It is not exactly chemistry part. As far as chemistry is concerned The active material gets converted to PbSO4. At the same time the specific gravty of the electrolyte
Reaches close to 1.0(water). Lead sulphate will start dissolving in the electrolyte and detach from the plates.
This leads to capacity loss. Deeper discharges closes the electrolyte paths(Pores) in the active materialand hence closes the charging paths. This is a parmanent death of the batteries and irricoverable. Are you the user of the above batteries.
Kavuri

Hai Kavuri

Thanks for the response

You are correct. I am discussing about submarine batteries.Recently in an experiment during deep discharge I analysed Copper in the electrolyte which has entered for CSM(Copper stretch material). The negative plate had partially disintregrated.

Varghese