.
Cycle life of NiMH under different operating conditions
Figure 3:
Cycle life of NiMH under different operating conditions
NiMH performs best with DC and analog loads; digital loads lower the cycle life. Li-ion behaves similarly.
Source: Zhang (1998)
Figure 4 examines the number of full cycles a Li-ion battery with a cobalt cathode can endure when discharged at different C-rates. At a 2C discharge, the battery exhibits higher stress than at 1C, limiting the cycle count to about 450 before the capacity drops to half level.
Cycle life of Li-ion with cobalt cathode at varying discharge levels
Figure 4:
Cycle life of
Li-ion with cobalt cathode at varying discharge levels
The wear-and-tear of a battery increases with higher loads.
Source: Choi et al (2002)
For a long time, Li-ion had been considered fragile and unsuitable for high loads. This has changed, and today many lithium-based systems are more robust than the older nickel and lead chemistries. Manganese and phosphate-type Li-ion permit a continuous discharge of 30C. This means that a cell rated at 1,500mAh can provide a steady load of 45A, and this is being achieved primarily by lowering the internal resistance through optimizing the surface area between the active cell materials. Low resistance keeps the temperature down, and running at the maximum permissible discharge current, the cells heat up to about 50ºC (122ºF); the maximum temperature is limited to 60°C (140°F).
One of the unique qualities of Li-ion is the ability to deliver continuous high power. This is possible with an electrochemical recovery rate that is far superior to lead acid. The slow electrochemical reaction of lead acid can be compared to a drying felt pen than works for short marking but needs rest to replenish the ink.
Simple Guidelines for Discharging Batteries
The battery performance decreases with cold temperature and increases with heat.
Heat increases battery performance but shortens life by a factor of two for every 10°C increase above 25–30°C (18°F above 77–86°F).
Although better performing when warm, batteries live longer when kept cool.
Operating a battery at cold temperatures does not automatically permit charging under these conditions. Only charge at moderate temperatures.
Some batteries accept charge below freezing but at a much-reduced charge current. Check the manufacturer’s specifications.
Use heating blankets if batteries need rapid charging at cold temperatures.
Prevent over-discharging. Cell reversal can cause an electrical short.
Deploy a larger battery if repetitive deep discharge cycles cause stress.
A moderate DC discharge is better for a battery than pulse and aggregated loads.
A battery exhibits capacitor-like characteristics when discharging at high frequency. This allows higher peak currents than is possible with a DC load.
