. However, the larger power requirements of faster CPUs, more RAM, larger drives, and near-universal adoption of active-matrix displays have left the battery life of a typical portable system approximately the same as it was several years ago.

The goal sought by many mobile users has been the same for many years: a full-featured portable system that can run on a single battery for the entire duration of a transcontinental U.S. airplane flight. Some airlines are now providing 12V automotive-type power connectors under the seat for use with automotive power adapters; in this case, battery life on a plane is no longer an issue. For those who are relegated to using batteries, you still typically must carry two or more battery packs for a 6-hour flight. Although mobile power technologies are improving, the demands of today's systems have also increased. However, although typical battery life has remained in the 2- to 4- hour range for most reputable manufacturers, the new notebooks have the advantage of larger hard drives, bigger and brighter screens, more RAM, faster processors, integrated speakers , and CD-ROM drives. System manufacturers have been able to better manage system power use by selectively turning off or shutting down components that are not being used, which is referred to as power management . Through better and better power-management techniques implemented in the latest processors, chipsets, and systems, overall power use has been maintained, even though consumption of individual components (when on) has risen. Such power management is the cornerstone of more recent designs, such as the Intel Centrino, which includes a processor, chipset, and wireless LAN component.

Based on the increasing popularity of these notebook systems, the buying consumer has clearly voiced that a more powerful notebook with continuing average battery life is more important and useful than a system that would run for 6 “10 hours but sacrifice features she has grown used to using.

For users who simply must have a system that can run for 6 “10 hours, check for airlines that are adding at-seat power connections designed for notebook computers.

Battery Types

Battery technology also plays a major role in the portable power issue. Most portable systems today use one of four battery types for their main power batteries:

• Nickel cadmium (NiCd)

• Nickel metal-hydride (NiMH)

• Lithium-ion (Li-ion)

• Lithium-ion polymer (LIP or Li-Poly)

Table 7.2 shows a comparison between the different types of rechargeable batteries used in portable computers.

Table 7.2. Rechargeable Battery Types and Specifications

Specification	NiCd	NiMH	Li-ion	Li-poly
Energy density (Wh/kg)	60	90	140	120
Avg. cycle life	1500	400	400	400
Cell voltage (nominal)	1.2	1.2	3.6V	3.6V
Memory effect	Yes	Yes	No	No
Relative cost	1	1.2	2	2
Fast charge time (hours)	1	3	3	3
Overcharge danger	Moderate	High	Very High	High
Self- discharge (per month)	20%	30%	10%	10%
First Used (year)	1950	1990	1991	1999

The following sections analyze the features and benefits of each of these types of batteries.

Nickel Cadmium (NiCd)

As the oldest of the four technologies, nickel cadmium (NiCd, often called Ni-Cad ) batteries are rarely used for main power in today's portable systems (although some use NiCd for standby batteries). They have a high current capacity but store less energy than other technologies, resulting in shorter laptop battery runtimes . Overall, NiCd is a very durable technology, offering greater cycle life (discharge/recharge cycles) with less degradation than other types, if properly maintained.

The battery runtime from a NiCd battery's charge can be shortened by as much as 40% if the battery is not periodically fully discharged, or if it is frequently overcharged. For NiCd batteries, permanent damage to the cells can be prevented by periodically discharging them to 1 volt or less, say once every month or so. Despite this so-called "memory" effect, NiCd batteries do hold a charge well when not in use, losing only about 20% of their charge per month. The memory effect is actually due to changes in the crystalline structures inside the battery. Normally the cadmium material is contained in very tiny crystal structures. When repeated shallow discharges and recharges occur, these crystal structures grow in size, reducing the surface area and thus the total battery capacity. Periodic deep discharges can actually rejuvenate a battery by breaking up the larger crystals, thus restoring the battery capacity to normal levels.

NiCd batteries can be recharged 1,500 times or more, which combined with their high current output, makes them ideal for portable two-way radios or other applications where high demands are made for short periods of time. NiCd cells are rated at a nominal 1.2V per cell, and in laptop systems were normally combined in packs with 6 or 7 cells combined to produce 8.4V to 9.6V. Despite their durability, their lower energy density (resulting in lower battery runtimes) has caused them to fall out of favor in laptop and portable computers, such that virtually no new systems use NiCd batteries today. The cadmium in NiCd batteries is also considered a toxic metal, and recycling is highly recommended if not required.

Nickel Metal-Hydride (NiMH)

Nickel metal-hydride (NiMH) batteries have become a near universal replacement for nickel cadmium (NiCd) batteries in portable computers. They have the same cell voltage (1.2V), come in the same physical form factors, and use the same charging equipment. Although they cost about 20% more, NiMH batteries have approximately a 40% higher energy density than NiCd batteries, resulting in longer battery runtimes for portable computers. They are less sensitive to the memory effect caused by improper charging and discharging, and they do not use the environmentally dangerous cadmium found in NiCd batteries.

In addition to their slightly higher cost, the main drawback of NiMH batteries is that they can withstand far fewer cycles (discharge/recharge) than NiCd batteries. In other words, while they support 40% longer battery runtimes, they have overall a much shorter life, generally lasting about 400 cycles before they lose their capacity to hold a charge. They also have a recommended discharge current that is lower than that of a NiCd battery (one-fifth to one-half of their rated capacity) and a higher self-discharge rate. NiMH batteries also require more than twice as long to recharge as NiCd batteries.

Although they are still used in many portable systems, NiMH batteries are now found mostly in computers at the lower end of the price range, having been replaced by lithium-ion batteries at the midrange and high end.

Lithium-Ion (Li-ion)

As the current industry standard, lithium-ion batteries support longer battery runtimes than NiCd and NiMH technologies, can't be overcharged (due to the special charging equipment required), and hold a charge well when not in use. Li-ion batteries have an extremely high energy density; that is, they store more energy than other types for a given size and weight. This is primarily because lithium is one of the lightest of all metals.

Lithium-ion batteries in laptop systems are really battery packs normally constructed of several individual cells. Each cell puts out between 2.5V (discharged) to 4.2V (fully charged) and is normally rated about 3.6V nominal. The cells are combined in battery packs of normally between 2 to 12 cells, arranged in serial and parallel connections resulting in 7.2V, 10.8V, or 14.4V (nominal) voltage ratings. Capacity ratings generally go from 2000 mAh (milli-amp hours) to 6600 mAh or more. The higher the capacity rating, the longer the battery lasts in a given application.

Lithium is an unstable substance, and when first released, rechargeable lithium batteries had problems. After a number of discharge/recharge cycles, it was found that metallic lithium plating would occur on the lithium electrode, causing a potential thermal runaway condition resulting in a violent reaction. A vent in the battery would prevent an outright explosion; however, extremely hot gases or flames would be released under these conditions, causing damage to the surrounding equipment or even burns to individuals nearby. Several incidents of batteries overheating and venting flames resulted in a number of recalls and redesigns in the early 1990s.

Subsequent models were manufactured using built-in protection circuits that limit the peak voltage of each cell during charge (usually to 4.3V or less), prevent the cell voltage from dropping too low (usually below 2.5V) on discharge, limit the maximum charge and discharge currents, and monitor the cell temperatures. With these electronic precautions , the possibility of thermal runaway and violent reactions has been virtually eliminated. But also because of the careful design requirements for Li-ion batteries, raw cells are not sold to individuals and instead are sold only in assembled battery packs, with the protection circuitry built in.

Various types of Li-ion batteries use different active materials for their positive and negative electrodes. For the positive electrode, higher-output versions use cobalt, whereas lower-output versions use manganese. Negative electrodes use either coke or graphite, each with slightly different discharge profiles.

The main advantages of Li-ion batteries are their high energy density, resulting in the longest battery runtimes in portable computers. They also have a lower self-discharge rate than other types, meaning they will hold a charge longer during nonuse or storage. Finally, they require no maintenance; that is, they do not need periodic deep discharges, as with NiCd and NiMH types. In fact, unlike NiCd and NiMH batteries, periodic deep discharges only serve to use up the fewer remaining charge cycles available in Li-ion batteries.

Li-ion batteries do have a few disadvantages, starting with their higher cost and the need for built-in protection circuitry in order to maintain safe operation. Similar to NiMH batteries, Li-ion types are also limited to about 400 total cycles, after which they no longer hold a charge. But perhaps the biggest drawback is uncontrollable aging. What this means is that there will be a noticeable loss in capacity after the battery ages one year, whether the battery is actually used or not. This continues such that over two or three years the battery will fail to hold a charge, even if it has never been used. Because of this aging characteristic, purchasing used Li-ion batteries should be avoided, and if you keep a laptop or notebook system for more than two years, you will most likely be forced to replace the expensive main battery, no matter how much (or little) the battery was actually used.

Finally, unlike NiCd and NiMH batteries, which can be used in the same system without modification to the circuitry, Li-ion batteries can be used only in systems specifically designed for them. Because of the special charging techniques required, merely inserting a Li-ion battery into a system designed for a NiCd or NiMH can result in the protection circuitry shutting the battery down (permanently in some cases).

Although lithium-ion batteries are the most expensive and finicky of the four technologies, they have come to be used in all but the very low end of the portable system market.

Lithium-Ion Polymer (LIP or Li-Poly)

Lithium-ion polymer is a fourth battery type that has been in development for several years, but which has only recently appeared on the market, and which has yet to achieve popularity in portable computer usage. Lithium-ion polymer batteries are manufactured using a lithium-ion and cobalt oxide cell with a proprietary polymer electrolyte and can be formed into flat sheets as small as 1mm thick and of almost any size. This allows a very thin battery shape, resulting in designs that can fit in places not previously thought possible (such as behind an LCD panel). Lithium-ion polymer batteries provide an energy density that is somewhat lower than standard Li-ion types, giving up capacity for their thinner and smaller size and weight. Besides allowing thinner designs with less weight, and a slightly lower energy density, Li-Poly batteries are similar to standard Li-ion batteries in most other respects and share most of the same characteristics.

Lithium-ion polymer batteries are finding most use in cell phones, PDAs, or other extremely small devices, but their lower energy density as compared to standard Li-ion types has prevented them from achieving widespread use in laptop and notebook systems.

Battery Maintenance

NiCd and NiMH batteries function best when they are completely discharged before being recharged. In fact, NiCd or NiMH batteries that have gone unused for several months should be reconditioned by putting them through two or three full discharge/recharge cycles. You can do this by leaving the system on until it automatically suspends or shuts down; alternatively, battery-conditioning utilities are available that can manage this process for you.

Although lithium-ion batteries do not need such conditioning (in fact, it would unnecessarily use up available charge cycles), there is a situation where a full discharge and recharge cycle may be beneficial. All modern laptop batteries are intelligent designs, containing special circuits used to indicate the state of charge to the system. These circuits can lose track of the actual state of charge if the battery remains topped off for a period of time. Then the battery gauge indicator becomes inaccurate, resulting in what seems to be shorter runtimes before the gauge drops to zero. Performing a full discharge and recharge cycle allows the intelligent battery "gas gauge" circuits to reset, placing them back in synchronization with the actual state of charge.

Another indication that you need to discharge-cycle your batteries is if charging stops before the charge indicator reaches 100%. Cycling the battery a few times usually corrects this situation and allows for proper charge indication. Lithium-ion batteries are affected the least by incomplete discharging and have a higher discharge cut-off point (2.5V “3V versus 1V for NiCd batteries), but due to the gas gauge circuits, the effect on the apparent life is still noticeable.

Unfortunately, buying a portable computer with a more efficient battery does not necessarily mean that you will realize longer run-times. Power consumption depends on the components installed in the system, the power-management capabilities provided by the system software, and the size of the battery itself. Some manufacturers, for example, when moving from NiMH to Li-ion batteries, see this as an opportunity to save some space inside the computer. They decide that because they are using a more efficient power-storage technology, they can make the battery smaller and still deliver the same performance.

Battery technology trails behind nearly all the other subsystems found in a portable computer as far as the development of new innovations is concerned . Because of the continuing trend of adding more features and components to mobile computers, their power consumption has risen enormously in recent years, and power systems have barely been capable of keeping up. On a high-end laptop system, a battery life of three hours is very good, even with all the system's power-management features activated.

One other way manufacturers are addressing the battery life problem is by designing systems capable of carrying two batteries. The inclusion of multipurpose bays within the system's case on some computers enables users to replace the floppy or CD-ROM drive with a second battery pack, effectively doubling the computer's charge life.