Need help designing off-grid system

[Xplode]

Instead of buying 16 batteries @ 225Ah rating, consider using bigger batteries. The T105 are 225Ah (from a quick google I did) but there are larger batteries in the 305Ah, or 430Ah ratings that you could use. Then you still only need 8 batteries, but because your Ah capacity has increased, you can increase your charging currents.

ie a Crown CR430 battery bank (430Ah @ 20hr rate) means you can put 43-53A into the batteries. So you could look to a 40-50A charger to push the charging capacity up.
a 430Ah battery bank @ 48V would push you up to 20.6KWh with your charging system returning that at a rate of about 2100Wh per hour +/- a few hundred. So if your solar system and/or generator are sized to support the 50A charging rate, you'd only need to run for an hour or two to replenish your daily usage.

That's getting into a large system... Your original estimates of ~10KWh (3 days @ 50% DOD) seem reasonably legit. You're going to get some charging capacity, even on cloudy days. Not much but it does add up a little. And if you run the generator for the clothes drying you sound like you should be on the right track. IF you really want to run the full gamut off the batteries, then consider investing in a 300W inverter for your basic lighting loads, and a 2000W inverter for your dryer. The reasoning for this is based on the "Idle losses" that all inverters have. If you look at the spec sheets of the inverters, they'll have it listed. It is the amount of energy wasted by the inverter PER HOUR to sit there idle, doing nothing. A small 300W inverter might be like 3-8W. a 2000W inverter is like 25W... That adds up to around 600Wh per day of losses just so you can run your dryer once in a while. That'll run the batteries and system harder and lead you to run your generator more often, as well as replace your batteries sooner. Many off grid users have 2 inverters. a large for big loads that you can switch on just when you need, and a small inverter you can leave running 24/7 for your lights and cell phone chargers/etc.

[Bigbus]

Based on the internal resistance equation for FLA batteries where max current = C/8, my max charging current is 26.9 amps. So I would need a charge controller that outputs less than 26.9 amps. No good. Somehow I need a higher amp hour rating for my bank. Probably by getting bigger batteries. Trojan's T-125 is 240 ah which would put me exactly at 30amps. (240/8 = 30)
Now, I have 1470 watts of panel. 6 245w panels. VOC is 37.27 and ISC is 8.61. If I wire my panels 2 in series 3 in parallel, my max voltage is 74.54v and my max current is 25.83a. Temperature could change that some but even at -10 F that puts me somewhere around 82v and 33a. Meaning I could do a charge controller that is 100v. My understanding is the the amps don't have to match exactly on the input, so I could still do a 30 amp charge controller. Something bigger like a 45a would be a waste. Right?

Xplode Someone along the way said 10kwh bank but my math had said 15kwh. Just so you know.

[citabria]

Just a comment on logistics. Is the remote site accessible by truck, atv or horses? Is it a fire lookout tower? Batteries may weigh in at about 125 pounds each, so that can be a handful when solo even with a dolly. FLA batteries are going to need distilled water, can the water be stored in an area not subject to freezing?

[jflorey2]

jflorey2 any reason you say to do 48v instead of 24?

12V is useful if 1) you have small loads, 2) because you can charge with a car's electrical system and 3) there are lots of accessories for 12V (RV market.) If, for example, you have an RV or trailer and small to modest loads, then 12V can work. And since your trailer is probably already wired for 12V the conversion costs are less.

If you want to go higher in power, then go to 48 volts. With either 24 or 48 volts you'll need a new inverter/charger and a new battery system. 48V will almost always be cheaper for the same power due to the smaller wire size. Except in very rare cases, 24V has no advantage over 48 volts.

[Bigbus]

Moving the battery bank is no problem. They will be permanently stored in my tiny home.
Can anyone confirm that my calculations are correct and I should be looking for a 100/30 charge controller?

[Xplode]

Since you've got 6 panels, I'd suggest going with a 3S2P configuration and a 150VOC CC setup. With MPPT the higher voltage is better for keeping the controller operation into the evening, and the higher voltage works better with higher voltage battery banks. If the panels are 2S3P then you'll only get about 65Volts unloaded, but once loaded and charging, they will drop down quickly when a cloud or the evening approaches. As soon as it drops below about 55Volts, the controller won't really be able to charge efficiently anymore (since a 48v battery will charge at 55v +).

Means you get lower losesl in your cabling any so may not need to upsize your wiring. Makes life a lot easier.

[littleharbor]

Since you've got 6 panels, I'd suggest going with a3S2P

That won't work with a 100 Voc. max controller. Panels have a Voc. of 37.27. Need a 150 volt max controller.

[Xplode]

Sorry littlehatbor - I am posting from my phone and accidently submitted before I was done writing. I edited to complete my thought which includes your concerns

[Sunking]

Can anyone confirm that my calculations are correct and I should be looking for a 100/30 charge controller?

You tell us. It is stupid simple math a 4th grader can do.

MPPT Output Current = Panel Wattage / Nominal Battery Voltage.

Gotta 300 AH 24 volt battery? Need 40 amps of charge current and need to know what panel wattage? . Again Stupid Simple math.

Panel Wattage = Charge Current x Battery Voltage.

You have 6 panels and that gives you 4 configuration possibilities which again is simple math.

6P last thing in the world you would want.
2S3P better than 6P but still a poor choice because it means larger wire and would be required to use combiners and fuses which cost real serious money.
3S2P is workable and most likely choice given you are hung up using low voltage 100 to 150 Voc controllers.
6S is most efficient and lowest cost. Best of all worlds. Trick is you must use a 600 Voc Controller.

At least you did not make a fatal mistake of using a Prime Number of panels. With 3, 5, 7, 11.... panels you have two possible configurations of all in series or all in parallel.

[Bigbus]

Sunking I've done those equations. My suggested setup fits perfectly. It tells me that I need 30amps. I just wanted to make sure!

Why would I want to do 6s if that means I need a very expensive CC? A 600v CC is easily over a grand vs a $300 150v option? I could do 3S2P and connect these using a MC4 connector for less than half the price, right?

Am I correct that if a CC is listed as 150/35, the output is 35a and can't be programmed to be only 30a?

[Xplode]

Some charge controllers (most of the food ones) can be programmed to go lower than their rating.

[Sunking]

Sunking Am I correct that if a CC is listed as 150/35, the output is 35a and can't be programmed to be only 30a?

Where are you coming up with this stuff? C/8 is a generic recommendation that is safe for any FLA battery. You can go up to C/6 on solar because solar is not capable of C/6 charge rate for more than 1 hour per day at noon.

I assume you are using golf cart batteries? Golf Cart Batteries are hybrids which means they can be charged and discharged quickly at C/4. Sorry but you cannot see the Forest because you are allowing the trees to blind you.

Now if we were talking about a AC charger running of the grid or generator, then we need to be a bit careful because when you use a charger on a stiff power source like the grid or generator is fully capable of supplying max charge current 24 x 7.

If you were to record charge current on a chart say every 5 minutes what you would see is in the early morning just a few amps. As the sun gets higher in the sky ,charge current will begin to rise slowly and keep rising until solar noon at say 40 amps. You will only see that 40 amps for just a few minutes around Solar Noon. Than the current starts going down and down until sunset when there is no current flowing.

[Sunking]

Sunking Why would I want to do 6s if that means I need a very expensive CC? A 600v CC is easily over a grand vs a $300 150v option? I could do 3S2P and connect these using a MC4 connector for less than half the price, right?

For your application yes you are correct. But that does not hold true as scale goes up. Electrical codes require you to use combiners/disconnects, and Over Current Protection Devices (fuses/breakers) with 3 or more strings of panels. Lets say you had 18 x 200 watt panels or 3600 watts into a 48 volt battery. That tells you need a 80 Amp Controller (3600 / 50 = 2 amps) . With me so far?

If you were to go with a 150 Voc model controller you would have to configure the panels in 3S6P configuration. That is 6 parallel strings. You now need a Combiner/Fuses/Disconnect unit which cost some big dollars, much more than the 600 volt controller. With the 600 volt controller, you configure the panels 9S2P which requires no combiners/fusses/disconnect and uses a a lot less and smaller wire. Not only would it be a lot less expensive, but also more efficient.

Understand? Now you know why with your application why you would avoid 2S3P like a naked ugly fat lady, You want a 3S2P model like a naked beauty with great big (.)(.)

[Bigbus]

I am thinking I will use Trojan's t-125. Its a 6v 240ah battery. According to the owners manual, the max charge current that should be used is 10% -13% of the AH capacity. Which is 24 - 31a. Are you saying that I should get a CC with a larger output because I won't normally see that peak charge current of 30a? In that case what would be the purpose of that? It seems like a 30a CC is exactly what I want. In the middle of the day I will be charging very close to the max at 30a and in the morning and evening I will get much less than that regardless of the CC max output.

[Sunking]

Bigbus listen very carefully. If you were to take a Trojan Battery to an authorized distributor of Trojan Battery with a warranty claim, they are going to put that battery onto a custom made Trojan Battery Analyzer. They will charge the battery up, and then do a full discharge capacity test. Now you maybe thing so what?

Well the Operator only has two choices of Charge/Discharge current Trojan built into the Analyzer. It is fully listed in the specs. If the battery is less than 200 AH they select 50 amps, anything over 200 AH and they use 75 amps.

So why the C/8 max recommendation? Simple answer, because consumers are stupid and have smart ambulance chasing lawyers to make companies pay for the public being stupid and lazy. Political Correctness term is Product Liability. Example a few years ago a person sued McDonalds when she spilled hot coffee in her lap. The poor person was to stupid to know coffee is hot and will burn skin. So McDonald's got sued for not educating the idiot telling her the coffee was hot.

The issue here is called Gassing and Water usage. When you charge at a higher rate, causes accelerated water use. For a FLA battery is not much of a problem because the water can be replaced, and is part of a normal maintenance routine. For AGM and Gel is fatal and the water cannot be replaced. So there is the problem, public is stupid and lazy and thus will not perform proper maintenance. So when a problem ppops up, they sue for damages and warranty replacement.

Now there is a point where there can be too much current coupled with too high of a charge voltage. That point is when the battery voltage gets to Gassing Voltage (roughly 2.4 vpc) with too high of a current, the bubbling becomes too violent. At that point the batteries are not only gassing, but spewing acid out which cannot be replaced and lost forever along with the damage acid causes getting onto things it should not like floor joist.

In fact you can charge most any FLA battery at 1C without any problems. Example say 100 amps on a 100 AH battery. How can that be. Real simple use Float Voltage like 13.6 volts on a 12 volt battery. A 12 volt battery Gassing voltage depending on temps is 14.2 to 14.6 volts. That is exactly what utilities do. I have countless DC Battery Plants in the country. Example a -48 VDC 10,000 AH battery is charged with 12,000 amps. Some of those batteries are approaching 40 years of age. So how do utilities get away with it? Because they are pros and know what they are doing. They have technicians that look at the batteries weekly and use lower charging voltages. Then when needed, which is rare, they will equalize the batteries, right after they check water levels because they know the batteries will gas heavily when they turn the voltage up from 2.25 volts per cell (54 volts) to 2.50 volts per cell (60-volts).

Now go look at the specs again on the Trojan Battery. Look for the Reserve Minutes at 50 and 75 amps. 50 amps is industry standard of what a 48 volt golf cart would use in the 150 to 200 AH battery size. 75 amps is what a 36 volt Golf Cart uses with a 200 to 250 AH battery.