Last Oct. I replaced my Parallax converter with a Progressive PD-4655 to maintain and charge my two relatively new Interstate 12V Batteries over Winter. When I checked the SG last week both were in the Red at 1.200 and below. VDC WAS 12.5 on bat. Pwr. and 13.5 on charge. The dealers question was:” Is the Hydrometer calibrated for Deep Cycle batteries”? Never heard of such a thing. I understand that ALL batteries used the same acid; the difference being construction of the internals. Was his question valid?