How is a battery aged?

A lead acid battery is most effectively aged if it is discharged. A new battery is then destroyed very quickly.

Also the wear rate increases, or the aging accelerates if desired, the deeper the battery is discharged before each new charge.

A battery that is constantly kept at high recharge rate and only be partially discharged from time to time (for example, spare power battery) may last for 8 – 15 years.
Depending on the battery type, a battery that is kept properly charged is capable of deliver approximately 200 – 1400 cycles.

Note: A battery cycle consists of an 80% discharge followed by a recharge to 100%.

Less expensive types of lead acid batteries last for 2-300 cycles, while AGM, GEL and Tube cells range between 500-1400 total cycles.
The same less expensive battery (capable of 200 cycles) at 80% discharge can hold over 1000 cycles at 25% discharge.

For each discharge cycle, a portion of the lead sulphate is released and falls off the plates and ends in the bottom of the container or in the gel of the GEL type. This material can never be re-formed into active lead and the capacity gradually decreases.
When a battery has fallen below 50% of its original capacity, it is usually considered to be worn out, but other limits can be set by a specific application. In the final phase of the battery aging a cell defect occurs, ie a cell gets close or completely interrupted or becomes short-circuited.