AGM's, solar panels, size of battery banks ?

Stu Jackson · Jul 30, 2012

The size of your house bank is based on two things, and two things only: your energy budget (which you've done) and how many days you want to go without charging.

In addition:

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).

FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.

heritage · Jul 30, 2012

I found while cruising in the Bahamas, with the combination of a fairly modest solar array and a wind generator that there is little use for a large battery bank. Our daily power needs were supplied directly by solar and wind, and there was very little need for energy storage. You need some for overnight in calm winds, and when it is cloudy, it is usually windy,, and in your case you have the generator for backup.

On the energy use side, you might consider adding a water maker in the future. Water can be bought at marinas, but it is typically $.50 a gallon and not as convenient as being able to make it.

Bob846 · Jul 30, 2012

I think 80-90 amp/hours per day is a little low given the area you will be in; especially if your refrigeration system is a little on the uninsulated side. I'm in So. Cal. and even with our cold water (rarely above 70 in the summer), my refrigeration will run 95% of the time on warm days. At 5 amps/hour, that's 114 amp hours/day and we haven't even gotten to lights/stereo/fans/instruments/electronics, or anything else. I've supplemented with 340 watts of solar power and this let's me break even and even have a little excess. But on cloudy days, I'll be running the engine some. I agree with those above...I think your battery bank is sufficient (I'd even go a little higher on the charger), but really look at your amp/hour spend and see what vehicles (solar, wind, generator, and/or alternator) best address that spend.

Maine Sail · Jul 30, 2012

One large bank for reasons beyond even what Calder touches on. It is more efficient to charge one bank than two unless using 100% free energy and even then "finishing" two banks is less efficient due to the longevity of the time acceptance limiting and "finishing" takes.. The larger bank will also not be as dramatically affected by Peukert and you'll actually get some more 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 a average load of 8A it looks like this if you were to draw the bank to 10.5V and use all the capacity.

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

This means that your bank will have 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 more out of the larger bank due to Peukert.

As for solar add as much as your real estate allows for. You may also consider wind power too. The Eclectic Energy D400 is really sweet as is the Superwind 350. I find the D400 to be quieter though to the point of almost inaudible.

The Electromaax alt will pump out some very good current...

Maine Sail · Jul 30, 2012

Bob 04 H260 said:
OK thanks guys, I think I got it ! It makes perfect sense. I got sidetracked to focus on getting the AGM's back to 100% on a daily basis. I'll go with the big battery bank, alternator, battery charger, monitor and then see what I have left over for solar panels and maybe a wind generator.
Good advice as always, Bob

Daily will be very hard but once weekly should be doable.. Problem is you can't equalize the East Penn AGM so as frequently as is possible to full will be the best case scenario.

Before you do anything read John Harries information on cruising with AGM batteries.

Morgans Cloud AGM's

willardj · Jul 31, 2012

Just a thought but your first step is to put in a good batt monitor. Then from that step do a few nights on it to see what your true power draw is going to be. That way you can a just your batt bank from there. Your solar should be based on your bank size. Normally you will want to have it 10-to 13% of your batt bank size. If you run much more than that will can heat your batts and if run less may not be enough to charge them.

And working backwards from the battery bank size, the recommended amount of solar panel for charging would be (based on 5% to 13% rate of charge):
450 AH * 14.5 volts charging * 1/0.77 panel+charger derate * 0.05 rate of charge = 424 watt minimum array
450 AH * 14.5 volts charging * 1/0.77 panel+charger derate * 0.10 rate of charge = 847 watt "nominal" array
450 AH * 14.5 volts charging * 1/0.77 panel+charger derate * 0.13 rate of charge = 1,059 watt "cost effective maximum" array

heritage · Jul 31, 2012

I'm thinking more like 2 140 watt panels and small wind generator. The cruisers I have talked to with a setup like that seem quite happy with the result, some seldom have to run the generator at all. I don't know if they can ever get their batteries up to 100% though while living aboard.

While cruising, I would try to keep the batteries between 50-80% charged. Anything less than 50% will shorten battery life, and anything over 80% requires too much charging time. The occasional cold fronts with several days of strong winds will drive the batteries to 100%, as will a couple of days at a marina hooked to ac power. By keeping the battery bank comfortable small, recharge to 100% is more likely. You can always add more batteries later if you feel the need.

willardj · Jul 31, 2012

That's why the monitor is so important to know what you need and that way you don't have to add($$$) no more than you need. The point is to not run the gen other then just the a/c. My buddy just put 3- 140's on his and seems to like them. I have a good batt monitor( Trimetric 2025rv) and after messing with it for a bit is why I went with my 145w panel. It is large enough to run every thing I have( couple radio's, auto tiller, fans, sounder,+ charging our phone and lap tops) and still keep my batts topped off. If you have davits of the back you could run 3-4 140's maybe 3-190's 3-240's really just depends on your budget. Being you mostly have LED's like we do you shouldn't need than much power.

jviss · Jul 31, 2012

How to size

Here's my shot at answering this, from an engineer's point of view.

First, as they say for engines that there's no substitute for cubic inches, for your house bank, there's no substitute for Amp-hours. Bigger is always better - giving shallower discharges or less frequent charing sessions. That said, there's a limit, and assuming infinite space, money, and weight capacity for batteries, the limit you'll hit is the capacity of your alternator. The house bank should be sized so that when it's as dead as it will ever get, the charging current during bulk doesn't exceed your alternator's rating. So, for a 150A alternator, you would size the house for conventional wet cells at about 450 Ah. A 450 Ah wet cell bank will draw about 150A, briefly, when it's at 50% SOC; I think - this is off the top of my head. But, the point is, for your battery technology, determine the max charging current the battery will draw at its lowest SOC, and size the bank to the alternator capacity.

AGM's, solar panels, size of battery banks ?

.

.

.

.

.

.

.

.

.

.

Attachments

.

.

.

.