Chapter 8: Choosing the Right Battery
What causes a battery to wear down — is it mechanical or chemical? The answer is ‘both’. A battery is a perishable product that starts deteriorating from the time it leaves the factory. Similar to a spring under tension, a battery seeks to revert to its lowest denominator. The rate of aging is subject to depth of discharge, environmental conditions, charge methods and maintenance procedures (or lack thereof). Each battery chemistry behaves differently in terms of aging and wear through normal use.
What’s the best battery for mobile phones?
When buying a replacement battery, the buyer often has the choice of different battery chemistries. Li-ion and Li-ion polymer batteries are used on newer phones, whereas the NiMH and NiCd are found in older models. If the buyer has a choice, the sales person may advise a customer to go for the highest capacity rating and to stay away from the NiCd because of the memory effect. The customer may settle for the slim-line NiMH because it offers relatively high capacity in a small package and is reasonably priced.
Seemingly a wise choice, an analysis in this chapter reveals that other chemistries may have served better. The NiMH offers good value for the price but falls short in expected cycle life. Although excellent when new, the performance trails off quickly after about 300 cycles due to decreased capacity and rising internal resistance. In comparison, the Li-ion can be used for about 500 cycles. The best cycle count is achieved with NiCd. Properly maintained, the NiCd delivers over 1000 cycles and the internal resistance remains low. However, the NiCd offers about 30 percent less capacity compared to the NiMH. In addition, the NiCd is being removed from the mobile phone market because of environmental concerns.
Switching to environmentally friendlier batteries is fitting, especially in the mobile phone market where the NiMH performs reasonably well and can be economical. The battery disposal issue is difficult to control, particularly in the hands of a diverse user group.
The NiMH and NiCd are considered high maintenance batteries, which require regular discharge cycles to prevent what is referred to as ‘memory’. Although the NiMH was originally advertised as memory-free, both NiCd and NiMH are affected by the phenomenon. The capacity loss is caused by crystalline formation that is generated by the positive nickel plate, a metal shared by both systems.
Nickel-based batteries, especially NiCd’s, should be fully discharged once per month. If such maintenance is omitted for four months or more, the capacity drops by as much as one third. A full restoration becomes more difficult the longer service is withheld.
It is not recommended to discharge a battery before each charge because this wears down the battery unnecessarily and shortens the life. Neither is it advisable to leave a battery in the charger for a long period of time. When not in use, the battery should be put on a shelf and charged before use. Always store the battery in a cool place.
Is the Li-ion a better choice? Yes, for many applications. The Li-ion is a low maintenance battery which offers high energy, is lightweight and does not require periodic full discharge. No trickle charge is applied once the battery reaches full charge. The Li-ion battery can stay in most chargers until used. The charging process of a Li-ion is, in many ways, simpler and cleaner than that of nickel-based systems, but requires tighter tolerances. Repeated insertion into the charger or cradle does not affect the battery by inducing overcharge.
On the negative side, the Li-ion gradually loses charge acceptance as part of aging, even if not used. For this reason, Li-ion batteries should not be stored for long periods of time but be rotated like perishable food. The buyer should be aware of the manufacturing date when purchasing a replacement battery.
The Li-ion is most economical for those who use a mobile phone daily. Up to 1000 charge/discharge cycles can be expected if used within the expected service life of about two to three years. Because of the aging effect, the Li-ion does not provide an economical solution for the occasional user. If the Li-ion is the only battery choice and the equipment is seldom used, the battery should be removed from the equipment and stored in a cool place, preferably only partially charged.
So far, little is known about the life expectancy of the Li-ion polymer. Because of the similarities with the Li-ion, the long-term performance of both systems is expected to be similar. Much effort is being made to prolong the service life of lithium-based systems. New chemical additives have been effective in retarding the aging process.
What’s the best battery for two-way radios?
The two-way radio market uses mostly NiCd batteries. In the last few years, environmental agencies have been attempting to discourage the use of NiCd, especially in Europe. NiMH have been tried and tested in two-way radios for a number of years but the results are mixed. Shorter cycle life compared to NiCd is the major drawback.
The reasons for the relatively short life of NiMH are multi-fold. NiMH is less robust than NiCd and has a cycle life expectancy that is half or one third that of the standard NiCd. In addition, NiMH prefers a moderate discharge current of 0.5C or less. A two-way radio, on the other hand, draws a discharge current of about 1.5A when transmitting at 4W of power. High discharge loads shorten the life of the NiMH battery considerably.
NiCd has the advantage of maintaining a low and steady internal resistance throughout most of its service life. Although low when new, NiMH increases the resistance with advanced cycle count. A battery with high internal resistance causes the voltage to drop when a load is applied. Even though energy may still be present, the battery cannot deliver the high current flow required during transmit mode. This results in a drop in voltage, which triggers the ‘low battery’ condition and the radio cuts off. This happens mostly during transmission.
The Li-ion has been tested for use with two-way radios but has not been able to provide the ultimate answer. Higher replacement costs, restrictions posed by the safety circuit and aging pose limitations on this battery system.
What’s the best battery for laptops?
Batteries for laptops have a unique challenge because they must be small and lightweight. In fact, the laptop battery should be invisible to the user and deliver enough power to last for a five-hour flight from Toronto to Vancouver. In reality, a typical laptop battery provides only about 90 minutes of service.
Computer manufacturers are hesitant to add a larger battery because of increased size and weight. A recent survey indicated that, given the option of larger size and more weight to obtain longer runtimes, most users would settle for what is being offered today. For better or worse, we have learned to accept the short runtime of a laptop.
During the last few years, batteries have improved in terms of energy density. Any benefit in better battery performance, however, is being eaten up by the higher power requirements of the laptops. It is predicted that the even more power-hungry PC’s of the future will counteract any improvements in battery technology, as marginal as they might be. The net effect will result in the same runtimes but faster and more powerful computers.
The length of time the battery can be used will get shorter as the battery ages. A battery residing in a laptop ages more quickly than when used in other applications. After a warm-up, the official operating temperature inside a laptop computer is 45°C (113°F). Such a high ambient temperature drastically lowers the battery’s life expectation. At a temperature of 45°C, for example, the life expectancy of a NiMH battery is less than 50 percent as compared to running it at the ideal operating temperature of 20°C (68°F).
The Li-ion does not fare much better. At this high ambient temperature, the wear-down effect of the battery is primarily governed by temperature as opposed to cycle count. The situation is worsened by the fact that the battery resides in a high SoC most of the time. The combination of heat and high SoC promotes cell oxidation, a condition that cannot be reversed once afflicted.
A fully charged Li-ion battery that is stored at 45°C suffers a capacity loss from 100 percent to about 70 percent in as little as six months. If this condition persists, the capacity degrades further to 50 percent in twelve months. In reality, the battery in a laptop is exposed to elevated temperatures just during use and the battery is in a full charge state only part of the time. But leaving the laptop in a parked car under the hot sun will aggravate the situation.
Some Japanese computer manufacturers have introduced a number of sub-notebooks in which the battery is mounted externally, forming part of the hinge. This design improves battery life because the battery is kept at room temperature. Some models carry several size batteries to accommodate different user patterns.
What then is the best battery for a laptop? The choices are limited. The NiCd has virtually disappeared from the mobile computer scene and the NiMH is loosing steam, paving the way for the Li-ion. Eventually, very slim geometry will also demand thin batteries, and this is possible with the prismatic Li-ion polymer.
Besides providing reliable performance for general portable use, the Li-ion battery also offers superior service for laptop users who must continually switch from fixed power to battery use, as is the case for many sales people. Many biomedical and industrial applications follow this pattern also. Here is the reason why such use can be hard on some batteries:
On a nickel-based charging system, unless smart, the charger applies a full charge each time the portable device is connected to fixed power. In many cases, the battery is already fully charged and the cells go almost immediately into overcharge. The battery heats up, only to be detected by a sluggish thermal charge control, which finally terminates the fast charge. Permanent capacity loss caused by overcharge and elevated temperature is the result.
Among the nickel-based batteries, NiMH is least capable of tolerating a recharge on top of a charge. Adding elevated ambient temperatures to the charging irregularities, a NiMH battery can be made inoperable in as little as six months. In severe cases, the NiMH is known to last only 2 to 3 months.
For mixed battery and utility power use, the Li-ion system is a better choice. If a fully charged Li-ion is placed on charge, no charge current is applied. The battery only receives a recharge once the terminal voltage has dropped to a set threshold. Neither is there a concern if the device is connected to fixed power for long periods of time. No overcharge can occur and there is no memory to worry about.
NiMH is the preferred choice for a user who runs the laptop mostly on fixed power and removes the battery when not needed. This way, the battery is only engaged if the device is used in portable mode. The NiMH battery can thus be kept fresh while sitting on the shelf. NiMH ages well if kept cool and only partially charged.
Selecting a Lasting Battery
As part of an ongoing research program to find the optimum battery system for selected applications, Cadex has performed life cycle tests on NiCd, NiMH and Li-ion batteries. All tests were carried out on the Cadex 7000 Series battery analyzers in the test labs of Cadex, Vancouver, Canada. The batteries tested received an initial full-charge, and then underwent a regime of continued discharge/charge cycles. The internal resistance was measured with Cadex’s Ohmtest™ method, and the self-discharge was obtained from time-to-time by reading the capacity loss incurred during a 48-hour rest period. The test program involved 53 commercial telecommunications batteries of different models and chemistries. One battery of each chemistry displaying typical behavior was chosen for the charts below.
When conducting battery tests in a laboratory, it should be noted that the performance in a protected environment is commonly superior to those in field use. Elements of stress and inconsistency that are present in everyday use cannot always be simulated accurately in the lab.
The NiCd Battery — In terms of life cycling, the standard NiCd is the most enduring battery. In Figure 8-1 we examine the capacity, internal resistance and self-discharge of a 7.2V, 900mA NiCd battery with standard cells. Due to time constraints, the test was terminated after 2300 cycles. During this period, the capacity remains steady, the internal resistance stays flat at 75mW and the self-discharge is stable. This battery receives a grade ‘A’ for almost perfect performance.
The readings on an ultra-high capacity NiCd are less favorable but still better than other chemistries in terms of endurance. Although up to 60 percent higher in energy density compared to the standard NiCd version, Figure 8-2 shows the ultra-high NiCd gradually losing capacity during the 2000 cycles delivered. At the same time, the internal resistance rises slightly. A more serious degradation is the increase of self-discharge after 1000 cycles. This deficiency manifests itself in shorter runtimes because the battery consumes some energy itself, even if not in use. [8.1] [8.2]
The NiMH Battery — Figure 8-3 examines the NiMH, a battery that offers high energy density at reasonably low cost. We observe good performance at first but past the 300-cycle mark, the performance starts to drift downwards rapidly. One can detect a swift increase in internal resistance and self-discharge after cycle count 700. [8.3]
The Li-ion Battery — The Li-ion battery offers advantages that neither the NiCd nor NiMH can match. In Figure 8-4 we examine the capacity and internal resistance of a typical Li-ion. A gentle capacity drop is observed over 1000 cycles and the internal resistance increases only slightly. Because of low readings, self-discharge was omitted for this test.
The better than expected performance of this test battery may be due to the fact that the test did not include aging. The lab test was completed in about 200 days. A busy user may charge the battery once every 24 hours. With such a user pattern, 500 cycles would represent close to two years of normal use and the effects of aging would become apparent.
Manufacturers of commercial Li-ion batteries specify a cycle count of 500. At that stage, the battery capacity would drop from 100 to 80 percent. If operated at 40°C (104°F) rather than at room temperature, the same battery would only deliver about 300 cycles. [8.4]