IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
Here's Nigels take..
Begin Quote - 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 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:
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)."
End Calder Quote:
My additional thoughts are below:
One large bank is best for reasons beyond even what Calder touches on.
#1 It is far more efficient to charge one bank rather than two unless using 100% free energy. Even then "finishing" two banks is less efficient due to the longevity of the time acceptance limiting and "finishing" the battery takes while using free energy..
#2 The larger bank will also not be as dramatically affected by Peukert and you'll actually get some 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.
Note: 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...
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 here 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"... This would be an average load of 20A on a 400Ah bank when really the average boater with a 400Ah bank is pulling 2A - 8A, on average. By using the bank split in half you lose much of these Peukert gains, especially on thick plate deep cycle batteries.
Rolls Battery Hour Rate Chart CH375:
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 Peukerts formula.
#3 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 batteries. With a single larger bank you not only combine Ah's of capacity but you also combine cranking amperage. 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.
#4 The batteries stay much better
balanced so that when you do need them in parallel they are all working as evenly as they can be.
#5 The shallower the discharges, for the same load, the less sulfation you create and this is exactly why a single large bank, cycled less deeply, yields more cycles.
#6 Wiring
*one large contiguous bank allows for more precision in keeping the bank wiring and current flowing through the bank balanced.
*Note: This does not mean that you can't have a means of
emergency battery isolation such as shown below, just that the "
emergency isolation switches" remain out of sight of guests and always remain ON,
unless there is a bank failure:
#7 Splitting/alternating a house bank makes for a monitoring nightmare with a battery monitor or even a Smart Gauge.
*Charge efficiency is better
*Cycle life is much better
*The batteries ability to support voltage is much better
*You get slightly more
usable capacity at the same average load
Battery Monitors:
If you are looking into a battery monitor they fall into two categories at this point.
Coulomb Counters = These count Ah's and require copious amounts of user input to get anywhere close to being accurate. They drift and become inaccurate over time. There are a lot of poor monitors out there so be sure to get one that fully allows you to program the Peukert's constant, charge efficiency etc... If splitting a bank you'd really need two battery monitors, one for each bank....
Smart Gauge = The Smart Gauge does not use an ammeter and is quite amazingly accurate. Connect two wires, program the battery type and then simply leave it connected. It gets more accurate over time. It does however need to remain connected to the house bank 100% of the time or it can get out of accuracy...
Both types of monitor will need to be
hard wired to the bank with no ability to switch or split the house bank
..
