The battery and the digital load（2）

The milliohm readings are related to the battery voltage. Higher voltage batteries allow higher internal resistance before the system fails 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 (IBM ThinkPad R50 battery).

Temperature also affects the internal resistance of a battery. The internal resistance of a naked Li‑ion cell 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 (IBM ThinkPad R60 battery).

Cold temperatures have a drastic effect on all batteries. At 0ºC (32ºF), the internal resistance of the same Li‑ion cell drops to 70mW. The resistance increases to 80mW at -10ºC (50ºF) and 100mW at -20ºC (-4ºF) (IBM ThinkPad R51 battery).

The internal resistance readings work best with Li‑ion batteries because the degradation follows a linear pattern with cell oxidation. The performance of NiMH batteries can also be measured with the internal resistance method but the readings are less dependable. There are instances when a poorly performing NiMH battery can also exhibit a low mW reading (IBM ThinkPad X41 Tablet battery).

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. Of course, high internal resistance can have sources other than memory alone (Toshiba PA3535U-1BRS battery).

Summary

Customer demand has compelled manufacturers to equip portable devices with batteries that provide a long talk-time, are small and are light in weight. By packing more energy into a pack, other qualities may be neglected, one of which is internal resistance and longevity (Toshiba PA3535U-1BRS battery).

Predictable low mW reading and long service life is found in the NiCd family. This chemistry has been replaced with higher energy dense batteries for many wireless applications. In addition, negative publicity about the memory phenomenon and concerns of toxic metals have caused a shift towards alternative choices (Toshiba PA3534U-1BRS battery).

For many applications, including biomedical devices, power tools and most notably the Tetra system, the NiCd may be the only battery that has the endurance of delivering high pulse current under continuous usage. Other chemistries are simply too fragile. The resistance on a NiMH rises after a few hundred charge/discharge cycles. In comparison, a properly maintained NiCd provides over one thousand cycles (Toshiba PA3399U-2BRS battery).