Jim writes:

Is there any procedure that I need follow to charge my deep cell
batteries? I have a "Marine" 12 volt charger. When I try to charge them
it seems to take a very long time and I have never charged them fully
because I am worried about the long charges. How long should it take to
charge a deep cell?

Besides the Vector brand already mentioned, check out the Schumacher brand
sold at auto parts stores. A sturdy 30 amp digital charger for less than
$100. I followed this thing with an ammeter, and you can see it finessing
the very top of the charge with a differential time-varying voltage.