How to increase the runtime of your wireless device (BU37A)

As the sponsor of www.BatteryUniversity.com, Cadex Electronics gets many interesting enquiries from battery users. One writes, "Hi, I am looking for an answer to a perplexing question. A co-worker and I have identical cell phones from the same provider. Moving into a new house, she complained of short battery runtime. I told her she was out of her mind, but then I noticed my battery behaving differently when I travel. Is there some mysterious force that's draining the battery?"

Yes, there is a force that's draining the battery. An active cell phone is in constant communication with the tower and consumes small bursts of energy once every second or so to check for incoming calls. The transmit power is adjusted to the signal strength. If the cell phone is close to a repeater tower, little energy is needed to communicate. Moving further away or entering an environment with high electrical noise, such as a shopping mall, hospital or factory, more energy will be required. An analogy can be made to sitting in a restaurant. In a quiet establishment the voice can be low, but as the crowd grows, everyone needs to talk louder to be heard.

Living in sight of a tower has advantages and your battery will run longer between charges. In essence, towers are the best friends to cell phone batteries. Even the placement of a cell phone in your house has an effect on runtime. At a recent meeting with a large cellular provider in the UK, a manager said that his son noticed short standby times after moving to his basement bedroom. If possible, leave your cell phone in an upstairs room facing a tower. When traveling by car, don't place your cell phone on the floor. Instead, raise it closer to window level but avoid direct exposure to the sun, as heat will harm the battery.

The same energy savings apply to TETRA and P25 radio systems, cordless telephones, Wi-Fi and Bluetooth devices. A wireless headset that is communicating with your cell phone on the belt will provide longer runtimes than placing the handset on the dining table while doing the cooking. The Bluetooth headset needs to work harder when farther away from the user, although the quality of communication may not be affected.

Just to clarify that the energy savings from the placement of a wireless device only apply when it's in the ON position. When OFF, the residual loads are very low; the battery needs only to supply power for housekeeping functions such as maintaining the clock. Housekeeping and self-discharge consume 5-10% of the available battery energy per month.

During the last few years, standby and talk-times have much improved. The lithium-ion battery has doubled its energy density since its introduction in the early 1990s. In addition, large energy savings are being achieved in the receiver and demodulator circuits. Figure 1 illustrates the reduction of power consumption in these circuits since 2002. We must keep in mind that this saving only applies to standby and receiving. Transmitting requires about five times the amount of power compared to receiving and demodulation. Modern handsets have also achieved better efficiencies in transmit circuits.

Figure 1: Reduction in power consumption. In addition to higher capacity batteries, cell phone manufacturers have achieved notable power savings in the receiver and demodulator circuits.(Sieber et al., 2004).

It's not always the battery's fault

When the cell phone quits, the battery often gets the blame. The battery is the only user-replaceable part on a cell phone and becomes an easy target. Service personnel often replace the pack without testing, only to have the fault recur.

Moving from nickel-based to lithium-ion batteries eliminated many problems. Lithium-ion packs are maintenance free and don't require periodic full discharges to restore capacity; there is no memory effect. Still, customers suspect the batteries as the reason of most problems. As a result, large volumes of good packs are replaced and discarded. This is costing the cell phone industry ten million dollars annually. Cell phone providers say that 90% of returned batteries can easily be serviced.

Technology is now available to rapid-test batteries at store level while the customer waits. If a replacement is needed, an exchange is given from a pool of batteries that had previously been serviced. On-site restorations eliminate courier charges and relieve manufacturers from the burden of handling tons of returned batteries.

Figure 2 illustrates the service flow, starting with the customer bringing in the cell phone, checking the battery and providing a replacement. The replacement pack is taken from a pool that had previously been refurbished on site with a battery analyzer. A recent pilot test by a large service provider using this exchange program worked well and no replacement battery ever came back due to failure.

Figure 2: A cell phone is brought in with a suspect battery. The battery is tested while the customer waits. If in need of service, a refurbished pack is given in return. Servicing batteries at point-of-sales saves the industry millions of dollars and adds to customer satisfaction.

According to a U.S. cellular provider, a typical store gets an average of ten returned batteries a day. The handling cost is estimated at $15US per pack. This amounts to a daily expense of $150 per store. Realizing this high expense and trying to cut cost, ten stores participated in a one-month experiment that involved examining and servicing incoming batteries using Cadex battery analyzers. During this study period, the stores saved 1981 batteries, resulting in a saving of about $30,000.

Battery rapid-testing

One of the key features of a modern battery analyzer is obtaining accurate test results when rapid-testing a battery. In the past, the battery state-of-health was mostly estimated by measuring internal resistance. As Figure 3 shows, the battery's ability to hold energy (capacity) may not correspond with resistance. On some lithium-ion batteries, the capacity can drop to half its original level while maintaining low

Figure 3: Relationship of capacity and resistance as part of cycling and aging. The state-of-health of lithium-ion cannot be obtained by measuring resistance alone.

For best results, a battery should be tested under similar conditions as used in the field. QuickSort™ by Cadex achieves this through a technology referred to as electrochemical dynamic response. This method can be compared to a mechanical arm under load. A strong arm remains firm, whereas a weak one bends and becomes sluggish when under load. This response can also be applied to estimating battery state-of-health. QuickSort™ provides a correct prediction 90% of the time over a wide population of lithium-ion batteries in various state-of-charge conditions.

A relatively high number of batteries fail due to over-discharge. We discovered this while checking 1000 customer-returned packs that had been sent to the Cadex lab for further evaluation. Among these packs, 30% had no voltage reading and appeared dead. This was due to over-discharge. At voltages between 2.5 and 2.8V, the internal safety circuit of a lithium-ion battery disengages and the battery goes into a sleep mode, making a recharge impossible. The Boost program of the Cadex C7000 Series battery analyzers activates the safety circuit and brings the battery back to life. The restoration is permanent and the pack can be returned to the customers. Figure 4 illustrates this process.
		Figure 4: Over-discharged battery receives a "Boost" current to raise the cell voltage into the operational threshold, re-engaging the safety circuits and enabling a charge.

To prevent a cell phone battery from inadvertently falling asleep, apply a 30-minute charge (or longer) after the "Low Batt" indicator comes on. Do not store the cell phone in a totally discharged condition. Peripheral loads, combined with self-discharge, will further discharge the battery. This can lead to an eventual disconnect in which the battery appears dead as described above.

Besides rapid-test and boost, most battery analyzers also offer full battery service programs that consist of charge and discharge cycles. Such programs provide the most accurate battery assessment and are the recommended methods to prepare replacement batteries for exchange purposes. Figure 5 illustrates the Cadex C7400.

Figure 5: Cadex C7400 battery analyzer provides QuickSort™, Boost and full service programs. The four battery stations accommodate virtually any portable battery.

Point-of-sale battery testing has only become practical with the introduction of advanced battery analyzers. Manufacturers support on-site testing, knowing that the service will reduce warranty returns. Organizations employing battery analyzers have reported a notable reduction in service related expenses. Testing batteries at storefronts also improves customer service and enhances customer satisfaction. Much of the success will depend on the effectiveness of battery test devices.

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Created: October 2007