BU-802: What Causes Capacity Loss?

The energy storage of a battery can be divided into three sections known as the available energy that can instantly be retrieved, the empty zone that can be refilled, and the unusable part, or rock content, that has become inactive as part of use and aging. Figure 1 illustrates these three sections.

As the rock content portion of the battery grows, the charge time shortens because there is less to fill. Quicker charging times on faded batteries are noticeable especially with nickel-based batteries and in part also with lead acid, but not necessarily with Li-ion. Lower charge transfer capability that inhibits the flow of free electrons prolongs the charge time with aged Li-ion(See BU-409a: Why do Old Li-ion Batteries Take Long to Charge?)

In most cases, the decrease is linear and capacity fade is mostly a function of cycle count and age. A deep discharge stresses the battery more than a partial discharge. It is therefore better not to discharge the battery fully but charge it more often. A periodic full discharge is only recommended on nickel-based batteries to control “memory” and on smart batteries as part of calibration. Lithium- and nickel-based batteries deliver between 300 and 500 full discharge/charge cycles before the capacity drops below 80 percent.

Specifications of a device are always based on a new battery. This is only a snapshot, which cannot be maintained over any length of time. As with any shiny new machine, the battery will fade and if left unchecked, the reduced runtime can lead to battery-related breakdowns.

A pack should be replaced when the capacity drops to 80 percent; however, the end-of-life threshold can vary according to application, user preference and company policy. Capacity measurement, a service that remains the best indicator for replacement, should be done every 3 months with active fleet batteries(See BU-909: Battery Test Equipment)

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 caught in time, an equalizing charge can reverse the condition. Grid corrosion can be reduced with careful charging and optimization of the float charge(See BU-403: Charging Lead Acid)

With nickel-based batteries, the rock content is often the result of crystalline formation, also known as “memory.” A full discharge/charge cycle often restores the battery to full service. A periodic full discharge while the battery is in service keeps the crystallization under control and prevents damage to the separator(See BU-807: How to Restore Nickel-based Batteries)

The aging process of lithium-ion is cell oxidation, a process that occurs naturally as part of usage and aging, and cannot be reversed(See BU-808b: What causes Li-ion to Die)

References

[1] Courtesy of Cadex