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  • 1 Post By Stu Jackson
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Old 03-16-2013
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Maine Sail - clarification? Dual battery w/ solar

Two questions.

1) I know you recommend drawing off both batteries since they will discharge a lower percentage than drawing off two separately. That makes sense but what about the fact you are also discharging each battery more often? On one hand you are drawing them down less but you are also discharging them more often. I may be missing something but it seems like a Catch 22.

2) If you are using a Morningstar Duo solar controller with two outputs (one to each battery) and drawing off both batteries, how would the controller handle the two batteries together? Each output has it's own "algorithm" based on the state of the battery and how you programmed it. Morningstar says each battery should be isolated.

"... this product (SunSaver Duo) will charge two separate and isolated batteries at the same time,..." Morningstar
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Re: Maine Sail - clarification? Dual battery w/ solar

You asked for the right guy, who knows better than I, but I'll offer an opinion. I don't think it matters that you draw down on a battery, it matters how much you draw and how well you recharge. Drawing on a single battery seems more likely that you will overdraw before you presumably flip a manual switch to the other.
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Old 03-16-2013
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Re: Maine Sail - clarification? Dual battery w/ solar

That's anew approach to an old, old subject.

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.
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Re: Maine Sail - clarification? Dual battery w/ solar

To add to what Stu said 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.

A single larger bank will also support higher voltages and suffer from less voltage sag. Every electrical device on your boat prefers good voltage, especially motors. Voltage sag is not a good thing and this is but one more benefit of the single large bank beyond life cycles, charging efficiency and Peukert benefits.

As for the Morningstar Duo I prefer to use those in the 90/10 mode with 90% going to a single large house bank and then the 10% going to the start battery.
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Re: Maine Sail - clarification? Dual battery w/ solar

Make sense, although I didn't provide all my information.

I should have explained I have two AGM's and no starter battery (8hp Honda outboard, pull start). Presently I have the Morningstar unit hooked to both batteries with an A/B switch. In the past I switched between the two as needed. After reading up more, I was going to switch to "Both" but the controller hookup as me stumped.

Quote:
Originally Posted by Maine Sail View Post
To add to what Stu said 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.

A single larger bank will also support higher voltages and suffer from less voltage sag. Every electrical device on your boat prefers good voltage, especially motors. Voltage sag is not a good thing and this is but one more benefit of the single large bank beyond life cycles, charging efficiency and Peukert benefits.

As for the Morningstar Duo I prefer to use those in the 90/10 mode with 90% going to a single large house bank and then the 10% going to the start battery.
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Re: Maine Sail - clarification? Dual battery w/ solar

Quote:
Originally Posted by mdbee View Post
Morningstar says each battery should be isolated.

"... this product (SunSaver Duo) will charge two separate and isolated batteries at the same time,..." Morningstar
Are you sure you are not misinterpreting that statement? I believe it indicates the dual controller's capacity to charge 2 isolated batteries/banks, not that the batteries must be isolated. You just have a redundant charge controller if it is hooked up to a single battery/bank. I take it you bought it because you have 2 panels?

I have one panel attached to one SunGuard controller attached to only one of 2 batteries in a bank and I leave my switch on "All" all the time. On a bright and sunny day, the battery which the controller leads to will show a slightly higher voltage than the other as it directly draws a charge. Eventually they even out since they are connected and stay fully charged at the same level.

Last edited by jameswilson29; 03-17-2013 at 08:34 AM.
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Re: Maine Sail - clarification? Dual battery w/ solar

In the manual it does say:
"• Each of the two SunSaver Duo Battery connections may be wired to one battery or a bank of batteries. The following instructions refer to a singular battery, but it shall be implied that each battery connection can be made to either one battery or a group of batteries wired in parallel to form a 12 volt battery bank."

[To get a bit of the subject... That being said, would that mean you couldn't hook it up to two 6 volt batteries is series to form a 12 volt battery bank? Maybe something lost in translation or they never thought of the use of multiple 6 volt batteries. ]

I purchased this model controller to use with a single panel but to charge two 12 volt batteries which were usually isolated. But after reading the advantages of running one battery bank, I was curious how to charge them together.

So if I run with the battery switch turned to A+B then a "standard" controller would work instead of using a dual output controller?

I was just concerned that with both batteries hooked together there might be some downside when charging with that controller. I would assume I wouldn't run the separate outputs to both batteries? What would happen if they both outputs were used?

In that case a plain standard controller would work the same. As long as I only had one battery bank. (with the same size & type of batteries)

Quote:
Originally Posted by jameswilson29 View Post
Are you sure you are not misinterpreting that statement? I believe it indicates the dual controller's capacity to charge 2 isolated batteries/banks, not that the batteries must be isolated. You just have a redundant charge controller if it is hooked up to a single battery/bank. I take it you bought it because you have 2 panels?

I have one panel attached to one Morningstar controller attached to only one of 2 batteries in a bank and I leave my switch on "All" all the time. On a bright and sunny day, the battery which the controller leads to will show a slightly higher voltage than the other as it directly draws a charge. Eventually they even out since they are connected and stay fully charged at the same level.

Last edited by mdbee; 03-17-2013 at 02:13 AM.
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Re: Maine Sail - clarification? Dual battery w/ solar

"Are you sure you are not misinterpreting that statement? I believe it indicates the dual controller's capacity to charge 2 isolated batteries/banks, not that the batteries must be isolated."
You've misread it, James. The charger is designed to charge two separate batteries at once, and "separate" means totally separated, as in ISOLATED. Not that you have to install an isolation device, but that the batteries are not cross-connected or paralleled to something else. If you put the battery switch on "both" and your two separate batteries are combined to run the lights? They're not isolated any more, are they?

Separated, isolated, separate, all the same thing in context. The controller wants to see two batteries that have no connections between them, in order to charge them properly. Typically that's for an RV where the starting battery is never connected to the house battery when the engine is off, and the RV is parked with the solar panels deployed.
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