Besides age-related losses,sulfation and grid corrosion are the main killers of lead acid batteries.Sulfation is a thin layer that forms on the negative cell plate if the battery is allowed to dwell in a low state-of-charge.If sulfation is caught in time,an equalizing charge can reverse the condition.With nickel-based batteries,the so-called rock content is often the result of crystalline formation,also known as “memory,” and a full discharge can sometimes restore the battery.The aging process of lithium-ion or lithium battery is cell oxidation,a process that occurs naturally as part of usage and aging and cannot be reversed.
Although the manufacturer specifies the runtime of portable equipment based on a battery performing at 100 percent,most packs in the field operate at less capacity.As time goes on,the performance declines further and the battery gets smaller in terms of holding capacity.A pack should be replaced when the capacity drops to 80 percent.This is only 20 percent down from 100 percent,and the end-of-life threshold may vary according to application and company policy.
Energy storage in a battery can conceptually be divided into three imaginary segments of the available energy,the empty zone that can be refilled, and the unusable part (rock content).Figure 1 illustrates these three sections.
Rising Internal Resistance
High capacity has limited use if the battery is unable to deliver the stored energy effectively.To bring the power out,the battery needs low internal resistance.Measured in milliohms (mW),resistance is the gatekeeper of the battery;the lower the value,the less restriction the pack encounters.This is especially important with heavy loads and high current pulses,as elevated resistance causes the voltage to collapse and trigger an earlyshutdown.The device turns off and valuable energy is left behind.Figure 2 illustrates batteries with low and high internal resistance as free-flowing and restricted taps.
Lead acid has a very low internal resistance,and the battery responds well to high current bursts lasting for only a few seconds.Due to inherent sluggishness,however,lead acid does not perform well with a sustained discharge at high current and the battery needs rest to recover.Sulfation and grid corrosion are the main causes of increased internal resistance.Temperature also affects the resistance;heat lowers it and cold raises it.
Alkaline,carbon-zinc and other primary battery has relatively high internal resistance,and this relegates their use to low-current applications such as flashlights,remote controls,portable entertainment devices and kitchen clocks.As the batteries discharge,the resistance increases further.This explains why regular alkaline cells have a relatively short runtime in digital cameras.The high internal resistance limits most primary batteries to “soft” applications,and using them to drive power tools that draw high amperage is unthinkable.
