Originally Posted by a64pilot
So can I put back the amps faster into a larger bank if it's discharged to a lower SOC or faster into a smaller bank discharged to a higher SOC, or is there not really enough difference to matter?
Is there any logic to having two similar banks, but alternating their use every other night?
Should I just put the battery switch to ALL and just have one large bank? I'm not worried about killing one bank and not being able to start the motor
Finally assuming I pull 100 AH out of a 660 AH bank nightly, is there an advantage to doubling up the chargers and being able to charge at 120 amps as long as the bank would accept that, would that significantly shorten charge time? Assuming a twice weekly use of the generator
allowing shorepower chargers to be used.
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I think this question has been well answered by Nigel Calder..
(By Nigel Calder - I did not write this quote)
Originally Posted by N. Calder
IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.
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 battery’s life.
This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a battery’s 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.
Let’s 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 5% increase in life expectancy using exactly the same batteries!
But now let’s 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:
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.
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).
My additional thoughts are below:
One large bank is best for reasons beyond even what Calder touches on.
(to 100% SOC) with two separate banks is less energy efficient due to the longevity of the time in acceptance limiting...
The larger bank will also not be as dramatically affected by Peukert and you'll actually get slightly more usable amp hours
out of a larger bank with the same load than you do with a smaller bank with the same load.
For example a bank with a Peukert of 1.2 and an average load of 8A looks like this. (if you were to draw the bank to 10.5V and use all the capacity)
Increases in capacity at slow rate discharge are from mathematical formula and usually do not = actual chemical capacity. Gains at slow
rates can range from 105% to 123% of capacity, but I've not seen much more than that...
100Ah bank, Peukert 1.2, load 8A = 91Ah's
200Ah bank, Peukert 1.2, load 8A = 209Ah's
400Ah bank, Peukert 1.2, load 8A = 480 Ah's
600Ah bank, Peukert 1.2, load 8A = 782Ah's
800Ah bank, Peukert 1.2, load 8A = 1104 Ah's
And below is an actual hourly rate chart from Rolls Battery that illustrates this extremely well. Note the usable capacity of this battery at a load of 4.86A vs. the 20 hour rated capacity load of 18.75A...
Boaters rarely if ever load their house banks at the 20 hour rate as an average load
... This would be an average load of 20A on a 400Ah bank when really the average boater with a 400Ah bank is often pulling 2A - 8A, on average
. By using the bank split in half you lose much of these Peukert gains, especially if you use thick plate deep cycle batteries with a high Peukert.
Rolls Battery Hour Rate Chart CH375: (NOTE: Lifelines
have a much lower Peukert but there is still Peukert effect)
This means that your bank will have even shallower discharges, not just because it is one large bank, but if the load stays the same and you increase bank size you will actually get a bit more out of the larger bank due to the Peukert effect.
An often overlooked benefit of a larger bank is the banks ability to support higher voltages for your equipment
for longer periods. Things like heaters and refrigeration
run more efficiently at higher voltages. Starter motors work better with less voltage sag as do windlass
or thruster or water
maker motors. With a single larger bank you not only combine Ah's of capacity but you also combine cranking amperage. More often than not a larger bank, even at 50% SOC, will start the motor
with less voltage sag than will a smaller bank at a higher state of charge. It is not uncommon to measure a house bank with a cranking ability in excess of 3000A - 4000A
The batteries stay better balanced
so that when you do need them in parallel they are all working as evenly as they can be.
The shallower the discharges, for the same load, the less sulfation you create and this is why a single large bank, cycled less deeply, yields more cycles.
one large contiguous bank allows for more precision in keeping the bank wiring and current
flowing through the bank balanced over the long haul. EnerSys (Odyssey TPPL AGM's) addresses this very well in their technical manual
Splitting/alternating a house bank makes for a monitoring nightmare with a battery monitor
or even a Smart Gauge.
Charging to 100% is less time consuming for one bank vs. two alternated.
Cycle life is better
The batteries ability to support voltage is better
You get slightly more usable capacity
at the same average load