Help me finalize my off grid system please

[sensij]

In cold, clear conditions you actually can produce STC outputs for a sustained period.

This is only true for panels that are mounted relatively normal to the angle of the sun. On a 4-12 roof (18 deg tilt), hitting STC is unlikely, except for some rare edge-of-cloud effects. Even high albedo from snow cover won't help much at that tilt, since the sun angle in winter (when it is cold) is very low. The panels would need to be mounted much more steeply to approach STC. See the PVWatts hourly output, which accounts for temperature. It doesn't account much for albedo, but again, in this situation, that isn't going to be much help.

[hammick]

I ended up buying the Crown CR330 batteries. Saved a bunch of money and hopefully will charge better with my proposed equipment.

All of the Crowns are June 2015 manufacture with resting voltages of 6.15 or 6.16 volts. One is at 6.13 volts. Any worries about getting them at about 60% SOC?

I would like to equalize these before putting them in my system. It may be Thanksgiving before I can install them. All I have available right now is a manual charger that will do 50a charge at 6 volt. Shall I just leave it charging for an hour or so after amps taper to 0?

I do have a CTEK 4.3a charger that has an equalization mode. I could put two batteries in series and use it but it would take forever to equalize all the batteries.

[mapmaker]

Any worries about getting them at about 60% SOC?

Yes. Get them charged ASAP. Time is of the essence. The batteries are getting on 4 months old.

I would like to equalize these before putting them in my system. It may be Thanksgiving before I can install them. All I have available right now is a manual charger that will do 50a charge at 6 volt. Shall I just leave it charging for an hour or so after amps taper to 0?

I do have a CTEK 4.3a charger that has an equalization mode. I could put two batteries in series and use it but it would take forever to equalize all the batteries.

I don't know what a 'manual charger' is. I don't know the particulars of the CTEK, except that it is apparently a 12 volt charger. Use it to charge the batteries 2 at a time... do not try to charge or EQ them while they are in parallel. By the way, do NOT equalize the batteries until after they are charged up.

While charging, monitor the voltage (and the current if you have a shunt or DC clamp ammeter) while charging to see what that charger can do.

--mapmaker

[Mike90250]

..All of the Crowns are June 2015 manufacture with resting voltages of 6.15 or 6.16 volts. Once is at 6.13 volts. Any worries about getting them at about 60% SOC?

That sucks, Below 75-80%, the sulfates harden and it's nearly impossible to restore to solution. So I would say they have lost capacity. Piss poor battery shop that does not do a monthly freshen charge on them. They sold you damaged batteries, and you should be able to take them back. The way to do it is go there TOMORROW, with the same meter, and firmly ask to check the rest of the batch they have on the shelf. If their stock on the shelf reads low with both your meter and theirs, it's time to ask for a partial refund.

[hammick]

That sucks, Below 75-80%, the sulfates harden and it's nearly impossible to restore to solution. So I would say they have lost capacity. Piss poor battery shop that does not do a monthly freshen charge on them. They sold you damaged batteries, and you should be able to take them back. The way to do it is go there TOMORROW, with the same meter, and firmly ask to check the rest of the batch they have on the shelf. If their stock on the shelf reads low with both your meter and theirs, it's time to ask for a partial refund.

I bought all six that they had and two more are coming from the warehouse for me to pick up Saturday. The warehouse batteries have July stickers.

Looking for more opinions on this. I am very familiar with stratified batteries and battery maintenance but they have probably only been below 80% for a couple months. Surely a good equalization will bring them back to 100%. Do the battery warehouses maintenance charge their batteries?

These are heavy mothers and I'm not looking forward to returning them.

Thanks.

[Mike90250]

The magic happens between 80 -75% . After a week, it's hardened, and not removable. At least that's the "rule of thumb" I've heard and obey. Maybe you will get lucky. But 60% for a month, is way down in the bad zone.
Good Luck.

[sensij]

I bought all six that they had and two more are coming from the warehouse for me to pick up Saturday. The warehouse batteries have July stickers.

Looking for more opinions on this. I am very familiar with stratified batteries and battery maintenance but they have probably only been below 80% for a couple months. Surely a good equalization will bring them back to 100%. Do the battery warehouses maintenance charge their batteries?

These are heavy mothers and I'm not looking forward to returning them.

Thanks.

I haven't gone through your posts to see where you've worked out the load these batteries need to serve, but at the beginning of the thread, you were ready to go with a 208 Ah battery. Perhaps even if the 330 Ah battery has lost some capacity, it will still be sufficient for your needs, and bought at a price that made it worthwhile.

[inetdog]

This wasn't great advice. Those 6 x 285 W panels will never produce 35 A, more like 25 A at best once you look at real world ratings and efficiency. They would have been reasonably well matched to a 208 Ah battery (C/8 max amperage).

I am having a little problem following your logic or arithmetic here.
six 285W panels will produce (at STC) 1710 W. If you assume that the MPPT CC is 100% efficient you do get ~35A.
I guess your normal estimate, factoring in CC efficiency and panel power drop at real temperatures, is about 70%. That seems a little low to me, but you have a lot more real world experience.

[sensij]

I am having a little problem following your logic or arithmetic here.
six 285W panels will produce (at STC) 1710 W. If you assume that the MPPT CC is 100% efficient you do get ~35A.
I guess your normal estimate, factoring in CC efficiency and panel power drop at real temperatures, is about 70%. That seems a little low to me, but you have a lot more real world experience.

I ran the numbers in this followup post. At 59 V bulk charging, 1710 W / 59 V = 29 A, which would be the charge current at STC and 100% efficiency. Yes, battery voltage will be somewhat less than that at the start of charging, but start of charging is typically in the morning, when the sun is not hitting the array directly and peak power is not available. By the time you get the sun overhead, hopefully the voltage has risen close to the bulk value and adsorb is starting (if thinking about it as a 3-stage charge).

86% of STC panel output + 95% MPPT efficiency gets you to 25 A. Reality is more complicated than that, and I used PVWatts to model what actual power would be available from the array, given the OP's location and his description of the array orientation, and assuming the panels + charge controller would function as efficiently as panels + a grid-tie inverter, which is where my real world experience is.

Too often, when we talk in design rules of thumb we treat PV power as this steady thing, but in reality, the max current is available for only a short time each day, and mostly it is less. I tried to present that in the histogram in the other post.

As discussed somewhat in the max smoke thread, it is much harder to overcharge a battery using PV than it is to undercharge it, and from all of the experience shared in the forum, it seems to me that sound advice would steer towards the overcharging direction, and away from undercharging.

[hammick]

I am going to call Crown and tell them the situation and see what my options are. Maybe eight fresh ones shipped from the factory.

[Sunking]

Sensi you do nbot use Bulk cost-off voltages. Use nominal or else you will be driving you CC into cut-off.

1710 watts / 48 volts = 35 amps

[sensij]

Sensi you do nbot use Bulk cost-off voltages. Use nominal or else you will be driving you CC into cut-off.

1710 watts / 48 volts = 35 amps

59 V may be too high, but 48 V is certainly too low. At what point in the charge cycle would you be able to put leads on the battery and measure 48 V?

The voltage at the time charging begins should be something around 50 Vdc if the system is being run properly. How long will it take for that voltage to rise to the charge voltage (which I hope we agree is ~59 V)? If you are charging from the grid, a generic curve suggests 3-4 hours. With solar, probably longer, because you aren't getting full current right away. This leads to the point I am trying to make. Ignoring everything else for the moment, even if you had an array rated for 1710 W of charge power, you won't get that until you are several hours into the day, when the sun is high, and you won't get that for very long. Several hours into the day, even at the slower charge in the morning, the voltage will still have risen above 50 V... maybe not all the way to 59, but using 48 V in the calculation is totally misleading.

I appreciate that rules of thumb can take what is a complicated combination of factors and simplify it enough to explain in a forum like this. Really, though, if the rules of thumb have convinced someone (or someones) that a 1710 W, 18 deg tilt array in Montana is too big for a 48 V 208 Ah battery, I think something is going wrong. The 330 Ah that the OP has now is not really going to be supported well by this array (if the batteries were in good condition), and the 420 Ah array he was ready to buy would have almost certainly been chronically undercharged or stratified unless they were closely monitored and maintained.

[Sunking]

59 V may be too high, but 48 V is certainly too low. At what point in the charge cycle would you be able to put leads on the battery and measure 48 V?

When they are discharged. When you apply a charge to a battery, it is not voltage, it is current. The voltage is only a product of the OCV of the battery plus IR voltage.

Example if you have say a 48 volt AH battery the Internal Resistance is around .04 Ohms. So with a 30 amp current supply on a battery with a OCV of say 48 volts is only going to rise to 48 + (30 amps x .02 Ohms) = 48.6 volts. That means your panel power was clipped from 1710 watts down to 30 amps x 48.6 volts = 1450 watts. Had you used a 40 amp controller, you would be able to use full panel power all the way through the Bulk Stage 1710 watts / 48 volts = 35.7 amps which is the constant current phase where the fastest and largest charge is obtained. Use the Bulk Voltage as your voltage and you screw yourself out of large percentage of power. The only time you would use full panel power is the last few minutes of the Bulk stage as the battery voltage rises to 1710 watts / 30 amps = 57 volts. You just wasted a lot of time, money, and energy. You would be better off just chunking 200 watts of panels into the trash as that is essentially what you are doing. using a high voltage.

Always use Nominal Battery Voltage or worse case for all calculations. You are sugar coating it, only fooling yourself and cheating your clients.

[sensij]

We are starting to talk past each other, so I'll take one more stab at this. Let's start with what we know:

1) OP is in S. Central Montana (I'm using Livingston weather file for modeling in this post)
2) The 1710 W (6 * 285 W panel) array is mounted 180 deg azimuth, 18.5 deg tilt.
3) OP has a 60 A high quality MPPT charge controller. CC clipping is not on the table in this discussion.
4) The PV output of the grid-tied array defined above can be modeled by PVWatts, using "Premium" panels, roof mount, 8% loss. These parameters fit real-world data for healthy grid-tied systems very well, and a good MPPT CC could have close to the same efficiency.
5) The OP was considering a 48 V, 208 Ah battery, and was told that 1710 W was too much array for this battery.
6) We don't know what the daily energy requirement is, but for the sake of conversation, let's say we start the day at 50% SOC.

Let's start to make some assumptions about the state of the battery. I'm sure these numbers can be nit-picked, and I'll be happy to adjust them as needed to improve the credibility of this thought process. I think they will be close enough to make the point.

7) A battery at 50% SOC has an OCV of 48 V.
8) OP's battery at 100% SOC has 48 V * 208 Ah = 9984 Wh of capacity (20 hr discharge rate) (call it 10000 Wh for the sake of rounding).
9) OP's battery at 50% SOC has 10000 Wh / 2 = 5000 Wh remaining of its capacity.
10) The round-trip efficiency is about 75%. To fully replenish 5000 Wh of capacity, 6667 Wh of energy are required.
11) The bulk stage of charging is the only stage in which the PV is able to deliver power without constraint.
12) The bulk stage of charging ends at about 85% SOC... that is when it shifts to adsorb and becomes voltage limited.
13) To get from 50% SOC to 85% SOC (bulk stage charging), 3500 Wh / 0.75 = 4667 Wh of energy are required.
14) The battery voltage at the transition from bulk to adsorb is 59 V.
15) During the bulk stage, as the 4667 Wh are being delivered, the voltage will climb linearly with energy from 48 V to 59 V (I know it isn't really linear, but I don't think the non-linearity screws this up)

Time for some pictures. First, scanning through the PVWatts model output, I looked for a cool, clear day with high peak power. May 24 fit the criteria, although there were a bunch of days in the April / May timeframe that would have worked for this. If the 1710 W array is truly oversized for this battery (because the C rate is too high), a day like this should show it right? If not, what type of day would?

PVWatts Hourly.GIF

Focusing on just the model of May 24, the grey lines below show when 4667 Wh have been generated, at 12:10 pm. Since it is in bulk, and the CC is not clipping, we'll assume that all of that energy is going towards charging the battery (with losses accounted for in the round trip efficiency).

PVWatts - Test Day.GIF

We know the power being delivered during bulk, and have guessed the voltage increases linearly with the energy delivered between the known 48 V and 59 V endpoints. With power and voltage defined, a plot of what the voltage and current during bulk might look like that day can be generated (P = I * V)

PVWatts - Test Day IV.GIF

Now that the system has gotten to adsorb at 12:10 pm, the voltage is held at 59 V and the current is allowed to taper back off as the SOC increases from 85% to 98% or so. The point has been made many times in this forum... during adsorb, the ideal case is to let the battery take whatever current it wants at that charge voltage, but with PV systems, usually the supplied current drops off faster than the natural decline in current drawn by the battery, and the system ends up undercharged. For that reason, having a larger array is better, as long as the current doesn't exceed C/8 in bulk for an extended period of time.

You can see in this model that the maximum current observed in bulk is 25.2 A, or C/8.2. From this, I conclude that in the situation that the OP described, a 1710 W array is sized well to support a 48 V, 208 Ah battery, and is not oversized, as another forum member suggested. Supporting the 330 Ah battery the OP ended up with is going to be a stretch, and careful maintenance / supplemental charging will be required.

[thastinger]

I understood the OP intends to leave the system unattended for a few months? Working off that assumption and knowing that his CC is going to reset at midnight, his batteries are going to see in excess of 35A at times, which I figured would boil off his water levels too quickly between visits. I did not pay attention to the 18 degree tilt so it wouldn't be as bad as I originally thought but would still be over 30A I would estimate. My array is pretty optimally placed for my Lat and on a 35 degree cloudless day my panels exceed their rated capacity by as much as 8 percent. I've never has an off-grid system where it gets as cold as where the OP is located nor tilted at 18 degrees but my feeling is that he could certainly approach his rated output.
I agree with Sensi on the real world aspects as my system output rarely exceeds 80% capacity on hot summer days with plenty of sun so extra panels would be good during those times, but in the winter my system cranks.