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solar panels and batts

Jim Healy

Wed, Sep 5, 2012 4:47 PM

Janice

There are two different subjects in your question. One is solar for maintaining refrigeration. The second is battery charging.

The ratio of 4:1 (150 amps for 600 amp hours) is a "guideline" or "folk wisdom" for flooded deep cycle lead acid batteries. It's based on the physical composition and characteristics of flooded lead acid batteries. This one be based on something called "Charge Acceptance Rate." For a 50% discharged flooded wet cell, the battery will accept recharge current at a rate in AMPS of approximately 25% of the AMP HOUR CAPACITY of the battery without overheating and outgassing. That's where your example (4x) came from. The number for a 50% discharged AGM or Gel battery is somewhat higher, around 40% to 50%. IT IS NOT A PROBLEM to charge batteries at a lower rate; it does not damage them. It just takes longer; potentially, a lot longer. This slow replacement operating mode is why deep cycle lead acid batteries can be left on solar PV cells or wind alternators for long periods of time in off-grid applications. So, if you're worried about how long you have to run the genset, you should do your best to match the output capability of the battery charger to the Charge Acceptance Rate of the battery technology, based on amp hour capacity of the battery bank you're charging. The most important function of a charging system is a modern 3-stage battery charger that will not over-charge the battery. Overcharging batteries will hurt them, and will do so quickly.

Bob McLaren mentioned in another post that when he cruises, he runs his batteries in a state-of-charge range from a low of 55% to a high of about 80% state-of-charge. That is a simplified summary, and I know he does more than that. No deep cycle lead acid batteries - flooded, AGM or Gel - should be discharged more than 50%. Well, at least not on a routine basis. A very occasional deeper discharge will not hurt them, but routinely discharging below 50% will shorten the life of the plates and decrease the energy storage capability of the battery in the process. So that covers the low end. The high end is very complicated. The Charge Acceptance Rate for lead acid batteries goes down as state-of-charge goes up. The last 15% of capacity can take as long to achieve as all of the first 35%; maybe longer. So many people stop the genset at that point to reduce genset runtime and to get on with whatever they had planned for their day. The problem is, not achieving that last 15% promotes sulfation, reduces the amp hour capacity of the battery, and shortens battery useful life span. All of these bad effects, of course, have matters-of-degree elements that make then very hard to quantify. Most charger manufacturers recommend re-charging batteries to 100% once a week AT A MINIMUM. Battery manufacturers always want batteries charged to 100%. So, you pays your money and you makes your choices. The only reliable way to measure state-of-charge of a battery bank is with a high quality "computerized" battery monitor. Most charger manufacturers make them. I prefer Magnum, but there are other good ones out there. For wet cells, it's possible and reasonably reliable to measure specific gravity of the electrolyte. Possible, not easy. Depending on battery terminal voltage to assess state-of-charge is not reliable. This is geekish stuff, but within the month I went to replace an AGM 8D. At $550 per battery, and the issues of physically handling a 168# hunk of lead, expected life span becomes a real issue. Discouraged by that price, I've changed over to flooded wet cell Golf Cart batteries from Sam's Club. The label is Duracell, they're made by East Penn in the USA, the model is EGC-2, and they're $92 per 230aHr 6V unit.

Relative to refrigeration, I'd suggest there is no equivalent ratio analogous to Charge Acceptance Rate. First, I ass/u/me you're looking at a scenario of leaving the boat on it's mooring for many days at a time, and not having to run the engine or the genset to charge the batteries. It's doable, but you have to calculate how many watts of energy DC devices will consume, and then convert that to amp hours. And there are a lot of difficult-to-quantify variables in the mix. Here's a methodology for doing it: Start by finding out how much energy your fridge needs to run? My refrigerator consumes 69 watts when running. You get that number from the specs/nameplate for the unit. 69 watts at 12.6 volts is 5.5 amps, so for 24 hours, that 5.5 amps becomes 185 amp hours. (185 amp hours is a worst case scenario, because there is a "duty cycle" factor that will come into play here; your fridge compressor probably does not run continuously for 24 hours.)

Solar cells rated in watts work the same way. You need 185 amp hours of replacement energy to the batteries to replace what the fridge has used. But you will not get usable solar output for 24 continuous hours... or 12 hours, even... So you need to make some more assumptions. How many hours a day will the solar cells produce usable output? PV cell manufacturers many not be entirely forthcoming about this. Depends on where you are; more in the Bahamas than in Maine. How many amps must they produce over their outputting time to replace the 185 amp hours back into the batteries? Well, if we ass/u/me 6 hours of usable solar output per day, then 186/6 means the PV cells must give you an average of 31 amps at 14.4 charging volts for 6 hours. That would be a PV array size of around 450 watts. Now unfortunately, that would not be enough, because battery efficiency plays a factor here, too. You generally have to put 15% to 20% more amp hours back into a battery to replace an amount you've used, so that takes your 185 amp hour number up to 220 amp hours, and your 6-hour average PV cell output to 37 amps, and your cell wattage to 530 or so. I'v ignored rounding here, but you should get the idea behind the general methodology. That's how you'd have to do it. I believe 500 watts of PV cells is possible on most cruising boats.

And by the way, the same method works for wind power. In the Bahamas, wind is more efficient that solar; more of it, more consistent, day and hight, etc. In some places, the reverse is true. Your best trade off may be a solution that includes both wind and PV cells. So that raises the question of where you plan to cruise.

This is complicated stuff with lots of tradeoffs and fudge factors. If it doesn't make sense to you, get a designer involved to help. You'll have to tell the designer where you plan to cruise and how long you want to go on solar/wind alone. You'll have to know - or have a very good estimate - of how much power you need to replace. You'll need to know how big the battery bank is, and you'll need to know it's underlying technology: wet, AGM or Gel, as they all have different Peukert exponents, terminal voltages and charge rates that will make small differences in your design objectives.

Hope this is useful.

Jim

Peg and Jim Healy aboard Sanctuary
Currently at Rock Creek, Pasadena, MD
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436

Janice wrote: "I'm adding solar (the quest for refrigeration continues) and wondered if there was a standard ratio for solar wattage to batts? For instance, with a 150 amp alternator the suggested battery capacity (minimum) is 600 amps (4x)." Janice There are two different subjects in your question. One is solar for maintaining refrigeration. The second is battery charging. The ratio of 4:1 (150 amps for 600 amp hours) is a "guideline" or "folk wisdom" for flooded deep cycle lead acid batteries. It's based on the physical composition and characteristics of flooded lead acid batteries. This one be based on something called "Charge Acceptance Rate." For a 50% discharged flooded wet cell, the battery will accept recharge current at a rate in AMPS of approximately 25% of the AMP HOUR CAPACITY of the battery without overheating and outgassing. That's where your example (4x) came from. The number for a 50% discharged AGM or Gel battery is somewhat higher, around 40% to 50%. IT IS NOT A PROBLEM to charge batteries at a lower rate; it does not damage them. It just takes longer; potentially, a lot longer. This slow replacement operating mode is why deep cycle lead acid batteries can be left on solar PV cells or wind alternators for long periods of time in off-grid applications. So, if you're worried about how long you have to run the genset, you should do your best to match the output capability of the battery charger to the Charge Acceptance Rate of the battery technology, based on amp hour capacity of the battery bank you're charging. The most important function of a charging system is a modern 3-stage battery charger that will not over-charge the battery. Overcharging batteries will hurt them, and will do so quickly. Bob McLaren mentioned in another post that when he cruises, he runs his batteries in a state-of-charge range from a low of 55% to a high of about 80% state-of-charge. That is a simplified summary, and I know he does more than that. No deep cycle lead acid batteries - flooded, AGM or Gel - should be discharged more than 50%. Well, at least not on a routine basis. A very occasional deeper discharge will not hurt them, but routinely discharging below 50% will shorten the life of the plates and decrease the energy storage capability of the battery in the process. So that covers the low end. The high end is very complicated. The Charge Acceptance Rate for lead acid batteries goes down as state-of-charge goes up. The last 15% of capacity can take as long to achieve as all of the first 35%; maybe longer. So many people stop the genset at that point to reduce genset runtime and to get on with whatever they had planned for their day. The problem is, not achieving that last 15% promotes sulfation, reduces the amp hour capacity of the battery, and shortens battery useful life span. All of these bad effects, of course, have matters-of-degree elements that make then very hard to quantify. Most charger manufacturers recommend re-charging batteries to 100% once a week AT A MINIMUM. Battery manufacturers always want batteries charged to 100%. So, you pays your money and you makes your choices. The only reliable way to measure state-of-charge of a battery bank is with a high quality "computerized" battery monitor. Most charger manufacturers make them. I prefer Magnum, but there are other good ones out there. For wet cells, it's possible and reasonably reliable to measure specific gravity of the electrolyte. Possible, not easy. Depending on battery terminal voltage to assess state-of-charge is not reliable. This is geekish stuff, but within the month I went to replace an AGM 8D. At $550 per battery, and the issues of physically handling a 168# hunk of lead, expected life span becomes a real issue. Discouraged by that price, I've changed over to flooded wet cell Golf Cart batteries from Sam's Club. The label is Duracell, they're made by East Penn in the USA, the model is EGC-2, and they're $92 per 230aHr 6V unit. Relative to refrigeration, I'd suggest there is no equivalent ratio analogous to Charge Acceptance Rate. First, I ass/u/me you're looking at a scenario of leaving the boat on it's mooring for many days at a time, and not having to run the engine or the genset to charge the batteries. It's doable, but you have to calculate how many watts of energy DC devices will consume, and then convert that to amp hours. And there are a lot of difficult-to-quantify variables in the mix. Here's a methodology for doing it: Start by finding out how much energy your fridge needs to run? My refrigerator consumes 69 watts when running. You get that number from the specs/nameplate for the unit. 69 watts at 12.6 volts is 5.5 amps, so for 24 hours, that 5.5 amps becomes 185 amp hours. (185 amp hours is a worst case scenario, because there is a "duty cycle" factor that will come into play here; your fridge compressor probably does not run continuously for 24 hours.) Solar cells rated in watts work the same way. You need 185 amp hours of replacement energy to the batteries to replace what the fridge has used. But you will not get usable solar output for 24 continuous hours... or 12 hours, even... So you need to make some more assumptions. How many hours a day will the solar cells produce usable output? PV cell manufacturers many not be entirely forthcoming about this. Depends on where you are; more in the Bahamas than in Maine. How many amps must they produce over their outputting time to replace the 185 amp hours back into the batteries? Well, if we ass/u/me 6 hours of usable solar output per day, then 186/6 means the PV cells must give you an average of 31 amps at 14.4 charging volts for 6 hours. That would be a PV array size of around 450 watts. Now unfortunately, that would not be enough, because battery efficiency plays a factor here, too. You generally have to put 15% to 20% more amp hours back into a battery to replace an amount you've used, so that takes your 185 amp hour number up to 220 amp hours, and your 6-hour average PV cell output to 37 amps, and your cell wattage to 530 or so. I'v ignored rounding here, but you should get the idea behind the general methodology. That's how you'd have to do it. I believe 500 watts of PV cells is possible on most cruising boats. And by the way, the same method works for wind power. In the Bahamas, wind is more efficient that solar; more of it, more consistent, day and hight, etc. In some places, the reverse is true. Your best trade off may be a solution that includes both wind and PV cells. So that raises the question of where you plan to cruise. This is complicated stuff with lots of tradeoffs and fudge factors. If it doesn't make sense to you, get a designer involved to help. You'll have to tell the designer where you plan to cruise and how long you want to go on solar/wind alone. You'll have to know - or have a very good estimate - of how much power you need to replace. You'll need to know how big the battery bank is, and you'll need to know it's underlying technology: wet, AGM or Gel, as they all have different Peukert exponents, terminal voltages and charge rates that will make small differences in your design objectives. Hope this is useful. Jim Peg and Jim Healy aboard Sanctuary Currently at Rock Creek, Pasadena, MD Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436

Janice Marois

Fri, Sep 7, 2012 3:48 AM

Thank you Jim -- your explanation is good however I need more if you
please...... the scenario I am working with is thus:

We live on the hook (I did tie up so far this year maybe 5 days total, so
essentially all power used is generated aboard. No generator. At anchor I
rely on an Air-Breeze (original unit, board replaced) and currently just 75
watts of solar. Between the wind gen and the 25 amps my solar panel gives
me per day, everything EXCEPT the refrigerator runs.

And yes, you are correct in that I can have 500 watts solar (I'll end up
with nearly 600 once the new panels are made/installed) thus I'll be
working with (roughly) 175 amps per day via the sun, and I intend to use
that additional capacity for my 1.7 cubic foot (a/c) refrigerator running
from an Aims1000 inverter using .6 amps to provide said power.

At present my batteries fluctuate between 13.2 and 12.3 (sometimes 12.2 --
never lower)
I see 12.5 - 12.8 mostly.
I do get HVD (high voltage disconnect) on my Morningstar30 (especially if
the wind is blowing) and perhaps once every three weeks or so I notice the
code for absorption charge -- it's not frequent though I don't always
look at the monitor.

What I'm trying to determine is how much additional battery storage I need.
The whole battery bit does confuse me ... I have 450 amps of capacity; can
I count on having about 150 amps usable each day? That's 1/3rd (from 50% to
80%) ... yes, I get the bit about replacing the amount of power used --
that's why I'm adding more solar panels.

Can I squeak by with my current 450 battery bank, or do I need to budget
for another battery or two? (plus cabling, et al)

Janice, still learning.
And thanks again for the information.

On Wednesday, September 5, 2012, Jim Healy wrote:

Thank you Jim -- your explanation is good however I need more if you please...... the scenario I am working with is thus: We live on the hook (I did tie up so far this year maybe 5 days total, so essentially all power used is generated aboard. No generator. At anchor I rely on an Air-Breeze (original unit, board replaced) and currently just 75 watts of solar. Between the wind gen and the 25 amps my solar panel gives me per day, everything EXCEPT the refrigerator runs. And yes, you are correct in that I can have 500 watts solar (I'll end up with nearly 600 once the new panels are made/installed) thus I'll be working with (roughly) 175 amps per day via the sun, and I intend to use that additional capacity for my 1.7 cubic foot (a/c) refrigerator running from an Aims1000 inverter using .6 amps to provide said power. At present my batteries fluctuate between 13.2 and 12.3 (sometimes 12.2 -- never lower) I see 12.5 - 12.8 mostly. I do get HVD (high voltage disconnect) on my Morningstar30 (especially if the wind is blowing) and perhaps once every three weeks or so I notice the code for absorption charge -- it's not frequent though I don't *always* look at the monitor. What I'm trying to determine is how much additional battery storage I need. The whole battery bit does confuse me ... I have 450 amps of capacity; can I count on having about 150 amps usable each day? That's 1/3rd (from 50% to 80%) ... yes, I get the bit about replacing the amount of power used -- that's why I'm adding more solar panels. Can I squeak by with my current 450 battery bank, or do I need to budget for another battery or two? (plus cabling, et al) Janice, still learning. And thanks again for the information. On Wednesday, September 5, 2012, Jim Healy wrote: > > The ratio of 4:1 (150 amps for 600 amp hours) is a "guideline" or "folk > wisdom" for flooded deep cycle lead acid batteries. It's based on the > physical composition and characteristics of flooded lead acid batteries. > This one be based on something called "Charge Acceptance Rate." For a 50% > discharged flooded wet cell, the battery will accept recharge current at a > rate in AMPS of approximately 25% of the AMP HOUR CAPACITY of the battery > without overheating and outgassing. That's where your example (4x) came > from. >

Jim Healy

Fri, Sep 7, 2012 2:20 PM

Janice,

I worked around this in your first note, but now I have to mention, I think you are mis-using some of the electrical terms. If that reflects any misunderstanding of the electrical theory involved, you'll need to work on that. For sure, I think you are confusing "amps" and "amp hours." Amps are a measure of how many electrons are running through a circuit at a given instant-in-time. Amp Hours are a theoretical construct that attempts to quantify the amount of potential energy (chemical to electrical) that is stored in a battery. Manufacturers rate battery capacity at several "STANDARD" drawdown rates. IN THE UNITED STATES, the retail standard for lead acid batteries is the "20 hour" rating, so we talk about a 6V, 225 amp hour battery being able to supply 11.25Amps for 20 hours before the battery is completely discharged. In the US, batteries also have 6 hour, 3 hour and 1 hour ratings, although not all manufacturers publish all of that data to retail consumers. Commercial batteries - 8Ds, L16s - would usually have that data. In the UK and EU, batteries are rated to a 10 hour STANDARD drawdown period. Why? Because they're not the US, I guess. So anyway, when you talk about a 450 AMP HOUR Battery bank, what you mean with US-made batteries is a battery bank that can supply 22.5 amps for 20 hours before becoming fully discharged, or 11.25 amps for 20 hours to 50% discharge, and so forth.

When you say, for example, "and the 25 amps my solar panel gives me," or when you say, "thus I'll be working with (roughly) 175 amps per day via the sun," those statements have no practical meaning in electrical theory. "Amps" are not a measure of energy; "amp hours" and "watts" are measures of energy use/capacity. And energy reserves are what you care about in most battery discussions. Amps can be low over a long time or high over a short time to result in identical amp hour energy equivalents. The distinction I'm making is not just a terminology difference; the concepts are very different and very important.

So now I'm going to launch into what I think you're asking about.

On Sep 6, 2012, at 11:48 PM, Janice Marois wrote:

Thank you Jim -- your explanation is good however I need more if you please...... the scenario I am working with is thus:

We live on the hook (I did tie up so far this year maybe 5 days total, so essentially all power used is generated aboard. No generator. At anchor I rely on an Air-Breeze (original unit, board replaced) and currently just 75 watts of solar. Between the wind gen and the 25 amps

AMPS or AMP HOURS or WATTS???

my solar panel gives me per day, everything EXCEPT the refrigerator runs.

And yes, you are correct in that I can have 500 watts solar (I'll end up with nearly 600 once the new panels are made/installed) thus I'll be working with (roughly) 175 amps

AMPS or AMP HOURS???

per day via the sun, and I intend to use that additional capacity for my 1.7 cubic foot (a/c) refrigerator running from an Aims1000 inverter using .6 amps to provide said power. THE INVERTER ADDS ANOTHER FUDGE FACTOR TO THE SYSTEM; INVERTERS ARE NOT 100% EFFICIENT.

At present my batteries fluctuate between 13.2 and 12.3 (sometimes 12.2 -- never lower). I see 12.5 - 12.8 mostly. I do get HVD (high voltage disconnect) on my Morningstar30 (especially if the wind is blowing) and perhaps once every three weeks or so I notice the code for absorption charge -- it's not frequent though I don't always look at the monitor.

I read this to say that you have very nearly what you need to handle what you have installed already. 12.2V is on the OK low margin, but voltage measurement is not generally a reliable measure of battery state-of-charge. Learn to depend on your battery monitor, set to measuring and report "amp hours."

The net additional PV bank capacity you're looking at adding is to pick up the net new load of the 1.7 cu ft fridge. So, let's look at just that. Let's ass/u/me that the fridge is rated at 120VAC, and 0.6 amps. Let's further ass/u/me you have flooded wet cells that are rated at 12.7 volts at the terminal, fully charged. So, amps DC from the battery = 120/12.70.6 = 5.7 amps DC from the battery. Throw in an inverter efficiency factor of 0.89% (could be better, could be worse; check it's spec) and that becomes 6.4 amps DC from the battery. Now ass/u/me that the fridge will have a duty cycle of 50% (could be more, could be less), so that's 6.4 amps12 hours = 76.4 amp hours per 24-hour day to run that refrigerator/inverter combination. It's also 6.4*12.7 = 81.3 watts per hour, or 975.4 watts/day. Your planned additional PV bank that generates 500 additional watts, for ass/u/me an output cycle of 6 hours, should cover the fridge, perhaps with some margin left over.

The HVD is probably more a factor of the design of the charge regulator on the windmill than of the battery bank. The raw internal voltages produced in the windmill vary enormously with the speed of rotation of the machine. If speed-of-rotation is high and load is low, I wouldn't be at all surprised that the regulator can't manage it.

What I'm trying to determine is how much additional battery storage I need. The whole battery bit does confuse me ... I have 450 amps

Here, I'm pretty sure you mean "450 AMP HOURS." Perhaps 4 6V Golf Cart Batteries rated at 225 aHr each? These batteries can produce many thousands of "amps" if you short circuit the terminals! They produce many hundreds of amps to operate the engine starter motor.

of capacity; can I count on having about 150 amps

I THINK YOU MEAN AMP HOURS HERE

usable each day? That's 1/3rd (from 50% to 80%) ... yes, I get the bit about replacing the amount of power used -- that's why I'm adding more solar panels.

Yes, you can count on having 150 aHr per day... Actually, you can probably have 225 aHr per day adhering to the 50% rule... But not the way you said it... Yes, 150 aHrs is 1/3 of the bank, but you can't take that 1/3 out of the middle. If you're discharging the batteries to 50%, and recharging to 80%, that's 1/2 discharged, not 1/3. And since I already discussed it, I won't belabor the bit about re-charging to 100%; if you only regularly recharge to 80%, you will hurt the batteries.

Can I squeak by with my current 450 battery bank, or do I need to budget for another battery or two? (plus cabling, et al)

Ahhh... the key question...

Well, two things will happen when you install a larger PV array. In electricity theory, there is a thing called Kirchoff's Law (laws, really.) You will benefit from them in this case. First, with a larger PV array, more of the energy to run the appliances will come directly from the PV array. Second, then, less of the required energy will have to come from the batteries. Therefore, with a larger PV array, you will wind up using less of the aHr capacity of the batteries. And therefore, you will have to replace fewer aHrs into the batteries. So, win-win. The down side is that you are adding electrical load to the boat. During periods of calm, cloudy weather, the existing batteries will not last as long as they do now, and may not be sufficient. More battery capacity will buy you more battery runtime, but in truly long periods of calm, cloudy weather, no battery bank will be big enough by itself. The magic is in guessing the balance point with which you can live comfortably!

Now, earlier, you said your current PV array is 75 watts. (You also said, "and the 25 amps my solar panel gives me per day..." DOES THAT MEAN YOUR 75 WATT PANEL GIVES YOU 25 AMP HOURS OF ENERGY DAILY??? That would be 5.9 Amps for 4-1/4 hours, which is a shorter output cycle duration than I have been ass/u/ming, at 6 hours. So, you'll need to re-work all this with whatever the actuals are for your specific equipment.) In any case, if you go from 75 Watts of PV to 600 Watts of PV, that should more than cover you for some cloudy days without adding battery capacity.

I would suggest you approach this in two phases. First, add to the PV array capacity and give it some time to see how you like it's performance. Then, if necessary, add to the battery bank. Since you say you "never" get below 12.2 volts today, I doubt it will be necessary to add more batteries.

OK. By now, we're both tired. I hope this all helps.

Janice, still learning.
And thanks again for the information.

On Wednesday, September 5, 2012, Jim Healy wrote:

Peg and Jim Healy aboard Sanctuary
Currently at Rock Creek, Pasadena, MD
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436

Janice, I worked around this in your first note, but now I have to mention, I think you are mis-using some of the electrical terms. If that reflects any misunderstanding of the electrical theory involved, you'll need to work on that. For sure, I think you are confusing "amps" and "amp hours." Amps are a measure of how many electrons are running through a circuit at a given instant-in-time. Amp Hours are a theoretical construct that attempts to quantify the amount of potential energy (chemical to electrical) that is stored in a battery. Manufacturers rate battery capacity at several "STANDARD" drawdown rates. IN THE UNITED STATES, the retail standard for lead acid batteries is the "20 hour" rating, so we talk about a 6V, 225 amp hour battery being able to supply 11.25Amps for 20 hours before the battery is completely discharged. In the US, batteries also have 6 hour, 3 hour and 1 hour ratings, although not all manufacturers publish all of that data to retail consumers. Commercial batteries - 8Ds, L16s - would usually have that data. In the UK and EU, batteries are rated to a 10 hour STANDARD drawdown period. Why? Because they're not the US, I guess. So anyway, when you talk about a 450 AMP HOUR Battery bank, what you mean with US-made batteries is a battery bank that can supply 22.5 amps for 20 hours before becoming fully discharged, or 11.25 amps for 20 hours to 50% discharge, and so forth. When you say, for example, "and the 25 amps my solar panel gives me," or when you say, "thus I'll be working with (roughly) 175 amps per day via the sun," those statements have no practical meaning in electrical theory. "Amps" are not a measure of energy; "amp hours" and "watts" are measures of energy use/capacity. And energy reserves are what you care about in most battery discussions. Amps can be low over a long time or high over a short time to result in identical amp hour energy equivalents. The distinction I'm making is not just a terminology difference; the concepts are very different and very important. So now I'm going to launch into what I think you're asking about. On Sep 6, 2012, at 11:48 PM, Janice Marois wrote: > Thank you Jim -- your explanation is good however I need more if you please...... the scenario I am working with is thus: > > We live on the hook (I did tie up so far this year maybe 5 days total, so essentially all power used is generated aboard. No generator. At anchor I rely on an Air-Breeze (original unit, board replaced) and currently just 75 watts of solar. Between the wind gen and the 25 amps AMPS or AMP HOURS or WATTS??? > my solar panel gives me per day, everything EXCEPT the refrigerator runs. > > And yes, you are correct in that I can have 500 watts solar (I'll end up with nearly 600 once the new panels are made/installed) thus I'll be working with (roughly) 175 amps AMPS or AMP HOURS??? > per day via the sun, and I intend to use that additional capacity for my 1.7 cubic foot (a/c) refrigerator running from an Aims1000 inverter using .6 amps to provide said power. THE INVERTER ADDS ANOTHER FUDGE FACTOR TO THE SYSTEM; INVERTERS ARE NOT 100% EFFICIENT. > > At present my batteries fluctuate between 13.2 and 12.3 (sometimes 12.2 -- never lower). I see 12.5 - 12.8 mostly. I do get HVD (high voltage disconnect) on my Morningstar30 (especially if the wind is blowing) and perhaps once every three weeks or so I notice the code for absorption charge -- it's not frequent though I don't *always* look at the monitor. I read this to say that you have very nearly what you need to handle what you have installed already. 12.2V is on the OK low margin, but voltage measurement is not generally a reliable measure of battery state-of-charge. Learn to depend on your battery monitor, set to measuring and report "amp hours." The net additional PV bank capacity you're looking at adding is to pick up the net new load of the 1.7 cu ft fridge. So, let's look at just that. Let's ass/u/me that the fridge is rated at 120VAC, and 0.6 amps. Let's further ass/u/me you have flooded wet cells that are rated at 12.7 volts at the terminal, fully charged. So, amps DC from the battery = 120/12.7*0.6 = 5.7 amps DC from the battery. Throw in an inverter efficiency factor of 0.89% (could be better, could be worse; check it's spec) and that becomes 6.4 amps DC from the battery. Now ass/u/me that the fridge will have a duty cycle of 50% (could be more, could be less), so that's 6.4 amps*12 hours = 76.4 amp hours per 24-hour day to run that refrigerator/inverter combination. It's also 6.4*12.7 = 81.3 watts per hour, or 975.4 watts/day. Your planned additional PV bank that generates 500 additional watts, for ass/u/me an output cycle of 6 hours, should cover the fridge, perhaps with some margin left over. The HVD is probably more a factor of the design of the charge regulator on the windmill than of the battery bank. The raw internal voltages produced in the windmill vary enormously with the speed of rotation of the machine. If speed-of-rotation is high and load is low, I wouldn't be at all surprised that the regulator can't manage it. > > What I'm trying to determine is how much additional battery storage I need. The whole battery bit does confuse me ... I have 450 amps Here, I'm pretty sure you mean "450 AMP HOURS." Perhaps 4 6V Golf Cart Batteries rated at 225 aHr each? These batteries can produce many thousands of "amps" if you short circuit the terminals! They produce many hundreds of amps to operate the engine starter motor. > of capacity; can I count on having about 150 amps I THINK YOU MEAN AMP HOURS HERE > usable each day? That's 1/3rd (from 50% to 80%) ... yes, I get the bit about replacing the amount of power used -- that's why I'm adding more solar panels. Yes, you can count on having 150 aHr per day... Actually, you can probably have 225 aHr per day adhering to the 50% rule... But not the way you said it... Yes, 150 aHrs is 1/3 of the bank, but you can't take that 1/3 out of the middle. If you're discharging the batteries to 50%, and recharging to 80%, that's 1/2 discharged, not 1/3. And since I already discussed it, I won't belabor the bit about re-charging to 100%; if you only regularly recharge to 80%, you will hurt the batteries. > > Can I squeak by with my current 450 battery bank, or do I need to budget for another battery or two? (plus cabling, et al) Ahhh... the key question... Well, two things will happen when you install a larger PV array. In electricity theory, there is a thing called Kirchoff's Law (laws, really.) You will benefit from them in this case. First, with a larger PV array, more of the energy to run the appliances will come directly from the PV array. Second, then, less of the required energy will have to come from the batteries. Therefore, with a larger PV array, you will wind up using less of the aHr capacity of the batteries. And therefore, you will have to replace fewer aHrs into the batteries. So, win-win. The down side is that you are adding electrical load to the boat. During periods of calm, cloudy weather, the existing batteries will not last as long as they do now, and may not be sufficient. More battery capacity will buy you more battery runtime, but in truly long periods of calm, cloudy weather, no battery bank will be big enough by itself. The magic is in guessing the balance point with which you can live comfortably! Now, earlier, you said your current PV array is 75 watts. (You also said, "and the 25 amps my solar panel gives me per day..." DOES THAT MEAN YOUR 75 WATT PANEL GIVES YOU 25 AMP HOURS OF ENERGY DAILY??? That would be 5.9 Amps for 4-1/4 hours, which is a shorter output cycle duration than I have been ass/u/ming, at 6 hours. So, you'll need to re-work all this with whatever the actuals are for your specific equipment.) In any case, if you go from 75 Watts of PV to 600 Watts of PV, that should more than cover you for some cloudy days without adding battery capacity. I would suggest you approach this in two phases. First, add to the PV array capacity and give it some time to see how you like it's performance. Then, if necessary, add to the battery bank. Since you say you "never" get below 12.2 volts today, I doubt it will be necessary to add more batteries. OK. By now, we're both tired. I hope this all helps. > > Janice, still learning. > And thanks again for the information. > > On Wednesday, September 5, 2012, Jim Healy wrote: > > The ratio of 4:1 (150 amps for 600 amp hours) is a "guideline" or "folk wisdom" for flooded deep cycle lead acid batteries. It's based on the physical composition and characteristics of flooded lead acid batteries. This one be based on something called "Charge Acceptance Rate." For a 50% discharged flooded wet cell, the battery will accept recharge current at a rate in AMPS of approximately 25% of the AMP HOUR CAPACITY of the battery without overheating and outgassing. That's where your example (4x) came from. Peg and Jim Healy aboard Sanctuary Currently at Rock Creek, Pasadena, MD Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436

Janice Marois

Sun, Sep 9, 2012 8:31 PM

I was glad to learn the additional solar panels should supply the
needed amp hours to power my reefer -- of course by the time all are
installed it will be winter (where does time go?) and less critical
than in 90+ degree weather of August (er, September)

You'd mentioned:

And in response: I'm at the 25th latitude. When I did the math
(admittedly a couple years back) the basics from the 20th to 30th
parallel were, take your wattage, divide by three and call it amp
hours per day. So, on a normal sunny day I can count on 25 amp hours
to play with/use. On my Morningstar the charging light comes on just
before dawn (showing at least enough incoming power to light the LED
and remains on until shortly after dusk -- far more than 6 hours. The
most incoming I've seen is 4.7 and 2-3+ is normal in the middle of the
day (from say 1100 until 1500))
It is half past three and I'm seeing 2.2 right now incoming.
Anyway, the figures work out though I'm sure there are plenty of folks
who are more precise -- out here, for me, just having it all work is
what counts. :)

As for cloudy days, that's why the wind gen is up there.
One thing I did do was buy a $30 weather station by Ambient on Amazon.
It's got a remote for "outside" temperature and humidity and that is
already inside the reefer. It's giving me readings already so we shall
see.

I do thank you for your attention to detail (and vocabulary) so I can
get this right when I write. Happy cruising.

Janice, in Bayou Chico

On Fri, Sep 7, 2012 at 9:20 AM, Jim Healy gilwellbear@gmail.com wrote:

Amps can be low over a long time or high over a short
time to result in identical amp hour energy equivalents. The distinction
I'm making is not just a terminology difference; the concepts are very
different and very important

The net additional PV bank capacity you're looking at adding is to pick up
the net new load of the 1.7 cu ft fridge. So, let's look at just that.
Let's ass/u/me that the fridge is rated at 120VAC, and 0.6 amps. Let's
further ass/u/me you have flooded wet cells that are rated at 12.7 volts at
the terminal, fully charged. So, amps DC from the battery = 120/12.70.6 =
5.7 amps DC from the battery. Throw in an inverter efficiency factor of
0.89% (could be better, could be worse; check it's spec) and that becomes
6.4 amps DC from the battery. Now ass/u/me that the fridge will have a duty
cycle of 50% (could be more, could be less), so that's 6.4 amps12 hours =
76.4 amp hours per 24-hour day to run that refrigerator/inverter
combination. It's also 6.4*12.7 = 81.3 watts per hour, or 975.4 watts/day.
Your planned additional PV bank that generates 500 additional watts, for
ass/u/me an output cycle of 6 hours, should cover the fridge, perhaps with
some margin left over.

Thanks to all who have responded, both on list and off... Jim is quite correct in that I use the term amp to mean more than one thing (amp = power consumed, amp (properly amp hour) meaning duration/availability of said power) and I'm pretty sure that to a professional the difference is sort of like nails on a chalk board. I was glad to learn the additional solar panels should supply the needed amp hours to power my reefer -- of course by the time all are installed it will be winter (where does time go?) and less critical than in 90+ degree weather of August (er, September) You'd mentioned: > Now, earlier, you said your current PV array is 75 watts. (You also said, > "and the 25 amps my solar panel gives me per day..." DOES THAT MEAN YOUR 75 > WATT PANEL GIVES YOU 25 AMP HOURS OF ENERGY DAILY??? That would be 5.9 > Amps for 4-1/4 hours, which is a shorter output cycle duration than I have > been ass/u/ming, at 6 hours. And in response: I'm at the 25th latitude. When I did the math (admittedly a couple years back) the basics from the 20th to 30th parallel were, take your wattage, divide by three and call it amp hours per day. So, on a normal sunny day I can count on 25 amp hours to play with/use. On my Morningstar the charging light comes on just before dawn (showing at least enough incoming power to light the LED and remains on until shortly after dusk -- far more than 6 hours. The most incoming I've seen is 4.7 and 2-3+ is normal in the middle of the day (from say 1100 until 1500)) It is half past three and I'm seeing 2.2 right now incoming. Anyway, the figures work out though I'm sure there are plenty of folks who are more precise -- out here, for me, just having it all work is what counts. :) As for cloudy days, that's why the wind gen is up there. One thing I did do was buy a $30 weather station by Ambient on Amazon. It's got a remote for "outside" temperature and humidity and that is already inside the reefer. It's giving me readings already so we shall see. I do thank you for your attention to detail (and vocabulary) so I can get this right when I write. Happy cruising. Janice, in Bayou Chico On Fri, Sep 7, 2012 at 9:20 AM, Jim Healy <gilwellbear@gmail.com> wrote: > Amps can be low over a long time or high over a short > time to result in identical amp hour energy equivalents. The distinction > I'm making is not just a terminology difference; the concepts are very > different and very important > The net additional PV bank capacity you're looking at adding is to pick up > the net new load of the 1.7 cu ft fridge. So, let's look at just that. > Let's ass/u/me that the fridge is rated at 120VAC, and 0.6 amps. Let's > further ass/u/me you have flooded wet cells that are rated at 12.7 volts at > the terminal, fully charged. So, amps DC from the battery = 120/12.7*0.6 = > 5.7 amps DC from the battery. Throw in an inverter efficiency factor of > 0.89% (could be better, could be worse; check it's spec) and that becomes > 6.4 amps DC from the battery. Now ass/u/me that the fridge will have a duty > cycle of 50% (could be more, could be less), so that's 6.4 amps*12 hours = > 76.4 amp hours per 24-hour day to run that refrigerator/inverter > combination. It's also 6.4*12.7 = 81.3 watts per hour, or 975.4 watts/day. > Your planned additional PV bank that generates 500 additional watts, for > ass/u/me an output cycle of 6 hours, should cover the fridge, perhaps with > some margin left over.