There are several wear-out mechanisms in a lead acid battery. The one the limited depth of discharge addresses is plate integrity. Basically, the discharge chemical reaction within a battery dissolves the lead off the anode and puts it into solution as lead sulphate. The cathode reaction dissolves the lead oxide of the cathode and puts it into solution as lead sulphate and water. Charging the battery drives the reaction in reverse and hypothetically replates the lead and lead oxide back on their respective plates. The deeper the discharge the more of the plates are put into solution and the plates become porous and spongy. As the material replates, it does not fill in all of the pores. There is also some tendency for the lead and lead-oxide to plate to impurities and precipitate to the bottom of the cell. Occasionally dendrils of lead can grow between the plates and short the cell. The precipitate of lead and lead oxide can build up in the bottom and short the plates as well.
By limiting the depth of discharge the integrity of the plates remain higher. The amount of lead in solution is limited so the amount of precipitate and dendril growth is limited.
This reaction is taking place all the time in a battery, whether or not the battery has a load connected. Charging the battery at the float voltage level forces the chemical reaction back as rapidly as it naturally occurs without a load (self discharge). This is why batteries don’t last forever, even with a float charge.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...