Off-grid system review

[BackwoodsEE]

Makes sense. The paralleled cells are permanently balanced with each other. You really are an invaluable asset to this forum, man.

Glad to know I don't really need the BMS. But spending $600 on an Orion Jr. for some peace of mind and a more accurate SOC info about an $8000 battery, and to avoid having to take everything apart to manually balance every year, seems reasonable enough to me.

I'm also thinking about building a box with about six 0.1F 80V electrolytics ($40/each from United Chemi-Con via DigiKey) to put in parallel with the battery (switched and fused, of course). I'd like to reduce the amount of micro-cycling that goes on when there are significant loads and PV power at the same time, and reduce the peak current out of the battery from the 120 Hz inverter load.

[Sunking]

Well, thanks,here I go down the old rabbit hole again, now researching CALB batteries. Actually they look pretty darn attractive for my situation. About half the cost of the slick-packaged SimliPhi units with built-in BMS. (One problem with that BMS: If its breaker trips open and disconnects the battery and you have nowhere else for the charge controller to put the current, you might wind up with a fried charge controller.)

You really do not need a BMS for 16S

Now, I understand paralleling strings of cells is frowned on with lead acid, but is that true with these as well?

Not at all, If done correctly you can parallel lead aicd. The problem with Pb aka Lead acid is most folks use 6 or 12 volt jars, rather than 2-Volt Lead Acid batteries. If you were to use two volt Pb batteries them you can parallel them exactly like you would Lithium cells. Pictures is worth a thousand words.



You parallel at the Cell Level. not string level. Example the Tesla Roadster uses 6831 18650 cells arranged 99S69P. That is 99 cells in series and 69 cells in parallel.

[BackwoodsEE]

If you are a Train Driver EE like me, Look into LFP cells made by Chi-Coms like CALB. Expensive initially around 50-Cents per Watt Hour vs say 15 to 20 cents of FLA, but if you Bottom Balance, and never fully charge them can give you 1500 to 2000 cycles. You can hit them with 1C charge rate, and unlike FLA have no need to fully charge them up. In fact you do not want to ever fully charge them. Good fit for solar because LFP works best in Partial State of Charge (PSOC), where AGM and FLA must be kept fully charged for maximum cycle life

Well, thanks,here I go down the old rabbit hole again, now researching CALB batteries. Actually they look pretty darn attractive for my situation. About half the cost of the slick-packaged SimliPhi units with built-in BMS. (One problem with that BMS: If its breaker trips open and disconnects the battery and you have nowhere else for the charge controller to put the current, you might wind up with a fried charge controller.)

Given these batteries' tolerance for fast charge and discharge, it looks like a single 16S string of 180 Ah cells (about $3700) would support an Outback Radian and a fully-loaded TS-MPPT-60. Two hours of sun hitting the panels between the trees in December? No problem, things happen fast, and then it's back to shadows and gloom.

Now, I understand paralleling strings of cells is frowned on with lead acid, but is that true with these as well? Thinking I could add a second string of them for 360 Ah and still be at around the cost of a bank of Trojans or Surrettes. Having two strings would allow one string to be taken out and balanced at a time.

[Sunking]

Yes, it does! Fascinating stuff. And it helps explain why there is such a narrow range of battery capacity vs maximum solar capacity.

What makes this all so frustrating, especially in a place like the Pacific Northwest where you have cloud cover for much of the year, is that the PV array is going to be producing much less than its full capacity most of the time. .

Easy solution, You have two options, but both are expensive, around 400% more expensive.

Those C/6 to C/8 max charge rates apply to FLA batteries, Does not apply to AGM or LiFePO4 (LFP). Most AGM's can easily handle C/4 and many C/2 to 1C. The catch is AGM's cost twice as much and last half as long as FLA.

If you are a Train Driver EE like me, Look into LFP cells made by Chi-Coms like CALB. Expensive initially around 50-Cents per Watt Hour vs say 15 to 20 cents of FLA, but if you Bottom Balance, and never fully charge them can give you 1500 to 2000 cycles. You can hit them with 1C charge rate, and unlike FLA have no need to fully charge them up. In fact you do not want to ever fully charge them. Good fit for solar because LFP works best in Partial State of Charge (PSOC), where AGM and FLA must be kept fully charged for maximum cycle life

Unfortunately where you are just piss poor for solar. It takes a 400 watt system in Gloomy Doomy Seattle to equal a 100 watt system in Phoenix. You get slapped twice. Once for extra panel cost and again for batteries.

[bcroe]

And it helps explain why there is such a narrow range of battery capacity vs maximum solar capacity.

What makes this all so frustrating, especially in a place like the Pacific Northwest where you have cloud cover for much of the year, is that the PV array is going to be producing much less than its full capacity most of the time. From observations of my toy 3-panel setup, the output current is around 15% of the nameplate figure for the kind of light overcast we often get out here. I went for weeks this spring without seeing a moment of full PV output. So you are then faced with the problem of too little charging capacity to maintain SOC on your batteries, and it becomes time for that nasty old fossil-fuel burning generator to keep SOC from turning into SOL.

Panels are relatively cheap now, so you could go heavy on them, but then you have to have charge controllers to handle the maximum current and, for the reasons you just explained, a battery to handle the most that the CC's can put out.

Extra panels can help a lot under cloudy skies. No problem if charging is limited by the controller, and
NOT by the panel rating. But extras won't do anything for you under good sun, if they are all pointed
the same direction. Point those extras E and W, and they will stretch your sunny production time,
instead of wasting it around noon. Bruce Roe

[LETitROLL]

Panels are relatively cheap now, so you could go heavy on them, but then you have to have charge controllers to handle the maximum current and, for the reasons you just explained, a battery to handle the most that the CC's can put out.

The only way I have found to kind of "have it all", (i am in the cloudy NW), is mostly what you elude to above, but with current limiting that most of the better controllers have, you can stay with a reasonable size/price battery bank and just adjust the controller to not let the battery have it all when the sun does come out full. In the end you want to design a system for the "normal conditions" that are expected at the site, if often cloudy then its okay to overpanel and put in other protections, not much different than a normal design system for a sunny location that experiences a rare 4-6 days in a row of clouds and everybody scrambles to get out the genny to save the battery bank, when i get alot of full sun (just as rare), my system just wastes potential energy to save the battery bank, it is still less of a waste then any of the other alternatives, since the panels are the cheapest and longest lasting part of the system.

[BackwoodsEE]

Hope that helps.

Yes, it does! Fascinating stuff. And it helps explain why there is such a narrow range of battery capacity vs maximum solar capacity.

What makes this all so frustrating, especially in a place like the Pacific Northwest where you have cloud cover for much of the year, is that the PV array is going to be producing much less than its full capacity most of the time. From observations of my toy 3-panel setup, the output current is around 15% of the nameplate figure for the kind of light overcast we often get out here. I went for weeks this spring without seeing a moment of full PV output. So you are then faced with the problem of too little charging capacity to maintain SOC on your batteries, and it becomes time for that nasty old fossil-fuel burning generator to keep SOC from turning into SOL.

Panels are relatively cheap now, so you could go heavy on them, but then you have to have charge controllers to handle the maximum current and, for the reasons you just explained, a battery to handle the most that the CC's can put out.

I could see the benefit of an adaptive arrangement that senses the amount of sunlight (or PV Isc) and switches in a second string of panels (pointing straight up) for cloudy days to each CC input, in parallel with the main string. But getting something like that to pass code seems like it would be a lot more of a challenge than any of the actual engineering involved.

Perhaps a better solution might be a DC dump load on the battery for when the PV array is maxed out and the house loads aren't keeping up. But then you are dependent on something switching in and actively consuming power to prevent your system from blowing up.

[Sunking]

Sunking I am curious to what extent the charge controller scales back on its charging current when the undersized battery raised its charging voltage in response to the current. If you shove 160 A into, say, a 800 Ah battery, won't that develop a higher voltage at the battery terminals, which would make the CC think it's time to switch to absorption mode?

Probably a lot of damage would occur before it makes the switch, though, right? Just curious to what extent the battery resistance and CC algorithm would work together in an undersized battery situation.

You are on the right track as Ri has a lot to do with it. But here is the issue you might be over looking, Gassing and Corrosion in excess. Plus the possibility of too much current through the battery plates and term post.

Allow me to elaborate. If you took say a 12 volt 100 AH battery @ 60% SOC the Open Circuit Voltage is roughly 12.2 volts with an Ri of .01 Ohms. Now lets say I have a 12 volt 200 Amp charger set to the recommended 14.8 volt setting. The instant you connect the charger you will have 200 amps of charge current, and the battery voltage will go to 12.2 + (100 amps x .01) = 14.2 volts. With me so far.

Well magic happens at 14.2 volts or 2.4 vpc called Gassing and Corrosion. You are still at 60% SOc and will now be at and above Gassing Voltage for a very extended period of time. The electrolyte will be boiling profusely, and unless you have special caps to collect the spewing electrolyte and return it to the battery, otherwise is spew out the vent caps and you cannot replace electrolyte.

Additionally it would likely warp the plates and possibly over heat the term post. No tonly that but you would be generating 200 amps x 200 amps x .01 Ohms = 400 watts of heat on a small battery easily over heating the battery beyond safe limits.

This is why battery manufactures limit max charge curent to C/8 to C/6. Som AGM's because they have much lower resistance can be charged at C/2. What I am driving at when you charge at a slower rate like C/8, you arrive at Absorb phase when SOC is 90%, not 60%, thus much less time at or above Gassing and Corrosion voltages.

Thisi is why utilities like Telephone, Electric, Data only use Float Voltages which is the safest and gentlest way to charge. They never ever reach Gassing voltages and why there batterries can last 10 years or more. Down side to Float Charging is it can take up to 24 hours.

The so called Smart Chargers are really hard on batteries, and solar is even harder. You have an 8 hour window to get recharged.

Hope that helps.

[BackwoodsEE]

You are going to fry your batteries with 160 amps of charge current. That is I assume you know a 8400 watt panel system requires two very expensive 80 amp MPPT charge controllers. The minimum size battery to support those panels are 1280 Amp Hours...

Sunking I am curious to what extent the charge controller scales back on its charging current when the undersized battery raised its charging voltage in response to the current. If you shove 160 A into, say, a 800 Ah battery, won't that develop a higher voltage at the battery terminals, which would make the CC think it's time to switch to absorption mode?

Probably a lot of damage would occur before it makes the switch, though, right? Just curious to what extent the battery resistance and CC algorithm would work together in an undersized battery situation.

[donald]

We have a brackish well, and the pump will push water through an elaborate water filter that will generate 120 gallons of clean water per hour at 50% efficiency. ..................

ah, I missed that. So you have the big storage tank, the RO system, and conventional pump and pressure tank to feed the water system (more or less)?

Considering my personal interests, I would have probably designed not including RO as part of the battery calculation. Since the need to make drinking water is not daily. This attribute is an unusual advantage.

An alternate design would have been to only run the RO on solar or generator. The battery bank could have been smaller. The advantages would be several. First you wouldn't have the efficiency loss of taking power in and out of the batteries to run RO, second the batteries could often be charged in the earlier part of the day, and be in absorption stage in the afternoon. During the afternoon the (now larger) RO system would run, so the efficiency of using the panel output late in the day would be improved.

A small Rolls bank could be discharged deeply, yet still sometimes complete absorption. The relatively large solar array would be able to bulk charge on less than ideal days.

The generator would be sized to both bulk charge the batteries and run the RO simultaneously.

To implement this solution automatically would require a couple hundred lines of code on a $150 home automation computer. (I use a product called Vera). This device would start and stop the RO, as well as report exceptions. Most people aren't going to be doing this sort of implementation themselves, but it is only a minor computer science problem. The logic of what would need to be done is not complicated.

My main point is that the RO didn't have to necessarily be part of the battery system. Something to consider in the future when it's time to replace the batteries. At that time you will know what is working well. You will have undoubtedly developed likes and dislikes for the various technologies you are implementing in your new home. Your choices at this point seem reasonable and considered for your experience level. I'm not trying to get you to second guess your decisions. Doing house systems you know are high quality, and can be serviced locally, should be your deciding factors at this point.

[Cult of Dionysus]

I don't understand the pump. There's a pressure tank, right? A commercial greenhouse might run a 3hp pump 4-6 hours per day, not a home. A home with near ground level water storage would run a 1 hp pump 10-15 minutes per day.

We have a brackish well, and the pump will push water through an elaborate water filter that will generate 120 gallons of clean water per hour at 50% efficiency. In other words, we need to push 4 gallons of feed water through the system per minute. Hence the pump, which is a $25k system (including the pump) from a company based in California which is owned by Grundfos.

Half the water is for residential use. The other will be for ag use, as we plan to have some fruit orchards (pineapple, citrus, banana, etc.). If we didn't filter the water, our appliances would need to be replaced every few years due to rust. That's what happened to the only other guy who built in the area. His laundry machine, dishwasher, pipes all were wrecked because he didn't install a good filter. So he needs to truck water in, which is not cheap either.

[donald]

I don't understand the pump. There's a pressure tank, right? A commercial greenhouse might run a 3hp pump 4-6 hours per day, not a home. A home with near ground level water storage would run a 1 hp pump 10-15 minutes per day.

[Cult of Dionysus]

better study up on AC motors.
My 1/2 hp motor consumes 1Kw according to the inverter internal meter. there are copper losses, eddy losses, and Power Factor, which all end up working against you. And generally, after 2 hp, motors go 3-phase for efficiency. Maybe it's time to consider a 3ph motor and a VFD to run it.

That's a LOT of water you are pumping. Maybe look at usage conservation too ?

The Grundfos CR1-27 does have a VFD. Could that explain the low kW draw...

[Mike90250]

better study up on AC motors.
My 1/2 hp motor consumes 1Kw according to the inverter internal meter. there are copper losses, eddy losses, and Power Factor, which all end up working against you. And generally, after 2 hp, motors go 3-phase for efficiency. Maybe it's time to consider a 3ph motor and a VFD to run it.

That's a LOT of water you are pumping. Maybe look at usage conservation too ?

[Hilltopsolar]

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