We have a 32Vdc boat. 4 x 8v batteries in series, make up each of 2 banks. Each bank runs half of the ship and cranks it own engine. Thru the many years we have been onboard I keep checking/adding distilled water and hydrometer testing the banks every 6 months. Plus our 24Vdc inverter bank, gen-sets and electronics 12Vdc batteries. Seems the 32Vdc banks last 4 years. Somehow their in a rotation, every two years one bank gets replaced. The top Battery (25 to 32Volts) always needs more water and gets weak first in the bank. This past week, the port top battery was reading weak one year ahead of the norm. As an experiment, I had this one weak top battery replaced. We will see what happens to the rest of the bank as they start their fourth year. I think the top 12v batteries in the 24V bank may use just a bit more water also. Is anybody else out there noticing top battery issues in their in-series battery banks? Adding extra water vs the rest of the bank? Any thoughts?