With the move from analog to digital devices, new demands are being placed on the battery. Unlike analog equipment that draws a predictable and steady current, digital devices load the battery with short, high current bursts (Sony Vaio VGN-FZ battery).

One of the urgent requirements of a battery for digital applications is low internal resistance. Measured in milliohms (mW), the internal resistance is the gatekeeper that, to a large extent, determines the runtime. The lower the resistance, the less restriction the battery encounters in delivering the needed power bursts. A high mW reading can trigger an early ‘low battery’ indication on a seemingly good battery because the available energy cannot be fully delivered (Sony VGP-BPS8 battery).

In this article we examine the current requirements of analog and digital communications devices. Figure 1 provides typical examples of peak current of the analog two-way and digital Tetra radio, as well as the AMP, GSM, TDMA and CDMA mobile phones (Sony VGP-BPL9 battery).

Why do seemingly good batteries fail on digital equipment?

Service technicians have been puzzled by the seemingly unpredictable battery behavior when powering digital equipment. With the switch from analog to digital wireless communications devices, particularly mobile communications equipment, a battery that performs well on an analog system may show irrational behavior when used on a digital unit. Testing these batteries with a battery analyzer produces good capacity readings. Why then do some batteries fail on digital devices but not on analog (Toshiba PA3535U-1BRS battery)?

The overall energy requirement of a digital mobile phone is less than that of the analog equivalent, however, the battery must be capable of delivering high current pulses that are often several times that of the battery’s rating. Let’s look at the battery rating as expressed in C‑rates (Toshiba PA3535U-1BRS battery).

A 1C discharge of a battery rated at 500mAh is 500mA. In comparison, a 2C discharge of the same battery is 1000mA. A GSM phone powered by a 500mA battery that draws 1.5A pulses loads the battery with a whopping 3C discharge (Toshiba PA3534U-1BRS battery).

A 3C rate discharge is acceptable for a battery with very low internal resistance. However, aging batteries, especially Li‑ion and NiMH chemistries, pose a challenge because the mW readings increase with use. Improved performance can be achieved by using a larger battery, also known as an extended pack. Somewhat bulkier and heavier, an extended pack offers a typical rating of about 1000mAh or roughly double that of the slim-line. In terms of C‑rate, the 3C discharge is reduced to 1.5C when using a 1000mAh instead of a 500mAh battery (Toshiba PA3399U-2BRS battery).

As part of ongoing research to find the best battery system for wireless devices, Cadex has performed life cycle tests on various battery systems. In Figures 2 to 4 we examine NiCd, NiMH and Li‑ion batteries, each of which generate a good capacity reading when tested with a battery analyzer but produce stunning differences on a pulsed discharge of 1C, 2C and 3C. These pulses simulate a GSM phone (Toshiba PA3399U-1BRS battery).

A closer look reveals vast discrepancies in the mW measurements of the test batteries. In fact, these readings are typical of batteries that have been in use for a while. The NiCd shows 155mW, the NiMH 778mW and the Li‑ion 320mW, although the capacities checked in at 113, 107 and 94 percent respectively when tested with the DC load of a battery analyzer. It should be noted that the internal resistance of a new battery reads between 75 to 150mW (ACER Travelmate 2300 Battery).

From these charts we observe that the talk-time is in close relationship with the battery’s internal resistance. The NiCd produces a long talk time at all C-rates. In comparison, the NiMH only works at a lower C-rate. The Li‑ion performs better but is marginally at a 3C discharge (ACER Aspire 3020 Battery).

How is the internal battery resistance measured?

A number of techniques are available to measure the internal battery resistance. One common method is the direct current (DC) load test, which applies a discharge current to the battery while measuring the voltage drop. Voltage over current provides the internal resistance (ACER Aspire 3000 Battery).

The alternating current (AC) method, also known as the conductivity test, measures the electrochemical characteristics of a battery. This technique applies either a fixed frequency, or a frequency range from 10 to 1000Hz to the battery terminals. The impedance level affects the phase shift between voltage and current, which reveals the condition of the battery. Some AC resistance meters evaluate only the load factor and disregard the phase shift information (ACER Aspire 5020 Battery).

Cadex uses the discreet DC method to measure internal battery resistance. Added to the Cadex 7000 Series battery analyzers, a number of charge and discharge pulses are applied, which are scaled to the mAh rating of the battery tested. Based on the voltage deflections, the battery’s internal resistance is calculated. Known as Ohmtest™, the mW reading is obtained in five seconds (IBM ThinkPad R50 battery).

Neither of the three methods is dead accurate. The discrepancies are reasonably small on a good battery but the readings get more diverse on weaker packs. Figure 5 compares the accuracy obtained using the three methods (IBM ThinkPad R60 battery).

Resistance measurements alone do not provide a reliable indication on the battery’s performance. The mW readings may vary widely depending on battery chemistry, cell size (mAh rating), type of cell, number of cells connected in series, wiring and contact type (IBM ThinkPad R51 battery).

When using the impedance method, a battery with a known performance should be measured and its readings used as a reference. For best results, a reference reading should be on hand for each battery type. Figure 6 provides a guideline for digital mobile phone batteries based on impedance readings (IBM ThinkPad X41 Tablet battery).



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