Reconditioning deep cycle 12v dc batteries?

If they are maintenance free it is extremely difficult....however, if you have individual cell access.. You might be able to flush each cell carefully with a hose and nozzle after the acid is emptied first. Blasting them with a jet of water can sometimes loosen and flush out the settled lead sulphide compounds that cause the "death syndrome"...once they are completely flushed. Rinse with distilled water, empty, and then replace the sulphuric acid. Chemical house supplied available stronger than normal battery acid..Fill to the right levels and trickle charge over night then switch to a regular higher AMP charger. If you had a chance at all... this would have done it......