Originally Posted by Maine Sail
I would urge folks to consider not doing this. It's not just me who does not advise this but Nigel Calder and most marine electrical experts advise against this as well.
I set up cruising banks all the time and the benefits of a single large bank are less cycling depth, which equals longer life, and the benefit of the Peukert effect which gets cut when you cut the banks in two.
I tend to suggest one large bank for reasons beyond even what Nigel Calder touches on. It is more efficient to charge one bank than two unless using 100% free energy. Even then with solar or wind the time allotted to "finishing" two banks is less efficient due to the longevity of the time in bank acceptance limiting and the time the "finishing" charge takes.
The single larger bank will also not be as dramatically affected by Peukert effect and you'll actually wind up with more usable amp hours out of a single larger bank, with the same daily load, than you do with two smaller banks with the same daily load. By cutting the bank in half you lose both the positives of size on DOD and the Peuekert effect.
For example a bank with a Peukert of 1.25 and a average load of 8A it looks like this:
100Ah bank, Peukert 1.25, load 8A = 88Ah's
200Ah bank, Peukert 1.25, load 8A = 211Ah's
300Ah bank, Peukert 1.25, load 8A = 351 Ah's
400Ah bank, Peukert 1.25, load 8A = 502 Ah's
600Ah bank, Peukert 1.25, load 8A = 834Ah's
800Ah bank, Peukert 1.25, load 8A = 1200 Ah's
By using a single larger bank and considering the Peukert effect it means that your bank will have shallower discharges, not just because it is one large bank, but if the average load stays the same, and you increase bank size, you will actually get more out of the larger bank due to Peukert.
And rather than go into a huge explanation repeating what Calder has already written about I will just post it here:
WOW! I had never heard of this concept.
Not to argue this, but we added the second bank of 1200 AH, besides the original 1200 AH, because the single bank was draining while charging, and therefore not getting back up to a full charge before nightfall.
So by the morning that bank was pretty weak, and we played catch up all day, until eventually I had to start an engine.
After adding the second bank, I would charge one bank, while draining the other, and would switch / reverse their roles each day.
That worked - theoretically endlessly.
So apparently what this Peukert affect means to that, is that I should have only added the 1200 AH / additional batteries to the same bank. Hmmmmmm...
I'll have to do some research on this, because I just loved being able to isolate and control each individual bank. And, when I chose to I could bring them both into the same "role".
I think I would still wire things that way, but would be able to control over 1000 AH bank(s) individually, as I found "worked" for me.