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post #21 of Old 02-11-2013
Maine Sail
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Re: Are portable generators safe?

Originally Posted by DougSabbag View Post
I can't over emphasize the value of having TWO BANKS of batteries to go along with a robust solar power system.
Otherwise you are setting yourself up for a guaranteed slow decline.

Two banks provides you a lot of wonderful benefits!
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:

Originally Posted by Nigel Calder
(By Nigel Calder)


The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the batteryís life.

This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a batteryís life expectancy is disproportionately shortened. A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Letís say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged. The batteries will fail after 380 cycles, which is 760 days (since each is used every other day). If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 50% increase in life expectancy using exactly the same batteries.

But now letís double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load. The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days. Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1. For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2. All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).


-Maine Sail / CS-36T

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