All batteries are affected by self-discharge.The battery self-discharge is not a manufacturing defect but a battery characteristic,although poor manufacturing practices and improper handling can increase the problem.The amount of electrical self-discharge varies with battery type and chemistry.Primary cells such as lithium battery and alkaline retain the stored energy best and can be kept in storage for several years. Figure 1 illustrates self-discharge in the form of leaking fluid.
Among rechargeable batteries,lead acid has the lowest self-discharge and loses only about five percent per month.With age and usage,however,the flooded lead acid builds up sludge in the sediment trap,which causes a soft short when this semi-conductive substance reaches the plates.Nickel-based rechargeable batteries leak the most and need recharging before use when placing them on the shelf for a few weeks.High-performance ni-mh battery and nickel-based batteries have a higher self-discharge than the standard versions.
Furthermore,self-discharge increases with use and age,and the contributing factors are crystalline formation (memory),permitting the battery to “cook” in the charger or exposing it to repeated harsh deep discharge cycles.Lithium-ion self-discharges about five percent in the first 24 hours and then loses 1 to 2 percent per month; the protection circuit adds another three percent per month.A faulty separator can lead to a high self-discharge and if critical,the electrical current will generate enough heat that can in extreme cases lead to a thermal breakdown.Table 2 shows the typical self-discharge of battery systems.
The energy loss is asymptotical,meaning that the self-discharge is highest right after charge and then tapers off.Nickel-based batteries lose 10 to 15 percent of their capacity in the first 24 hours after charge,then 10 to 15 percent per month.Figure 3 shows the typical loss of a nickel-based battery while in storage.
The self-discharge on all battery chemistries increases at higher temperature and the rate typically doubles with every 10°C (18°F).A noticeable energy loss occurs if a battery is left in a hot vehicle.High cycle count and aging also increase self-discharge.Nickel-metal-hydride is good for 300-400 cycles,whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance.The self-discharge on an older nickel-based battery can get so high that the pack loses its energy through leakage rather than normal use.
Under normal circumstances the self-discharge of Li-ion 18650 battery is reasonably steady throughout its service life; however a full state-of-charge and elevated temperature increase the self-discharge.These very same factors also affect longevity,a phenomenon that applies to most batteries.Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge.The high self-discharge at full state-of-charge may come as a surprise to many.This explains in part the asymptotical self-discharge characteristic when removing a battery from the charger.
