Very interesting post. Thanks for making this a separate thread.
I'm a bit dismayed and not a bit disturbed by some of the comments and "advice" I've seen on the subject of paralleling batteries. In particular, to state that this is "never a good idea" is pure poppycock.
For many cruising sailboats, particularly the larger ones, paralleling house batteries is the only practical solution to obtaining the needed AH capacity, where both cost and space considerations prevent the use of large industrial batteries. For many sailors, a large house bank constructed from, e.g., Trojan T-105 golf cart batteries (225AH 6V) in series/parallel is the most cost-effective solution.
Furthermore, the idea of separating your house batteries into, e.g., bank 1 and bank 2 and using them independently is not good practice. Paralleling all your batteries into a single large bank has numerous advantages, including faster charging and longer life. A large capacity bank may be charged more rapidly (more AH replenished per unit of time) than a smaller capacity one. This means shorter run times for your engine or generator, with attendant savings.
The percentage of discharge on a large capacity battery bank per unit of time as a result of the typical house load (lights, instruments, refrigeration, etc.) will be less than the percentage discharge caused by the same load on a smaller capacity battery bank. Typically, this will mean that the house batteries are drawn down less when banks are combined than they are when banks are used separately, resulting in longer life (more discharge/charge cycles) for all the batteries.
Finally, all house batteries are treated identically in the combined pattern. If they are treated well, their life will be greatly extended and overall costs will be reduced.
Starting batteries, of course, need to be treated separately. I favor an approach wherein a large capacity alternator/smart regulator charges the combined house battery bank, and a small EchoCharger is used to maintain the starting battery which is completely separate. All battery charging devices (shore power, onboard generator, wind powered generator, solar panels, etc.) also charge the combined house battery bank.
Batteries on sailboats don't die. They are murdered by bad installations, improper maintenance, and ignorant practices.
BTW, there's no such thing as a battery with no discharge. They all are self discharging, even with no load attached. Just a matter of rate!
Equally, they are all sulphating. From the moment after manufacture when sulphuric acid is added, they begin sulphating. How far and how fast this chronic process progresses depends on how the battery is treated. Proper 3-stage charging will help minimize sulphation. Failure to fully charge a battery and/or letting it sit without charging for any appreciable time contributes mightily to sulphation.
Just my $.02 :-)
S/V Born Free