Originally Posted by sea_hunter
I agree with your point 1 and half of point 2 of your response, however, unless both start and house banks are completely isolated (which in most cases are not) it's better if the batteries are rated the same. Once you start adding external charging sources IE solar, it only takes a short time for the start battery to get cooked if not switched or correctly isolated. There's been many a posting here ( perhaps hundreds, even thousands) over the years with battery/charging issues suggesting there's obvious issues with the way many boats are wired. Keeping the ratings the same prevents over charging of the lesser battery.
1. On most cruising boats other than very small power or sailboats, the house battery bank is normally MUCH LARGER in capacity than the start battery. It would be totally impractical to have a start battery bank the same capacity as the start battery on these boats, and extremely expensive and wasteful. From both an engineering and a practical standpoint, this would be ridiculous.
2. On such boats the start battery is totally isolated
from the house battery bank. Sometimes this is via a 1-2-Both-Off switch and a battery combiner or voltage follower device -- these are preferred methods.
Sometimes the isolation is just manual, with the boat operator using the battery switch itself to combine the batteries. This is NOT a preferred method, though it can work for some folks if they take care with the battery switching regime, if the batteries are not badly depleted, etc., etc.
By far the preferred method is to use an automatic charging relay ACR or, better IMHO, a voltage-follower device. These are totally automatic and, when no charging is taking place, they totally isolate the two banks.
BTW, the idea that you will fry a flooded or AGM battery which is already near full charge by exposing it to voltages in the charging range of 14.4-14.8VDC for a few hours is totally bogus. So long as you don't let the voltage climb above that level, a battery is going to take what it's going to take and no more.