Off-grid system review

[sensij]

I feel that is a trick question, but I'll bite the bait because getting hooked is the only way to find out what is at the other end of the line. I put up 300W because I want to use the 200 watts to provide the correct charging currents to the battery, and the other 100 watts to run the daytime load. Or I want the other 100 watts facing east. Or I want the other 100 watts because I usually have a lot of clouds. But that is part of the 'Dumbest Question Ever' thread I'm fixing to ask when I have given up answering it myself. Go ahead, reel me in.

So, there is some stuff to deal with here.

Most entry level charge controller do not have a facility for limiting amps, beyond the hardcoded limit at the charge controller's rating. Yes, if you are using controllers at that end of the market, you need to replace the controller when you want to change the charge current limit.

More advanced controllers have the ability to set a current limit less than that controller's rating. However, that feature does you no good for operating daytime loads, since the available current will not be increased even if some of it is being routed off away form the battery. It does give more design flexibility and better future-proofing for the different battery configurations you might consider using.

Some controllers have relays or control signals available to connect with other equipment that may control opportunity loads. The more typical use case is for those loads to kick in when the battery is fully charged. Trying to run those loads in parallel with charging, on the assumption that there is still enough sunlight in the day to achieve both, is more thinking than the charge controller will offer.

I fully agree with the idea that the charge controller amp limit (whether it is the rating limit, or a lower one set by the user) is an effective tool to lengthen the charging day and safely over-panel the system, and builds more margin into a design than strictly limiting the PV rating to the controllers rating would allow.

[Mike90250]

........Well that is what you want out of your controller. You want to install 300 watts of panels, and only use 200 watts. Why?

I put up a 1200w array and limited to 30A @ 24V, because that's all the batteries can take in the summer. Winter, in the clouds. fog and rain, I need every inch of that array and more. wife does not want a generator there.

[Sunking]

I feel that is a trick question, but I'll bite the bait because getting hooked is the only way to find out what is at the other end of the line. I put up 300W because I want to use the 200 watts to provide the correct charging currents to the battery, and the other 100 watts to run the daytime load. Or I want the other 100 watts facing east. Or I want the other 100 watts because I usually have a lot of clouds. But that is part of the 'Dumbest Question Ever' thread I'm fixing to ask when I have given up answering it myself. Go ahead, reel me in.

FWIW you are making this harder than it has to be.

I am OK with you over driving a charge controller and using the controller to limit maximum charge current. I think you are getting to rigid or hung up on exact charge C-Rate of some number like C/10 or C/8 . A good Deep Cycle battery like Trojan can take C/6 to C/4. They just use more water, and with solar you want to Run on Heavy Metal and Hot charging since solar does not do a good job of charging batteries, just not enough hours in a day. So run hot to limit deficit charging cycles. For as short of a period max current is actually produced, you really do not want to limit unless you have to or have batteries that do not require being fully charged like Mike has with NiFe. Use every bit of power the panels have to get recharged ASAP. A few minutes of a high rate is not going to hurt anything.

As always consult your battery manufacture for rmax charge rates, but you can get away with greater than C/8 for limited time. If you start seeing higher water use back off just abit if you are reaching 100% on a hydrometer.

[Sunking]

I put up a 1200w array and limited to 30A @ 24V, because that's all the batteries can take in the summer. Winter, in the clouds. fog and rain, I need every inch of that array and more. wife does not want a generator there.

Yep Mike I know there are times when you need to limit charge current and over size panels. You have a bit of a unique circumstance using NiFe. As you know you must limit charge rate on them. That nasty ole internal Resistance thing sends them to gassing voltage in no time.

To bad you cannot charge them slower so by the time they reach gassing voltage, should be close to fully charged, and thus spend a lot less time at gassing voltage aka Absorb Phase. Would save you lotta water.

[AzRoute66]

Sensij, Thanks. That is exactly the subject matter that is somehow nebulous to my common sense of a charge controller.

Sunking, OK that's a fair treatise and I did learn something new. Thank you. So, is it not fair to say that if I do not size my array to 'bulk battery charge current PLUS daytime load current' I am basically dooming myself to failure? Or perhaps better, is this an argument for not starting ANY frivolous load until we get out of Bulk?

Third Question: Per Sunking, "As always consult your battery manufacture for rmax charge rates, but you can get away with greater than C/8 for limited time. If you start seeing higher water use back off just abit if you are reaching 100% on a hydrometer."
Isn't that hard to do ("just back off") if you don't have a maximum charge current setpoint and are allowing CC max to dictate bulk charging current?

[Mike90250]

.... is it not fair to say that if I do not size my array to 'bulk battery charge current PLUS daytime load current' I am basically dooming myself to failure?

yes. no. yes. not nay

I'm sure that if you don't allow for proper charging AND all the normal loads, your batteries will NOT be happy, and will die young

[Sunking]

Isn't that hard to do ("just back off") if you don't have a maximum charge current setpoint and are allowing CC max to dictate bulk charging current?

No, you turn down or up the voltage setpoint silly.

You are still missing the most important point of a battery system. Step one is determine worse case loads on the worse day of the year. Design for that and everything falls in place and is downhill from there. When you design a battery system you design it to work for just one day of the year under the worse possible conditions. The other 364 days a year are a piece of cake.

Battery size is determined by your total daily load or 24 hours, not just day or night. Then your battery is sized for 5 days. Step 2 is size the panels to replace your power on the shortest day of the year. Third step is to size your generator to recharge your battery from 50% DOD in 3 to 4 hours.

Like taking off in a pane my friend. you had better know what you are doing and make damn sure you have more than enough fuel or you are going to crash and burn. If you would just take some time and read the STICKIES answers all your questions. Like this one is a good start.

[AzRoute66]

6 DETERMINE CHARGE CONTROLLER SIZE IN AMPS

Again Very Simple. MPPT Amps = Panel Wattage / Battery Voltage.

Tucson is 1400 watts / 48 volts = 30 amps or 60 amps at 24 volt battery. A 30 amp MPPT Controller is a lot less expensive than a 60 amp controller..

The above came from your sticky, where you changed the 'rules' and decided that your charge controller Amperage would be Array Size/Volts. Something you have absolutely derided in many places on this forum. You came up with a 30 Amp controller, very smugly, for your 500 Ah battery bank. What is that? C/16.7? Undercharging, in the one area (off grid) you want to overcharge as much as possible?. For a 24/7 constant demand load even. I'm glad you picked that as your sticky of the day, I can come back to it for as long as I need to in dozens of threads. [right now I'm so scared I missed something obvious and am shooting myself in the foot, I proceed either bravely or foolishly]

You know a lot, and you explain it well, but you pick and choose the parts you wish to ignore FOR THE MOMENT. Don't get me wrong, I LIKE that, it makes me think, and you have done that to me multiple times - I just don't respect it.

The road we're heading down is that you say something, I question it, or something it implies, and you ignore the question. Instead, you look for some way to go sideways (again, makes me think which is fine). I worded the all three of the questions in post #125 the way I did (in a amateurish manner) to get you to go in a straight line but that didn't even work. That is fine, I was just trying to get you to say something, it was a long shot, and you didn't. Touche'.

[Sunking]

The above came from your sticky, where you changed the 'rules' and decided that your charge controller Amperage would be Array Size/Volts. Something you have absolutely derided in many places on this forum. You came up with a 30 Amp controller,

You know a lot, and you explain it well, but you pick and choose the parts you wish to ignore FOR THE MOMENT. Don't get me wrong, I LIKE that, it makes me think, and you have done that to me multiple times - I just don't respect it.

What are you talking about?

Where did I change rules?

In the example what changed? 1400 watts / 48 volts = 29 amps. A 30 amp controller is perfect.

[AzRoute66]

Fair enough. 30 Amp / 500 Ah is a C/16.7 charge rate - perfect as per Sunking. I AM missing something. Back to my burrowing. Thanks.

[AzRoute66]

No problem. No 'war' being waged. Just some logical fallacies (from time to time).

[Sunking]

Fair enough. 30 Amp / 500 Ah is a C/16.7 charge rate - perfect as per Sunking. I AM missing something. Back to my burrowing. Thanks.

I have no clue what you are talking about. What does 500 AH battery have to do with it? C/16.7 is far from a perfect charge rate for any battery except maybe a Gel Battery which cannot be charged fast.

All I said is with 1400 watt panel and 48 volt battery you need a 30 Amp Controller. Nothing more, nothing less. Has nothing to do with a battery other than the nominal voltage. Hell 30 amps of charge current is perfect for a Lithium Ion LFP battery of 60 AH, or perfect for a 120 AH AGM battery, or perfect for a 300 AH FLA battery.

What are you talking about?

1. For an off-grid system the first step is determining worse case Daily Watt Hour Usage. Example say 2000 watt hours or 2 Kwh per day.

2. Primarily you want to size the battery for 2.5 to 3 days run time without Sun. If you are using a lead acid battery, you size it for 5 days. Using 5 days reserve capacity gives you 2.5 to 3 days of usable run time without a recharge or taking the battery below 50% DOD which is a No-No. For a lithium Ion battery you size it to 4 days for an equivalent of 5-day Lead Acid batteries. Example with a Pb battery using 2 Kwh per day gives us a 10 Kwh battery, or if you use LFP 8 Kwh.

2. What battery Voltage? Answer is high as you can so you do not spend a fortune on expensive charge controllers and wiring. Lower the voltage, the more expensive it gets. The real answer lies in a couple of things mostly determined by panel wattage. More on that in just a minute.

3. So what panel wattage? That depends on location and time of year use. We need to know the lowest month Sun Hour. In the USA is the months of December and January. Two extreme examples. Say Phoenix AZ = 4.0 Sun Hours in January, and Seattle WS = 1.8 Sun Hours in January. In order to generate 2000 watt hours usable in a day requires the panels to generate 3000 watt hours to overcome all the losses. In other words 1/5 times more than used. So panel wattage required:

PANEL WATTAGE = [DAILY WATT HOURS x 1.5] / SUN HOURS

Phoenix = [2000 wh x 1.5] / 4.0 sh= 750 watts, using 3 x 250 watt panels wired in series.
Gloomy Doomy Seattle = [2000 wh x 1.5] / 1.8 sh = 1667 watts round down to 1650 watts using 6 x 275 watt panels configured 3S2P

4. Charge Controller and Battery Voltage Selection. Everything above assumes using a MPPT Controller because it is the most efficient and lowest cost option. PWM is cost prohibited and foolish. So with MPPT Controllers charge amps:

Amps = Panel Wattage / Nominal Battery Voltage

Real damn simple. Let your wallet decide. In Phoenix we have a panel wattage of 750 watts which means we could use 12, 24, or 48 volt battery. But what voltage. This hould help

750 watts / 12 volts = 62.5 Amps
750 watts / 24 volts = 31.25 Amps
750 watts / 48 volts = 15.625 Amps.

So what does your wallet think? A 62 amp controller cost you roughly $550 to $625. A 30 amp model around $225 to $300, and no clue what a 15 amp controller for 48 volts is because they do not make them that small for 48 volt battery. The right answer is 24 VOLTS with a 30 amp controller.

What about Doomy Gloomy Seattle?

1650 watts / 12 volts = 138 amps. There are no CC that do 138 amps. It would take two very expensive $600 65 amp controllers.
1650 / 24 volts = 68.75 Amps doable with a $600 65 amp Controller
1650 / 48 volts = 34 amps, doable with a 30-Amp MPPT Controller for 48 volts like a Morningstar or Midnite Solar 30 amp controller.

Right answers ar 24 volt for Phoenix, and 48 volt for Seattle.

Now it is time for batteries. We know we need 10 Kwh for Pb batteries or 8 Kwh for Lithium.

Battery Amp Hours = Battery WH Capacity / Nominal Battery Voltage

For Phoenix we need a Pb sized to 10,000 wh / 24 volts = 416 AH round up to 420. Anywhere from 400 to 450 will work. For LFP 8000 wh / 24 volts = 333 AH or 300 to 350 AH. OK in Phoenix with 750 watts of panels and 31 amps of charge current do we meet the min/max charge rates of our batteries. Pb is C/8 to C/12, and for LFP no more than C/2. Well if we use a 400 AH battery 400/31 = C/13 a bit low at C/13 but close enough. To get C/12 would require a panel wattage of 800 watts with 33 amps of charge Current. LFP no issues at C/11 charge current because it is less than C/2.

For Seattle we need a Pb battery sized to 10,000 AH / 48 volts = 208 AH or 200 AH, for LFP 8000 wh / 48 = 166 AH. However what is the charge rate on the batteries with 1650 watts? On Pb would be 208 / 34 amps = C/6 which might be a bit to high forcing us to select either a AGM battery of one that can tolerate the higher charge rate. With LFP no issues again as it is below C/2.

FWIW all your questions are answered in the Off-Grid Stickies and I would not have to keep explaining to you over and over again. Try reading them sometime.

[AzRoute66]

The 500 Ah battery was the same battery you specified in the YOUR sticky that specified the 30 Amp Charge Controller. YOUR sticky being the one that you authored, provided the link to, and told me to read. Did you read my first paragraph in post #128?

[Sunking]

The 500 Ah battery was the same battery you specified in the YOUR sticky that specified the 30 Amp Charge Controller. YOUR sticky being the one that you authored, provided the link to, and told me to read. Did you read my first paragraph in post #128?

Now I understand. My bad. I used two extremes way outside the norms. I used the two most extreme Contrast locations in the USA. Tuscon has the highest Winter Irradiance in the continental USA, and Seattle has the lowest. Both fall outside the Window of 2.25 to 4 Sun Hours in winter will place the Charge C-Rate outside of C/8 to C/12 window. Today with the competition and slight improvement we can probable raise the max to C/6.

Normal national average for winter Sun Hours is greater than 3 less than 4 Sun Hours. So the example I used in the tutorial was outside normal limits for purposes of demonstration how important location is. I lost the chance to address what to do when you fall outside limits. But covered that topic in other Stickies. My bad.

So if you had FLA and only needed a panel wattage that produced C/16 charge current, you would have to increase panel wattage and controller size to meet C/12 minimum requirement. That is one extreme where they are blessed with Sun year round. The other Extreme is Seattle where charge currents get to high and we have to consider using AGM or today lithium since they are a bit closer in cost.

All checks and balances in the design.

[AzRoute66]

[from post #113]

In the top end controllers, there is, both my Mornigstar and Classic. have deeply buried settings to limit max output current to less than full power.

I haven't got to the Classic yet, but I have been trying to fully understand the Morningstar line. I have read the operators manual, and all of the others I can find, and it seems to me I keep coming back to "For a full list of settings, refer to the Help Section of the MS-View software."

Is there any documentation for these settings, or do I really have to install the MS-View software? (assuming it will run without some kind of 'key' or network contact with the actual Morningstar product)

I'm fixing to look at the Classic, how about those?