I have a question to help with the understanding of this... if one has the original stock alternator, using either 2 deep cycle batts vs using 4 deep cycle batts, if one was to draw the bank down to 40% of charge (NOT 50amps), would it take the same time to bring these banks back to full charge, or even within and hour of each other?
With the same charge source current the larger bank takes longer due to the longer bulk time. The absorption time will be similar though the larger bank will attain the limit voltage a bit higher in the SOC curve thus utilizing the max potential of the alt for a bit longer.
Let's assume a C-310 with a 55A stock alt and a voltage regulation point of 14.4V. Let's also assume that in bulk the alt can't produce more than about 45A due to heat and we are using 50% of bank caapcity.
The 225Ah bank is charging at 0.2C or about 20% of its Ah rating. This battery will attain the limiting voltage of 14.4V at approx 75% SOC and bulk charging will take roughly 1 hour. From this point forward the charge acceptance rate at 14.4V and XX SOC is governed by the battery and determines how long it takes to get to full. With a brand new battery you are upwards of 7 hours and as it sulfates closer to 10 hours or more.
You used 112.5Ah's and in 1 hour you were able to return about 43Ah's before your charging speed began to decline to a point where you were no longer using your alternator in an effective or efficient manner. If you do 1 hour per day of charging then you essentially have about 43 usable Ah's when off cruising, unless you want to run the motor and alt in a less efficient manner. You are also discharging the bank and cycling it between 50% SOC and approx 75% SOC.
The 450Ah bank is charging at 0.1C or about 10% of its Ah rating. This battery will attain the limiting voltage of 14.4V at approx 85% SOC and bulk charging will take roughly 3.5 hours. From this point forward the charge acceptance rate at 14.4V and XX SOC determines how long it takes to get to full. It will essentially be the same duration as the smaller bank once absorption voltage has been attained.. With a brand new battery you are upwards of 7 hours and as it sulfates closer to 10 hours or more.
If you used 225Ah's over a few days in 1 hour you still return 45Ah's. If you run the full bulk period this will take approx 3.5 hours but you are returning approx 157.5Ah's in bulk vs. 43Ah's. Also due to the C rate being smaller you get to a higher SOC before the alt begins cutting back current based on battery acceptance. Due to the lower C rate your alternator is getting the batteries to a slightly higher SOC point before voltage limiting begins. With 1 hour per day of charging you are still easily able to get 43-45Ah's of use but if you keep up with that 1 hour per day your 450Ah bank can remain at an overall higher cycling SOC
every day than does the 225 Ah bank..
This means longer life from the larger bank plus the ability to go a few days in between charging episodes, if you are willing to run the engine longer for bulk replenishment after not running it for two or three days. Usually this can be planned around a windlass day but you get the
option and the bank, if you keep up with 1 hour per day, remains at a higher SOC than the smaller bank which can lead to dramatically longer life...
Course Peukert has also not been accounted for and the larger bank will be drawn down at a lower percentage rate so it can actually deliver more Ah's than the 450Ah face value rating.. If we figure 100Ah's per day then the average load is about 4.2A and your Peukert corrected bank size, assuming 1.27 Peukert, at that load, grows. The 225 Ah bank also grows at that load. The larger the bank and the lower the load the more effective gain you get from Peukert. While both banks have a net Peukert
gain at a low discharge rate, the large bank gains more effective capacity, as a percentage, due to Peukert in relation to the same discharge rate.
This is a good battery manufacturer chart that sums this up:
.
I can understand the principal behind the so called smart chargers and the super alternators with the external charge regulators, and even good solar chargers being able to have better balanced and much quicker charge cycles, but I didnt think it was possible for a stock 55-85 amp alternator to accomplish this.
Given the time yes even a dumb regulated alternator can full charge a battery. Bulk is bulk and absorption is absorption. A 14.4V limit from a smart regulator will not charge any faster than a 14.4V limit from a dumb source if the current source is the same. Of course other things such as voltage sensing can affect this but 14.4V at the battery terminals is 14.4V.. The battery has no clue who or what is putting that 14.4V there.... Often times a dumb regulator (a truly dumb one not a "super dumb" one eg: Hitachi/Yanmar) can actually charge a bank faster than a smart regulator due to the
lawyer safe settings and "premature floatulation".. For a more in-depth understandingof regulation this is a good read:
Musing Regarding External Regulation
but if it were possible, then one would think that having 2 banks of 4 GC batteries each would be the best option available for our little boats... to let one of the banks run down to 50% before switching banks is a LOT of power delivered with the other bank still full to run the components for the same amount of time that the spent bank gave us. and even at 50% or 40% the 4 GC batteries will still have plenty of amps left to crank our small 4 cyl diesels over quite strongly...
and still be brought back to full charge within a reasonable time frame that a 12v start batt and 2 GC batteries would be?...
Here is Nigel Calder's take:
Nigel Calder said:
IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDN’T write this!!!)
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).
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 still a less efficient use of the free energy due to the longevity of the time acceptance limiting and "finishing" the battery takes while using free energy. What goes out is not what goes back in. We need 110% to 130% of removed capacity to go back in and this simply takes time. If this takes 7-10 hours for each bank you need 14-20 hours to get both banks full. If it is one large bank the absorption time is still dictated by voltage and SOC so you literally cut the time to full for one large bank nearly in half vs. two split banks..
#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.
#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 the same or even 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. Wiring pathways can make a difference in the intra-bank balancing over time.
#5 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.
#6 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 better
*The batteries ability to support voltage is better
*You get slightly more
usable capacity at the same average load