24 versus 48 volt and panel array size

[yeahgofigure]

Am in the process of ordering parts for my off grid cabin system. My installer has me going with a 24 volt setup with a Xantrex XW4024 inverter and XW MPPT charge controller with six 235 watt panels. I saw the charge controller can do 24 or 48 volts meanwhile the XW4548 inverter at 48 volts is the same price as 24 volt inverter. I asked why not do the 48 volt system as it is supposed to be more efficient however my installer said not as the charge side would be less efficient and I'd need to double the size of my solar array to 12 panels to get same charge effect.

My understanding is the wiring of panels and batteries for 48 volts would halve the number of amps but would double the volts, thus provide roughly the same wattage. So what I don't get is why would that cause the battery charging to become less efficient and cause me to have to double the # of panels? He said running at 48 volts is like doubling the size of a gas tank so you'd need twice the gas to fill it up (i.e. twice the panels). I'm confused, can anyone shed more light on this?

[Sunking]

Find a different installer because he has no idea what he is talking about.

But your assumption of

My understanding is the wiring of panels and batteries for 48 volts would halve the number of amps but would double the volts, thus provide roughly the same wattage.

is wrong. For example if you have a fixed panel wattage of 1000 watts and use a MPPT controller the maximum charge current vs battery voltage is:

12 volts @ 80 amps
24 volts @ 40 amps
48 volts @ 20 amps

OK if you had a 1000 watt panel, the minimum size battery for a C/8 charge rate where C = the battery AH capacity is

12 volt @ 640 Amp Hours
24 volt @ 320 Amp hours
48 volt @ 160 Amp Hours

All three batteries above have the exact amount of energy expressed in Watt Hours. With respect to batteries battery Watt Hour Capacity = Battery Voltage x Battery Amp Hour. So try all three and you will get 7680 Watt Hours. In addition all three example will receive the exact same charge rate of C/8, but that C/8 current is the same.

What you are missing is the relationship of Power, Voltage, and Current. Power or Watts = Voltage x Amps. For example the largest MPPT Charge Controller is 80 amps. So the maximum panel wattage vs battery voltage is thus"

1000 watts @ 12 volt
2000 watts @ 24 volt
4000 watts @ 48 volts.

To conclude operating at higher voltages give you too things
1. The higher the voltage, means less current for a fixed power. This means it is more efficient because line losses are less.
2. Allows higher power inputs for a Charge Controller rating. Example above if using a 80 amp controller you are limited to 1000 watts max at 12 volt battery. With 48 volts you can grow up to 400o watts.

So your installer comment is pure rubbish I can give you any amount of battery capacity you want at any voltage. Capacity is the size of the TANK he referred to. You want 10,000 watt hours at 12 volts you need a 833 AH battery. If you want that same 10,000 watt hours @ 48 volts is 208 Amp Hours. Both are the exact same size TANK. The difference is at 12 volts takes 4 times more current than at 48 volts, but the lower current makes 48 more efficient.

Here is a good example that will click and you can ask your installer. Let's say you need a battery capacity of 4800 Watt Hours. At 12 volts the battery is 400 Amp Hours, and at 48 volts is 400 AH. We go to the store and we find 12 volt 100 AH batteries at a good price and you buy 4 of them . Well for 12 volt system we configure all 4 batteries in parallel which makes 12 volts @ 400 Amp Hours. Or we take the exact same 4 batteries and configure them in series to make 48 volts @ 100 Amp Hours. Now you tell me which TANK is bigger?


Conclusion, fire your contractor, he is dumber than a rock.

[inetdog]

He said running at 48 volts is like doubling the size of a gas tank so you'd need twice the gas to fill it up (i.e. twice the panels). I'm confused, can anyone shed more light on this?

Well, if the 24 volt system he put together for you was something like two 12 volt 150AH batteries, he may be thinking that the 48 volt system will have to be 4 of those same batteries because he does not know of any good 75AH batteries. In that limited sense it would in fact be like a gas tank twice the size.

But if the 24 volt system was 4 12 volt batteries in series parallel and you take those same 4 batteries and put them in series, you will have twice the voltage and half the amperage, just as you think it should be. That will end up being the same battery wattage, and therefore the same panel wattage. The charge controller will also be putting out twice the voltage and half the current, so you could consider it a waste of the output capacity of the CC. But not a waste of power.

[yeahgofigure]

More details.. to start will have 24 batteries with Xantrex XW MPPT 60 amp charge controller. I believe the batteries are 6V Trojan golf cart flooded batteries, I know a lot of peeps will say that's wrong but the locals swear by them so let's please not go off topic.

My guy does all the solar installs in the local small off-grid community so I need to use him. He repeatedly said I'd need the amps to get the batteries to charge at proper rate, so if at 48 volts I wouldn't have enough amperage to charge the batteries, something to do with sulfate, and would ultimately perform poorly. After a long conversation my head was spinning and basically ended with... he'd install as 48 volt system but it simply won't work and he highly recommended 24 volt.

Not sure how to have the discussion that 48 is the way to go. I spoke of 48 volt is simply half the amps and twice to volts for same wattage, but he kept repeating it's all about the amps when charging the batteries. I'm no expert on electricity and hard to get into the details with him. Maybe there's a simple way to convince him, or maybe he's right for some oddball reason?

[inetdog]

I spoke of 48 volt is simply half the amps and twice to volts for same wattage, but he kept repeating it's all about the amps when charging the batteries. I'm no expert on electricity and hard to get into the details with him. Maybe there's a simple way to convince him, or maybe he's right for some oddball reason?

I can't think of any way to convince him, based on what you say so far.
But: 24 6 volt GC batteries to make up a 24 volt system. That is putting six strings of four batteries each in parallel. That requires careful attention to every detail of the wiring along with unlikely good luck and constant monitoring to keep the six strings balanced when charging and discharging.
Using the same batteries (I respect your wanting to avoid getting into that) as three strings of 8 batteries each would have a much better chance of staying balanced.
That arrangement would require only half the amps (since it is three strings instead of six in parallel). And, amazingly enough, with an MPPT controller, half the amps is exactly what you will get at 48 volts.

If you draw out a circuit diagram and show him that each of the 24 batteries would still be getting exactly the same current in the 48 volt configuration as it would in the 24 volt configuration, it might have some chance of getting through to him. But I will not hold my breath.

[yeahgofigure]

My understanding is at 48 volts the 24 volt panels would need to be tied together doubling volts and halving amps and the 24 batteries would be 3 strings of 8 batteries each, meanwhile at 24 volts would be 6 strings of 4 batteries each. So did his gas tank analogy mean that strings of 8 batteries have twice the amps being fed by half the amps from array which is why he meant wouldn't charge properly? I'm struggling with this as to me it's still same power/watts which to me should charge just as well, even better / more efficiently since lower amps.

2. Allows higher power inputs for a Charge Controller rating. Example above if using a 80 amp controller you are limited to 1000 watts max at 12 volt battery. With 48 volts you can grow up to 400o watts.

At current configuration 6 panels puts me below safety margin of charge controller 60 amp limit and if went to 8 panels would need to reconfigure system, which with MPPT controller isn't cheap. Does this mean that at 48 volts I'm half the amps and I could easily grow to 8 panels while staying well below the 60 amp limit of single MPPT controller? If so that's a huge advantage.

[Naptown]

My understanding is at 48 volts the 24 volt panels would need to be tied together doubling volts and halving amps and the 24 batteries would be 3 strings of 8 batteries each, meanwhile at 24 volts would be 6 strings of 4 batteries each. So did his gas tank analogy mean that strings of 8 batteries have twice the amps being fed by half the amps from array which is why he meant wouldn't charge properly? I'm struggling with this as to me it's still same power/watts which to me should charge just as well, even better / more efficiently since lower amps.



At current configuration 6 panels puts me below safety margin of charge controller 60 amp limit and if went to 8 panels would need to reconfigure system, which with MPPT controller isn't cheap. Does this mean that at 48 volts I'm half the amps and I could easily grow to 8 panels while staying well below the 60 amp limit of single MPPT controller? If so that's a huge advantage.

Charge controllers are limited and rated on their output amps. If you go to a 48V system the amps on the system will be half of what they are on a 24v system If say the array was putting out 30A at 24V it would be 15A at 48V. If you currently have 6 panels for the 24V system by going to a 48V system you could double that number of panels and still be within the charge controller limit.

[Sunking]

My understanding is at 48 volts the 24 volt panels would need to be tied together doubling volts and halving amps and the 24 batteries would be 3 strings of 8 batteries each, meanwhile at 24 volts would be 6 strings of 4 batteries each. So did his gas tank analogy mean that strings of 8 batteries have twice the amps being fed by half the amps from array which is why he meant wouldn't charge properly?

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.

[yeahgofigure]

That helps a lot! He mentioned would need a proper sized generator to provide backup plus powerful enough to also periodically burn the sulfate off the plates, guess that's why. Originally was going to save with a 120V AC inverter but the 240V accepts both 120 legs from the generator so can save on generator plus have inverter that can grow with my needs.

Ran some #s 12 Trojan L16RE-2V provides 26640 watt hours. At approx $400 each w/ shipping is $4800 (1110 amps * 2 volts = 2220/batt * 12 batteries = 26640, correct?) meanwhile 24 T-105 provides 32400 watt hours and they get them $140 each so $3600. So other than balance issues seems I'm getting more with the cheaper batteries. Everyone in community swears by the T-105s, even the guys that don't use this installer.

I've got to speak with him more about this 48V thing!

[Naptown]

That helps a lot! He mentioned would need a proper sized generator to provide backup plus powerful enough to also periodically burn the sulfate off the plates, guess that's why. Originally was going to save with a 120V AC inverter but the 240V accepts both 120 legs from the generator so can save on generator plus have inverter that can grow with my needs.

Ran some #s 12 Trojan L16RE-2V provides 26640 watt hours. At approx $400 each w/ shipping is $4800 (1110 amps * 2 volts = 2220/batt * 12 batteries = 26640, correct?) meanwhile 24 T-105 provides 32400 watt hours and they get them $140 each so $3600. So other than balance issues seems I'm getting more with the cheaper batteries. Everyone in community swears by the T-105s, even the guys that don't use this installer.

I've got to speak with him more about this 48V thing!

Cheaper initially When they go dead from imbalance in 4 years and need to be replaced vs a proper battery that will last twice as long.
Maybe your installer is looking for recurring income every few years in battery replacement.

[Mike90250]

More details.. to start will have 24 batteries .....

Run away, run away !
You are being fed a STORY concocted by an IDIOT. An idiot with electricity is more dangerous than a drunk behind the wheel of a school bus.

[Naptown]

The point you and he are missing is that at that size battery bank either at 48V or 24V you do not have enough panels to charge the batteries.
your installer said you need a generator to knock the sulphate off the plates. Sorry but not going to happen. Pretty much if the batteries sulphate from a constant defecit charge no amount of generator or charger is going to help.What he was referring to was a generator to run an equalization charge which you will have to do constantly as the charge rate on any configuration of batteries at that size with the panels you have will cause the batteries to stratify. meaning acid on the bottom and water on the top. the charge rate as recommended by the battery manufacturer is to stir this up a bit during normal charging with an occasional equalization.
No matter how you cut it you either need twice as much panel or half as much battery.

[Sunking]

Ran some #s 12 Trojan L16RE-2V provides 26640 watt hours. At approx $400 each w/ shipping is $4800 (1110 amps * 2 volts = 2220/batt * 12 batteries = 26640, correct?) meanwhile 24 T-105 provides 32400 watt hours and they get them $140 each so $3600. So other than balance issues seems I'm getting more with the cheaper batteries. Everyone in community swears by the T-105s, even the guys that don't use this installer.

What you are experiencing is followers who only know 1 answer: 1 size fits all. That is all they know.

LISTEN my friend you need to quit being a Follower. You are being given horrible advice. Naptown, Mike, and myself are professionals not trying to sell you a damn thing. Think about it. FIRE your CONTRACTOR NOW. He is an idiot. Well maybe not an Idiot, just looking to get deep in your pockets every year which makes you an IGNORANT FOLLOWER. What he is recommending is STUPID. So are you a stupid ignorant follower or a leader looking for a better way?

Your whole system is mismatched. Nothing is designed to work with each other. What moron came up with this?

With a 1410 watt panel system can only support a 24 volt battery system of 480 AH (C/8) 720 AH (C/12). The moron has suggested a 24 volt 1350 AH battery or over twice what a 1410 watt system can support. If you do that with parallel batteries you will be replacing your batteries every year or two.

Another thing if designed properly based on a panel wattage of 1410 watts withing the range of battery it can support only gives you around 2.4 Kwh of daily power or about 30-cents of electricity. Taht means based on what you have had recomendation mean with 3 year battery replacement will cost you some $3?Kwh. You can buy it for 10 cents per Kwh. Design it correctly and you can bring that cost down to $1/Kwh over a 10 year life.

Do not be a fool and follow the pack of BAD advice you have been given.

[Chewbacca]

Golf Cart Batteries

Cheaper initially When they go dead from imbalance in 4 years and need to be replaced vs a proper battery that will last twice as long.
Maybe your installer is looking for recurring income every few years in battery replacement.

T-105's are fine. I have had mine over 5 years and they are starting to show their age, but still work. I only get 75% the discharge time out of them. I bought them from costco for $80 each. I don't know of ANY battery that lasts longer than 7 years when used daily. How many years do you get out of the L16RE-2V? Costco has the interstate golf cart battery for $83 +TTL.

[lkruper]

T-105's are fine. I have had mine over 5 years and they are starting to show their age, but still work. I only get 75% the discharge time out of them. I bought them from costco for $80 each. I don't know of ANY battery that lasts longer than 7 years when used daily. How many years do you get out of the L16RE-2V? Costco has the interstate golf cart battery for $83 +TTL.

Perhaps, but are you putting six strings of four of those batteries each in parallel? In addition the OP did not have enough solar to properly keep them charged.