The life of a lead-acid battery depends on keeping the active material securely held on its plates, and this is governed by how gently the charge and discharge currents are handled.

The active material (lead and lead-dioxide paste) is stressed by the magnitude of current passing through it; large currents generate heat and internal gassing that mechanically disturb the plates.

Rapid, high-rate charging and discharging force very large currents through the cells, causing excess heat, heavy gassing and buckling that loosen and shed the active material and weaken the plate structure.

Because this directly damages the plates, rapid charging and discharging is the practice to be strictly avoided for long life.

Charging at a moderate rate over long periods keeps currents and heat low, so it is gentle on the plates and actually prolongs life.

Discharging under light loads draws only small currents, and holding a steady constant voltage during use avoids sudden surges; both are healthy, not harmful, so they are not what should be avoided.

Hence, the answer is Charging and discharging the battery rapidly.