Chapter 9: Internal Battery Resistance
With the move from analog to digital devices, new demands are being placed on the battery. Unlike analog equipment that draws a steady current, the digital mobile phone, for example, loads the battery with short, high current bursts.
Increasingly, mobile communication devices are moving from voice only to multimedia which allows sending and receiving data, still pictures and even video. Such transmissions add to the bandwidth, which require several times the battery power compared to voice only.
One of the urgent requirements of a battery for digital applications is low internal resistance. Measured in milliohms (mΩ), 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 mΩ reading can trigger an early ‘low battery’ indication on a seemingly good battery because the available energy cannot be delivered in an appropriate manner.
Figure 9-1 examines the major global mobile phone systems and compares peak power and peak current requirements. The systems are the AMP, GSM, TDMA and CDMA. [9.1]
- Some TDMA handsets feature dual mode (analog 800mA DC load; digital 1500mA pulsed load).
- Current varies with battery voltage; a 3.6V battery requires higher current than a 7.2V 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 phones, a battery that performs well on an analog device may show irrational behavior when used on a digital device. Testing these batteries with a battery analyzer produces normal capacity readings. Why then do some batteries fail prematurely on digital devices but not on analog?
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.
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.
A 3C rate discharge is fine for a battery with very low internal resistance. However, aging batteries, especially Li-ion and NiMH chemistries, pose a challenge because the mW readings of these batteries 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.
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 Figure 9-2, Figure 9-3, and Figure 9-4, we examine NiCd, NiMH and Li-ion batteries, each of which generates 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. [9.2]
A closer look reveals vast discrepancies in the mΩ 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 was low when the batteries were new. [9.3] [9.4]
From these charts we can see that the talk-time is in direct relationship with the battery’s internal resistance. The NiCd performs best and produces a talk time of 140 minutes at 1C and a long 120 minutes at 3C. In comparison, the NiMH is good for 140 minutes at 1C but fails at 3C. The Li-ion provides 105 minutes at 1C and 50 minutes at 3C discharge.
How is the internal battery resistance measured?
A number of techniques are used to measure internal battery resistance. One common method is the DC load test, which applies a discharge current to the battery while measuring the voltage drop. Voltage over current provides the internal resistance (see Figure 9-5). [9.5]
The AC method, also known as the conductivity test, measures the electrochemical characteristics of a battery. This technique applies an alternating current to the battery terminals. Depending on manufacturer and battery type, the frequency ranges from 10 to 1000Hz. The impedance level affects the phase shift between voltage and current, which reveals the condition of the battery. The AC method works best on single cells. Figure 9-6 demonstrates a typical phase shift between voltage and current when testing a battery. [9.6]
Some AC resistance meters evaluate only the load factor and disregard the phase shift information. This technique is similar to the DC method. The AC voltage that is superimposed on the battery’s DC voltage acts as brief charge and discharge pulses. The amplitude of the ripple is utilized to calculate the internal battery resistance.
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. Figure 9-7 shows the technique used. [9.7]
Figure 9-8 compares the three methods of measuring the internal resistance of a battery and observe the accuracy. In a good battery, the discrepancies between methods are minimal. The test results deviate to a larger degree on packs with poor SoH.
Impedance measurement alone does not provide a definite conclusion as to the battery performance. The mW readings may vary widely and are dependent on battery chemistry, cell size (mAh rating), type of cell, number of cells connected in series, wiring and contact type.
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 9-9kl; provides a guideline for digital mobile phone batteries based on impedance readings.
The milliohm readings are related to the battery voltage. Higher voltage batteries allow higher internal resistance because less current is required to deliver the same power. The ratio between voltage and milliohm is not totally linear. There are certain housekeeping components that are always present whether the battery has one or several cells. These are wiring, contacts and protection circuits.
Temperature also affects the internal resistance of a battery. The internal resistance of a naked Li-ion cell, for example, measures 50mW at 25°C (77°F). If the temperature increases, the internal resistance decreases. At 40°C (104°F), the internal resistance drops to about 43mW and at 60°C (140°F) to 40mW. While the battery performs better when exposed to heat, prolonged exposure to elevated temperatures is harmful. Most batteries deliver a momentary performance boost when heated. [9.9]
Cold temperatures have a drastic effect on all batteries. At 0°C (32°F), the internal resistance of the same Li-ion cell drops to 70mW. The resistance increases to 80mW at -10°C (50°F) and 100mW at -20°C (-4°F).
The impedance readings work best with Li-ion batteries because the performance degradation follows a linear pattern with cell oxidation. The performance of NiMH batteries can also be measured with the impedance method but the readings are less dependable. There are instances when a poorly performing NiMH battery can also exhibit a low mW reading.
Testing a NiCd on resistance alone is unpredictable. A low resistance reading does not automatically constitute a good battery. Elevated impedance readings are often caused by memory, a phenomenon that is reversible. Internal resistance values have been reduced by a factor of two and three after servicing the affected batteries with the recondition cycle of a Cadex 7000 Series battery analyzer. Of cause, high internal resistance can have sources other than memory alone.
What’s the difference between internal resistance and impedance?
The terms ‘internal resistance’ and ‘impedance’ are often intermixed when addressing the electrical conductivity of a battery. The differences are as follows: The internal resistance views the conductor from a purely resistive value, or ohmic resistance. A comparison can be made with a heating element that produces warmth by the friction of electric current passing through.
Most electrical loads are not purely resistive, rather, they have an element of reactance. If an alternating current (AC) is sent through a coil, for example, an inductance (magnetic field) is created, which opposes current flow. This AC impedance is always higher than the ohmic resistance of the copper wire. The higher the frequency, the higher the inductive resistance becomes. In comparison, sending a direct current (DC) through a coil constitutes an electrical short because there is only a very small ohmic resistance.
Similarly, a capacitor does not allow the flow of DC, but passes AC. In fact, a capacitor is an insulator for DC. The resistance that is present when sending an AC current flowing through a capacitor is called capacitance. The higher the frequency, the lower the capacitive resistance.
A battery as a power source combines ohmic, inductive and capacitive resistance. Figure 9-10 represents these resistive values on a schematic diagram. Each battery type exhibits slightly different resistive values. [9.10]