24 versus 48 volt and panel array size

[zed]

...

[gmanInPA]

Apparently the US Military also has no true educators either - especially those "maroon" drill sergeants . I get it... some people here like Sunking comment rather aggressively - perhaps even insulting at times - but they're also sparing the OP some significant headache and financial loss in doing so - and for zero cost to the OP other than a tongue lashing now and again. It would be great if some here sent flowers and chocolate and inserted rainbows and unicorn emoticons into their posts, but bear in mind, it is free professional advice that people such as the OP voluntarily requested. Beggars can't be choosers. Which is better, getting some minor insults and a wake up call, or spending $30k in error only to find out later, you made some big mistakes?

[SunEagle]

Quit pretending your are an educated individual with adding fake credentials like MSEE and PE. A true educator would never insult someone by calling them a maroon or an idiot as you have. You may know what you are talking about when it come to solar, but then again it doesn't take a rocket scientist to figure this out, only someone with a least a 5th grade education, which I assume you must have as you seem to know how to add and divide. But I have got to tell you that your language is offensive and insulting to me and I have to believe most others in this blog. I'm guessing you do have at least a 5th grade education as you recognize the fact to figure out the difference between 12, 24 or 48 volt systems you only need a 5th grade education.

While Sunking may not have the workings of being Politically Correct he does have a vast amount of knowledge when it comes to batteries and electrical codes.

Try not to get offended by his "approach" and maybe you will learn something.

[sensij]

Quit pretending ...

Oh boy. If you've only just now caught up to things Sunking said in 2013, you'll be in for quite a bumpy ride as you read through the last three years!

[Ripsaw]

OK lets see if I can straighten you out.

It does not matter if the 24 batteries are configured as 24 or 48 volt. Either way each battery will have the exact same amount of charge current. As you stated if configured as 48 volts requires you to make 3 parallel string with 8 batteries in each string. This means the current from the charge controller is divided evenly by 3. If configured as 24 volts you will have 6 parallel strings with 4 batteries in each string which means the current from the controller is divided by 6. Are you with me so far?

OK it is very simple 5th grade math from here. You have a panel wattage of 1410 watts right? (6 panels x 235 watts) If the charge controller is set up for 48 volts, the output current is 1410 watts / 48 bolts = 29.375 amps. OK you have 3 strings with a 48 volt battery setup, so that 29.375 amps is divided by 3 so each string has 29.375 amps / 3 strings = 9.79 amps/sting. That means every battery configured at 48 volts has 9.79 amps charge current. Understand?

OK if configured at 24 volts the current out from the charge controller is 1410 watts / 24 volts = 58.75 amps. With 6 strings of batteries means the current is divided by 6 right. So we have 58.75 amps / 6 strings = 9.79 amps /string.

Now you tell me which number is greater than the other: 9.79 amps/string in the 48 volt configuration, or or 9.79 amps/string in the 24 volt configuration?

Here is the real scary part. Your installer is a moron and only knows enough to be very dangerous. He is correct that too low of a charge current can cause sulfation in flooded lead acid batteries. The minimum charge current to prevent sulfation is C/12 where C = the battery amp hour capacity. Your batteries are 225 AH batteries correct? So C/12 for your batteries is 225 AH / 12 hours = 18.75 amps minimum charge current required. Guess what, no matter if you configure them for 24 or 48 volts you only have 9.79 amps of a C/23 charge current. What this means is you will destroy your batteries and neither your installer or you have a clue why. There is two ways to fix that problem. Either double your panel wattage and configure it for 48 volts, or only use 12 batteries and configured for 24 volts. Which one do you choose.

Last warning. You need to understand fully what I just told you, or else you are going to make a huge mistake. BTW, fire your contractor, he is a moron. Anyone who told you what he did and to use either 3 or 6 parallel battery strings is an idiot. If it were using 1410 watts of panels you would be using a single sting of 12 Trojan L16RE2V batteries @ 24 volts, which is the same capacity you are looking at now, just less expensive and will last longer than your T-105 batteries. But hey it is only money right, your money. Use the batteries your contractor wants will last a year or two at best. My way 5 or 6 years.

Quit pretending your are an educated individual with adding fake credentials like MSEE and PE. A true educator would never insult someone by calling them a maroon or an idiot as you have. You may know what you are talking about when it come to solar, but then again it doesn't take a rocket scientist to figure this out, only someone with a least a 5th grade education, which I assume you must have as you seem to know how to add and divide. But I have got to tell you that your language is offensive and insulting to me and I have to believe most others in this blog. I'm guessing you do have at least a 5th grade education as you recognize the fact to figure out the difference between 12, 24 or 48 volt systems you only need a 5th grade education.

[thastinger]

I recently did a comparison of batteries on this forum compared to Costco and at $83.99 for a 6v 207AH battery that is said by Interstate to get 1220 cycles at 50% DOD, I came to the conclusion that they are the lowest cost in cents / kWh. That being said, their 1220 cycles may be a bit optimistic.

I went with Trojan T-605s, far less expensive than the 105s and only down 15AH in capacity to the 105s, for my fist battery bank figuring I would murder them. That hasn't happened and I can still get them up to 1.275SG on an EQ cycle about 2.5 years later now. I really only use about 60% of the power I designed the system for so the batteries have had a pretty easy life to date.
My setup is in a remote garage on a vacation property which we go to a couple time a month, my situation is unusual, but it would have cost me 35 a month to be hooked up to the grid whether I used a single Kwh or not. The batteries have now paid for themselves, if they will make it 7 years they will have paid for the whole system...which is probably doubtful.

[lkruper]

I paid lets say $100ea for 18 batteries $1800 + $3600 for solar panels + wire was cheap or free so $100 + inverter(s) $500 + $1000 + $500 + charge controller $100 = $7600 self installed. Note to self: "yes you are stupid enough some mornings to plug in grid power before turning off the circuit breaker" - hence the 3 inverters. All of you guys math is crazy because it does rain here and some days the solar just doesn't put out. My practical experience would say that I have saved an average $40/mo for 5 years over using grid power to do the same thing. $7600-$2400=$5200 The good news is the batteries are paid for. The bad is the solar system hasn't paid for itself and I will need new batteries soon.

When I was trying to analyze this for myself, I prorated solar panels over 25 years and electronics for 10 years and calculated the batteries based on 1220 cycles at 50% DOD (which for me is optimistic). I got about 11 cents / kWH for the batteries and about 1 cent extra each for the charger and inverter. Also, I was calculating the cost of electricity for charging batteries to be 1 cent because of the baseline allotment for Time of Use (TOU). I was barely breaking even at that. But it was encouraging, because technically I can break even on a back-up system that I want anyway. But then I was also looking at a tiny system compared to you.

[SunEagle]

I paid lets say $100ea for 18 batteries $1800 + $3600 for solar panels + wire was cheap or free so $100 + inverter(s) $500 + $1000 + $500 + charge controller $100 = $7600 self installed. Note to self: "yes you are stupid enough some mornings to plug in grid power before turning off the circuit breaker" - hence the 3 inverters. All of you guys math is crazy because it does rain here and some days the solar just doesn't put out. My practical experience would say that I have saved an average $40/mo for 5 years over using grid power to do the same thing. $7600-$2400=$5200 The good news is the batteries are paid for. The bad is the solar system hasn't paid for itself and I will need new batteries soon.

Unfortunately it will be a long time for that system to pay for itself due to the "short life" and "high cost" of replacement batteries.

I wish there was a lower cost energy storage system but until that time happens a solar / battery system will have a negative savings path.

[Chewbacca]

Cost savings

If we ignore your two 12v batteries, I count 16 6v batteries. At Costco's price that would be $1344 or $1584 with core. At $50 savings per month, that would be 27 months to break even just on the cost of the batteries, if core is not considered. So, if you have gotten 3+ years out of them so far at 100% DOD you have done pretty well. Have you done any calculations as to your total cost in terms of cents / kWh for your solar system?

I paid lets say $100ea for 18 batteries $1800 + $3600 for solar panels + wire was cheap or free so $100 + inverter(s) $500 + $1000 + $500 + charge controller $100 = $7600 self installed. Note to self: "yes you are stupid enough some mornings to plug in grid power before turning off the circuit breaker" - hence the 3 inverters. All of you guys math is crazy because it does rain here and some days the solar just doesn't put out. My practical experience would say that I have saved an average $40/mo for 5 years over using grid power to do the same thing. $7600-$2400=$5200 The good news is the batteries are paid for. The bad is the solar system hasn't paid for itself and I will need new batteries soon.

[SunEagle]

I have wondered myself how to factor in the direct power to calculate cents / kWh. But consider this:

$1344 for 36 months
16 x 6 X 207AH x .50 DOD = 9936w = 9.9kW / day
9.9kW * 30 = 298kW / mo
298 * 12 = 3577 kW / year
3577 * 3 = 10731 kW / life
$1344 / 10731 = 12.5 cents / kWh

My calculation of the 1220 cycles to 50% DOD comes out to about 11 cents / kWH.

So, it really comes down to what the utility charges. The OP has grid power and is not producing that much of his total power needs. So if he is in the higher tiers, the reduction could be much more than 11-12 cents / kWH.

But he also said he is draining the batteries down to what appears to be 100%. If that is calculated correctly then he may have sacrificed his batteries, but gotten the cent/ kWH down to 6 cents / kWH and they still have lasted 3 years.

Your math is correct and based on it the cost of 11 cents / kWh is pretty good. But as you indicated, it is still an assumption that you will get 50% DOD for 1220 cycles for each of those batteries. It may be possible for a couple of batteries to last that long but with all of those parallel charging/discharging paths I would think one or more of those batteries will die before getting close to 1220 cycles at 50% DOD.

I would agree that if it was just 2 of those 6v batteries, they could generate 50% DOD in kWh over 1220 cycles which at $85/each would come out to just over $0.11/kWh. But to expand that to 16 would IMO be a stretch.

[lkruper]

I suspect that some of that $50 savings comes from energy generated directly from the panels during the mid day and not just the batteries. I seriously doubt the batteries (even at the low Costco price) will pay for themselves in the 3+ years owned.

Also since the battery system kept being bolstered by new ones over a period of months it might be hard to get a true cost/kWh on any of them.

I have wondered myself how to factor in the direct power to calculate cents / kWh. But consider this:

$1344 for 36 months
16 x 6 X 207AH x .50 DOD = 9936w = 9.9kW / day
9.9kW * 30 = 298kW / mo
298 * 12 = 3577 kW / year
3577 * 3 = 10731 kW / life
$1344 / 10731 = 12.5 cents / kWh

My calculation of the 1220 cycles to 50% DOD comes out to about 11 cents / kWH.

So, it really comes down to what the utility charges. The OP has grid power and is not producing that much of his total power needs. So if he is in the higher tiers, the reduction could be much more than 11-12 cents / kWH.

But he also said he is draining the batteries down to what appears to be 100%. If that is calculated correctly then he may have sacrificed his batteries, but gotten the cent/ kWH down to 6 cents / kWH and they still have lasted 3 years.

[SunEagle]

If we ignore your two 12v batteries, I count 16 6v batteries. At Costco's price that would be $1344 or $1584 with core. At $50 savings per month, that would be 27 months to break even just on the cost of the batteries, if core is not considered. So, if you have gotten 3+ years out of them so far at 100% DOD you have done pretty well. Have you done any calculations as to your total cost in terms of cents / kWh for your solar system?

I suspect that some of that $50 savings comes from energy generated directly from the panels during the mid day and not just the batteries. I seriously doubt the batteries (even at the low Costco price) will pay for themselves in the 3+ years owned.

Also since the battery system kept being bolstered by new ones over a period of months it might be hard to get a true cost/kWh on any of them.

[lkruper]

I agree that some of my batteries are 80% or less now, but I am not ready to cough up $1,488 for 9 extra hours of battery power. Solar is still saving me about $50 a month in air-conditioning grid charges. And yes mixing batteries is a bad thing, but they were cheap and big! I took the risk. The point is why buy the expensive trojan 2 volt battery? If they last 10 years they still aren't cost-effective over a 5 year Costco one. Batteries are a stupid way to store power anyways cost wise. It takes a huge number of them to power a typical house and then 5 or 7 years later you start over, and have to buy new ones. Not to mention the frickin maintenance! Now that this experiment has taught me, my future (dream) setup will use water pumped to a water tower or pond via solar panels to store energy and use a hydro generator to keep a relatively small battery bank at a constant voltage state. I haven't finished doing the math on that one though. How many gallons at what height produces... So, anyways if all I can power with virtually the same setup as the OP is a small ac unit. How is he going to power an entire house?

If we ignore your two 12v batteries, I count 16 6v batteries. At Costco's price that would be $1344 or $1584 with core. At $50 savings per month, that would be 27 months to break even just on the cost of the batteries, if core is not considered. So, if you have gotten 3+ years out of them so far at 100% DOD you have done pretty well. Have you done any calculations as to your total cost in terms of cents / kWh for your solar system?

[lkruper]

I was hoping someone on this forum had experience with the Trojan 2v battery they could impart.

Not me, you have more real experience than I do

[inetdog]

I was hoping someone on this forum had experience with the Trojan 2v battery they could impart.

The Trojan 2V is a pretty specialized battery, so there are not going to be too many forum members who have used it or even evaluated it in depth.
Your best bet would probably be Sunking (Dereck) if he is around at the moment.

Some large 2V batteries are actually several independent cells, with their own electrolyte reservoirs (check SG and level through each of the caps), that are paralleled by internal interconnecting bars rather than wired in series.
Except for being identical at the start and being thermally linked and electrically connected with very low series resistance, I would expect those batteries would be subject to some of the disadvantages of paralleled batteries.