Understanding the importance of low conductivity
High battery capacity is of limited use if the pack cannot deliver the stored energy effectively. To supply power, the battery needs low internal resistance. Measured in milliohms (mΩ), resistance is the gatekeeper of the battery; the lower the resistance, the less restriction the pack encounters. This is especially important on heavy loads such as power tools and electric powertrains. High resistance causes the voltage to collapse on a load, triggering an early shutdown. Figure 1 illustrates a battery with low internal resistance in the form of a free-flowing tap and a battery with elevated resistance with a restricted tap.
Figure 1: Effects of internal battery resistance
A battery with low internal resistance delivers high current on demand. High resistance causes the battery voltage to collapse. The equipment cuts off, leaving energy behind.
Courtesy of Cadex
Lead acid has a very low internal resistance, and the battery responds well to high current bursts that last for a few seconds. Due to inherent sluggishness, however, lead acid does not perform well on a sustained high current discharge and the battery needs a rest to recover. Some sluggishness is apparent on all batteries at different degrees. This hints that power delivery is not based in internal resistance alone but also on the responsiveness of the chemistry. Nickel- and lithium-based technologies are more responsive than lead acid.
Sulfation and grid corrosion is the main contributor to the rise of the internal resistance with lead acid. Temperature also affects the resistance; heat lowers it and cold raises it. Heating the battery will momentarily lower the internal resistance to provide extra run time. This, however, does not restore the battery but will add stress.
Crystalline formation, also known as "Memory", contributes to the internal resistance on nickel-based batteries. This can often be reversed with deep-cycling. (See BU-807: How to Restore Nickel-based Batteries) The internal resistance Li-ion also increases with use and aging, but much improvement have been made with electrolyte additives to keep the buildup of films on the electrodes under check. (See BU-808b: What causes Li-ion to Die?)
Alkaline, carbon-zinc and most primary batteries have a relatively high internal resistance, and this limits its use to low-current applications such as flashlights, remote controls, portable entertainment devices and kitchen clocks. As these batteries discharge and deplete, the resistance increases further. This explains the relative short runtime when using ordinary alkaline cells in digital cameras.
Two methods are used to read the internal resistance of a battery: Direct current (DC) by measuring the voltage drop at a given current, and alternating current (AC) that takes reactance into account. Measuring a reactive device such as a battery, DC and AC resistance results can vary greatly and neither one is right or wrong. The DC method looks at pure resistance (R) while the AC method includes reactive components and provides an impedance (Z) reading. (See BU-902: How to Measure Internal Resistance)
Resistance values lend themselves well to predicting power delivery to a DC load, whereas Z provides more accurate results when working with a digital load. The battery has reactive qualities in form of capacitance (alike a giant capacitor) and performs well with digital loads. In comparison, the electric grid is resistive and helps to cook our meals at the end of a long day.
A battery pack has resistive and reactive values that are connected in series. Below is a DC resistance summary of a single-cell mobile phone battery and a power pack in which the cells are connected in parallel and series.
Internal Resistance of a Mobile Phone Battery
|Cell, single, high capacity prismatic||50mΩ||subject to increase with age|
|PTC, welded to cable, cell||25mΩ||18–30 mΩ according to spec|
|Protection circuit, PCB||50mΩ|
|Total internal resistance||ca. 130mΩ|
Internal Resistance of a Power Pack for Power Tools
|Cells 2P4S at 2Ah/cell,||18mΩ||subject to increase with age|
|Connection, welded, each||0.1mΩ|
|Protection circuit, PCB||10mΩ|
|Total internal resistance||ca. 80mΩ|
Courtesy: Siemens AG (2015, München)
Figures 2, 3 and 4 reflect the runtime of three batteries with similar Ah and capacities but different internal resistance when discharged at 1C, 2C and 3C. The graphs demonstrate the importance of maintaining low internal resistance, especially at higher discharge currents. The NiCd test battery comes in at 155mΩ, NiMH has 778mΩ and Li-ion has 320mΩ. These are typical resistive readings on aged but still functional batteries reflecting the chemistry. (See BU-208: Cycling Performance that demonstrates the relationship of capacity, internal resistance and self-discharge.)
Figure 2: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the NiCd battery is 113%; the internal resistance is 155mΩ.
Figure 3: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the NiMH battery is 94%, the internal resistance is 320mΩ.
Figure 4: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the Li-ion battery is 107%; the internal resistance is 778mΩ.
All three figures courtesy of Cadex
Notes: The tests were done when early mobile phones came with NiCd, NiMH and Li-ion. The performance of Li-ion and NiMH has since improved. The maximum GSM draw is 2.5A, representing 3C from an 800mAh pack, or three times the rated current.
Last updated 2015-06-18
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