Newbie looking for advice for dual battery bank solar setup

[Tarlak333]

Hello,

I'm looking to design a 24V off grid system using 24 T105 batteries in 2 banks of 12. My thinking is to let one bank run down to 50% then while that is charging run off the other bank, and switch back and forth. One charging while the other is being depleted by the inverter. (First question, is this a good way to run a off grid system?) Next is there a charge controller/inverter combo that could run the system automatically like this for me so I don't have to manually flip switches? I plan on tying this system into my house wiring via a unused 50A breaker (with main line shut off obviously) in the event of a power outage emergency. I've been looking at 1000W of the solar world panels are these good? (I want quality panels not cheep junk ones). Could you recommend good quality inverters and charge controllers. (Looking at about 2000W max load @ 120V, prob be around 2000W/Hr per day of power consumption, I live in the SW in a very high sun area & would like to be able to charge up 1 bank for 50% to 100% in 2days or less.)

Thanks,

Tarlak333

[solar pete]

Howdy Tarlakj333 and welcome to Solar Panel Talk. I am no off-grid expert but I dont think you generally have or need 2 banks running, having said that I dont know your requirements. I think you should start with reading up in he off-grid secion.

https://www.solarpaneltalk.com/forum/off-grid-solar/off-grid-solar-panel-systems

[Tarlak333]

Great , thanks for the link. I'll start reading up and be back once I've educated myself more thoroughly.

[Amy@altE]

I would discourage that. Instead, wire them in 2 parallel strings of 2 in series. This will result in a battery bank twice as big, and half the discharge of the batteries. Instead of running each bank to 50% DoD, you could run the whole bank to 25% DoD, making all of the batteries much happier and living longer.

[Tarlak333]

As you see this just shows my "current" (haha) naivety on this subject. I was under the impression that depleting the battery bank only a small amount (say 10-25%) (short cycling) and charging back up was harmful to the lifespan of the batteries. I thought it was better to run them down to ~50% and bring them back up.

One big bank would be much easier to wire and maintain.

I'm looking for a ~5 day battery reserve and with 24 T105s that would put me at 32400W/hr (16200W/Hr @ 50%) divided by house draw 2000W/Hr/Day = 8.1days X system inefficiencies of...say 30% = (8.1 x .7 = 5.67days). In addition I should have about ~20-30% battery reserve down to ~70-80% if I really had to push the battery bank.

Does this sound correct to you.

So if I go with 1 big bank of 24 T105s should I run a 24V or 48V system?

I guess what I'm getting at is where is the sweet spot for charger, inverter, wiring efficiencies vs. component cost?

Thanks for the info....I'll have alot more ?s in the days to come.

[Amy@altE]

Nice pun!
2000Wh x 2 (50% DoD) x 5 days x 1.19 temperature deration (I guessed 50F temp in battery room) / .9 inverter efficiency (another guess not knowing what inverter you are using) / 48V = 551Ah 48 volt battery bank. T105s are 6V 245Ah, so 3 parallel strings of 8 in series gives you 735Ah @ 48V. Three strings is the absolute highest limit for wiring batteries, and not recommended by many people (SunKing is screaming right now). You may want to consider a higher Ah battery instead, like the L16RE. 6V 370Ah, you could do 2 strings of 8 instead with 16 batteries.

24V is too small for this size system.

[Tarlak333]

Ok well I just did some math and apparently I messed up on my budget, I didn't want to spend over $5K for this system and 24 T105s or 16 L16REs would blow it completely. So scaling back to 16 T105s (6V) or 12 L16REs (6V) (~25920W/Hr) the LREs cost ~$4,200 vs. ~$2,800 for the T105s (~24000W/Hr). So why would the L16RE be better at 50% per W/Hr more cost than the T105s? Confused

[Amy@altE]

So why would the L16RE be better at 50% per W/Hr more cost than the T105s? Confused

Because the L16s would allow you to do 2 strings in parallel instead of 3 strings with the T105, and therefore will live longer. So cost over the life of the system will be better.

[sensij]

Your use of W/Hr is incorrect. Energy can have units of Wh, Watt-hours. Those units come from the fact that energy = power x time. If you use 1000 W for 2 hours, you will have consumed 2000 Wh of energy.

When you are comparing prices, keep in mind the total lifecycle cost, not just the upfront cost. An inexpensive battery that fails in a couple years due to heavy cycling or poorly balanced parallel strings is likely to cost more in the long run than a true RE deep cycle single series string.

The original idea of only running one string of batteries at a time is not completely without merit. If the swaps were made at 20% instead of the 50% suggested, it would be a way to avoid balancing problems with parallel strings. The catch is knowing when 20% discharge is reached. The battery voltage under load will be a function of both the battery SoC and also the draw of the load, and even the overall battery capacity will vary as a function of the discharge rate. Specific gravity is the best way to measure SoC, but I am unaware of a way to monitor it in real time to make the switching decision.

[Amy@altE]

Your use of W/Hr is incorrect. Energy can have units of Wh, Watt-hours. Those units come from the fact that energy = power x time. If you use 1000 W for 2 hours, you will have consumed 2000 Wh of energy.

Nice catch on the loads. I took the 2000Wh as fact, I missed how he came to it. Tarlak, use a loads list calculator to determine what your actual loads are throughout the day. Or get a Kill-a-Watt meter to measure them.

[Living Large]

The very first step is to figure out what your daily usage is in KWH (not W per hr), if you can eliminate any current loads, and what your peak use will be. This is critical to sizing the system to withstand 5 days of power outage. You're almost like off-grid at that point. You may also need a generator to account for sun don't shine. But do step one first, then figure out how many panels and batteries you need. Without step one, it's a waste of time.

[Mike90250]

So, first, while you are considering off grid. you know you will need to have a backup generator (in the last 15 days, I've had 2 days with "some" sun). you CAN intelligently use your backup generator for a couple (1 or 2) hours a day, in foul weather, and not need to have 5 days. You can chose to burn $30 of fuel per year (OK, maybe $200) and use a smaller battery bank. And since most folks tend to fry a bank while learning the ropes, you won't be crying so much when you r bank fails

[PNjunction]

As you see this just shows my "current" (haha) naivety on this subject. I was under the impression that depleting the battery bank only a small amount (say 10-25%) (short cycling) and charging back up was harmful to the lifespan of the batteries.

That's not really short-cycling. Less than 10% is, and that is where normal, yet irregular sulfation deposits can occur instead of a smooth coating during repetitive charge and discharge. If you aren't doing at least 10%, then the cost factor of just having too large of a bank comes into play. The most cost-effectiveness lies in between about 10-25% DOD.

[Logan005]

T-105's nor 24 volts are not for you, If you truly need that much power. consider 8 X L16's in series for 48 volts, you will be set. of course you will need a 48 volt inverter and a 48 to 12 volt step down converter, but you will have a system that has enough punch to keep you going through the night, and will last many years. I assume you do not have grid access, If you do, it is not an off grid system you need, but a hybrid system. If you are truly in need of an off grid system, do you really need AC power from an inverter? almost everything in your house actually runs from DC, major appliances being the exception and even then there are DC options.

[Tarlak333]

Thank you all for your replies.

1st off, yes this will be a 100% off grid system.

Ok got a power meter and have started getting actual readings on my appliances. It looks like worst case scenario I will need 4KWh / day in the hot southern NM summer months, mostly for refrigerator, and small swamp cooler. (Winter I'll actually need much less power since I could keep my refrigerator on my screened in porch in the cold and it would barely run, & wood heat for home. I'm being more realistic with a 3 day Autonomous reserve for the battery bank....running through the math for a ocasional 50% DoD and system efficiency losses, I get ~36KWh for the battery bank. I was looking at the J185P-AC (12V) (3 strings of 4 for a 48V system) so far I have found these online w/ shiping for $0.11/WHr ($3981.20 for 12 of them).

Also I was looking at these: SolarWorld SW320 Panels (1240W) for my power system.

What do you think?

I do want to get a backup generator but the only ones I trust are YAMAHA and the price is ~$1000 for a 2KW.....and I would get the dual fuel option and tie this into my 1000 gal propane tank.

Can you suggest a reliable dual-fuel (Gasoline & LP) generator for this size system?

Thanks