Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

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catamount
catamount Registered Users Posts: 12
Howdy all! I'm new here, 1st post. I have however done a lot of reading here and elsewhere. PS I did try to search here for this question and couldn't find it.

I am a former military electrician and do my own wiring projects, have some familiarity with the NEC and have been reading about PV design for the past few years. There is/was a lot for me to learn in PV design; it's just not like supplying an outbuilding with 240 volts! But the point is, I'm not a complete noob, so don't dumb it down too much for me. ;)

I live in a place where the grid is not an option. Currently I stay in a camper while I build a small cabin and I use a generator for most power. I do have a couple L16 batteries, 300 watts in 12V panels, and a chinese 30 amp PWM CC. While I am in the process of building my house I am also planning for the PV system. We cannot or will not get a loan, so everything we do is saved for and paid for up front. Which has an influence on my future PV system; for example I can't shell out 1,000s of dollars to get a bulk deal on panels.

So, because I have to spend money to build a place to live, my plan is to build slowly or by steps up to a 5kw array size. I do understand that this is not the ideal way to proceed, but it is what it is and I can't change it.

I was planning on Outback equipment because of it's reputation, the fact that the CC and inverter can talk to each other, and my neighbor has a 5kw Outback system and I thought we could help each other if we need tech support. I've recently learned here that the Outback CC causes EMI interference and I use a cell phone repeater so I've changed my mind on that. But anyway, before I learned this I was at a trade show a little while ago...

Here is the first question: I was at the trade show talking to a salesman about the Outback inverters and I mentioned I was planning to get an Outback 48 volt inverter. He (who was not an Outback representative, but an employee of the local off-grid store) suggested that I start with a 24 volt battery bank and get the Outback 24 volt inverter. He said in the future I could get another 24 volt inverter and with the two I could wire them to a 48 volt battery system. I've never heard of this anywhere but I'm new to the equipment side of things. I knew you could stack them and get a 240 volt output, but I am skeptical about this ability to stack to a different battery bank voltage. So is it true that you can stack inverters to input higher voltages?

Second question: How important is it that the inverter and charge controller can communicate with each other?

Third question: I know it is not ideal to incrementally upgrade from a small system to a 5kw array and 48 volt bank, especially in terms of mixing old and new batteries, but does anyone have any advice on starting small and adding on without wasting money on things that will become obsolete or redundant in my system?

Thanks for any help.

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Welcome to the forum.

    Directly, no you can not wire the DC side of two 24 Volt inverters in series to get a 48 Volt system. You can't even use the mid point of a 48 Volt battery bank to be the mid point of two inverters in such a configuration. The reason is the negative sides inevitably will be connected together somewhere, and that's called a dead short across half the battery bank.

    I think this fellow was confused by the fact you can parallel or series stack the output of two FX inverters to provide either double the output current or double the output Voltage. The same does not hold true for the DC input side.

    I do not believe the FM series charge controllers will cause any problem in the cell phone frequencies. However, the MidNite Classic has the best FCC rating of any controller at this point.

    It is not necessary to have the controller and inverter communicate. In fact many systems do not have this because so many inverters and controllers utterly lack the ability. I have OB both, but don't have them connected together at all. The only down side I experience is discrepancy in Voltage when charging with the inverter because it doesn't share the RTS info of the controller. Since it is only necessary to Bulk this way it isn't a worry. If you have lots of equipment (multiple controllers and inverters) communication between them becomes more advantageous.

    As for starting small and building up, design the system both ways and see what components can be shared. A good MPPT controller, for example, will work on 12, 24, or 48 Volts. Not so with the inverter. Also be aware that the panels you buy today may not be available tomorrow and finding exact matches may prove a challenge.

    One of the ways you can start small and build up is to resign yourself to having two separate systems: a small 'for now' system to run a few lights that will be retained in that capacity when you can afford to install a large 'runs the big stuff' system later.

    Unfortunately systems do not lend themselves to being scaled up easily, and most of this has to do with the need to increase system Voltage and power demands go up and the whole business with trying to keep the array all of the same panels.
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    So, because I have to spend money to build a place to live, my plan is to build slowly or by steps up to a 5kw array size. I do understand that this is not the ideal way to proceed, but it is what it is and I can't change it.

    That's not too much of a problem but be aware that at some point you will have to replace (not add on to) significant parts of your system to continue to expand. Fortunately the panels are 100% reusable. Inverters are generally not. Good charge controllers are pretty flexible; cheap ones are not reusable. Batteries SOMETIMES are, but since they are limited life you're going to be replacing them regularly anyway.
    Here is the first question: I was at the trade show talking to a salesman about the Outback inverters and I mentioned I was planning to get an Outback 48 volt inverter. He (who was not an Outback representative, but an employee of the local off-grid store) suggested that I start with a 24 volt battery bank and get the Outback 24 volt inverter. He said in the future I could get another 24 volt inverter and with the two I could wire them to a 48 volt battery system. I've never heard of this anywhere but I'm new to the equipment side of things. I knew you could stack them and get a 240 volt output, but I am skeptical about this ability to stack to a different battery bank voltage. So is it true that you can stack inverters to input higher voltages?

    You can with a very severe caveat - you can then not common their outputs. In other words, you cannot connect their outputs or their controls (including via MATE) but can use each output independently. It is very rare that this is OK. (For example, you'd need two distribution panels, two sets of distribution wiring etc and one inverter could not be grounded, which would be a serious safety issue.)

    The commonly used term for this is "positive grounded system." In that case you could ground the center of the battery pack. The "top" inverter would be standard negative ground and the "bottom" inverter would be positive ground. Outback's installation manual says specifically that you can't do this, but it also says to contact them for more info. Might be worth a phone call.

    If you can find an inverter that accepts both positive and negative grounding, then your scheme might work and allow common use of the outputs.
    Second question: How important is it that the inverter and charge controller can communicate with each other?

    Not that important. It is rare that one cares what the other is doing. There is a small value in coordinating charging, so for example they both don't try to equalize (thus resulting in twice the equalizations you would have otherwise.)
    Third question: I know it is not ideal to incrementally upgrade from a small system to a 5kw array and 48 volt bank, especially in terms of mixing old and new batteries, but does anyone have any advice on starting small and adding on without wasting money on things that will become obsolete or redundant in my system?

    If you can afford it, choose charge controllers/battery monitors/switchgear that will work at 12/24/48. Panels can be rewired.
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Thanks that all helps a lot.
    I do not believe the FM series charge controllers will cause any problem in the cell phone frequencies. However, the MidNite Classic has the best FCC rating of any controller at this point.

    Hmm, I wonder why the store here lists only the Morningstar and the Conext XW MPPT 60 Amp Solar Charge Controller as having the FCC ratings? Just need to update the web page I guess?

    The Midnite Classic seems to get good reviews around here but I had not considered it recently because I couldn't find out about the FCC rating so thanks for that info (I just found it in the manual myself). A charge controller will be my next big purchase. Knowing the little that you do about my plans, perhaps I should go with the Classic? The on board web server is an attraction.

    And it IS sexy looking :D
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    Not that important. It is rare that one cares what the other is doing. There is a small value in coordinating charging, so for example they both don't try to equalize (thus resulting in twice the equalizations you would have otherwise.)

    Thanks for the help. How do we control this? Is it important enough to worry about?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    Thanks for the help. How do we control this? Is it important enough to worry about?

    Turn off automatic equalization on both charge sources. End of problem.
    EQ should be done manually anyway, as far as I'm concerned.

    Our host's web site has been going through some bad times recently. They tried to update it but there were so many things that needed correcting they had to revert to the old version for now. For the latest info on any product contacting the manufacturer will be best.
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    I guess I have another important question as well. The costliest one-time purchased part of my system will be the inverter. I don't want to get a 24 volt inverter if it turns out I need a 48 volt inverter, but the higher voltage I go, the more upfront money I need because of the amount of batteries I will need.

    With a 5kw panel array, and a 4000 watt inverter, do I need to go 48 volts or can I keep it at 24? In the future I may stack another inverter if it makes any difference.

    I know I'm going about this wrong and I should size my battery bank then go from there. But I want to get this going as soon as possible, and due to cost I would like to keep the bank at 24 volts. Four L16s would give me 24 volts and 380 AH. I know that's not a lot of AH but I feel that this is where I am going whether I like it or not. My electrical usage may very well be determined by the size of my battery bank, but I can live with that. When I do upgrade the battery bank, I can make it much better, but at that point will I need to be at 48 volts, rendering my inverter useless?

    For what it's worth if I do run into a situation where I have equipment that's become obsolete, I can put it at the well pump and get that baby on solar (1HP, 10A 240V Grundfos). So it won't be a total loss. But in the near future I would like to decrease the generator run time for every day living.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    I've recently learned here that the Outback CC causes EMI interference and I use a cell phone repeater so I've changed my mind on that.

    I have an Outback system and a cell phone repeater... no problem.

    All switching electronics (inverters and controllers) make some electrical noise. Just as bad, in my opinion, is the audible noise they make. Search the forum and you'll find lots of complaints about audible noise.

    The solution is to build a power shed (I use my detached garage) for the batteries and electronics. That removes the electrical and audible noise and the battery fumes from your home. It may make your house a safer place in the event of a fire.

    btw, will your panels be mounted on your house?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Okay so when picking the system there's things that need to be thought about
    .
    One is how many Watt hours stored capacity you need. More Watt hours = higher Voltage because it's better to supply the power with Voltage than with current. A bit about system Voltages: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    Do you need 240 VAC output? If so, for what? Because there's more than one way to provide it. Outback's FX inverters are 120 VAC and need either stacking or an autotransformer on the output. You want to run a 1 HP 240 VAC pump, which can demand quite a bit of power depending on the design.

    Which brings us to the total Watts needed at any given time. 4kW? Or 2kW? Or 6kW? Another important number when it comes to picking an inverter.

    If you want you could buy an inexpensive 24 Volt inverter such as the Samlex now, and see how that suits your needs rather than lay out $2,000 and have to do it again if you need to go up to 48 Volts. A 1kW 24 Volt Samlex is about $400: http://www.solar-electric.com/sa1wa24vosiw.html
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    Thanks for the help. How do we control this? Is it important enough to worry about?

    Not too important. As CC mentioned it can easily be done manually when it needs to be done. That way you can plan to EQ from your charge controller on sunny days when you are close to full charge, or from your inverter during generator run time when you are not.
    With a 5kw panel array, and a 4000 watt inverter, do I need to go 48 volts or can I keep it at 24? In the future I may stack another inverter if it makes any difference.

    You can go either way with that. I used an SW4024 (4000 watt 24 volt inverter) for years. The Outback VFX3648 is another good option, which gives you 3600 watts at 48 volts. At 24 volts the difference is tiny - there is a VFX3524, only 100 watts difference.

    If you plan to expand significantly in the future I'd consider 48 volts instead of 24 volts. If not choose it based on your battery configuration so you don't have a lot of parallel strings.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    .....I don't want to get a 24 volt inverter if it turns out I need a 48 volt inverter, but the higher voltage I go, the more upfront money I need because of the amount of batteries I will need.

    With a 5kw panel array, and a 4000 watt inverter, do I need to go 48 volts or can I keep it at 24? In the future I may stack another inverter if it makes any difference......

    NO. no battery penalty if you start with 48V You just buy 2x as many smaller batteries. 900# of lead, is the same wattage
    24v @ 800ah is the same wattage as 48V @ 400ah.

    with a 4KW inverter, you want to have at least 100ah of battery for every 1Kw you expect to pull. If you need 3Kw to start a pump, you should have 300ah. (usually)
    BUT - with a 5KW array @ 60V charging, you will have 80 amps and your batteries have a max charge rate spec, that you have to stay within, or you can cook them with over charge amps.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Thanks for all the input folks.
    mike95490 wrote: »
    NO. no battery penalty if you start with 48V You just buy 2x as many smaller batteries. 900# of lead, is the same wattage
    24v @ 800ah is the same wattage as 48V @ 400ah.

    Yup that makes sense.

    I don't foresee a need for 240VAC. If I want to run my welder, etc, I don't plan on doing it from the PV system.

    FYI, if/when I put the well pump on solar it will be an independent system due to the distance from the house (500-600 ft). That is why I said if I do have left over equipment it can go to that setup.

    But since we are talking about the well pump, what will be the most cost effective way to run it?

    I pump to a storage tank and if need be can only do this on sunny days.

    I REALLY wanted to get the Grundfos SQF pump, but it was cost prohibitive. So I have the Grundfos SQ (I think) 1HP 240VAC 10 amp soft start pump. It is 520' deep and I am not changing it out unless it fails.

    I'm thinking something like 3kw in panels directly powering (2) inverters to get the 240 volts. No batteries. Pump when the sun is out once a day for an hour, or once a week for 3 hours, something like that.

    Is that possible? What inverters would have the best price/performance ratio?

    I'm sure it has been discussed before, but I'm still new at getting around the site.

    Thanks guys.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    To just run a standard deep well pump: generator.
    I know that sounds odd, but to build up a solar power system that can run a 1HP pump and then have it sit there doing nothing most of the time is a waste of money.
    My shallow well pump and 1HP digester pump both run from my tiny system by virtue of being activated only midday when full power is available and not running for very long (well pump 6 minutes for a day's worth of water, digester about 30 seconds).
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Can someone kindly point me to the formula used to calculate battery bank size, and then the formula to calculate the solar array size needed to charge it? I have looked for it here but didn't find it. I'm trying to reverse-engineer some calculator web sites and don't feel comfortable doing it that way.

    I understand there are many inputs like daily WH load, hours of peak sun, reserve days, etc. I also know there are inefficiency factors to add to the calculation.

    I'm trying to get a complete understanding of this myself, rather than be spoon fed, if you know what I mean.

    For an example, lets say my daily WH usage is 5630.

    Thanks for the help.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    Can someone kindly point me to the formula used to calculate battery bank size, and then the formula to calculate the solar array size needed to charge it? I have looked for it here but didn't find it. I'm trying to reverse-engineer some calculator web sites and don't feel comfortable doing it that way.

    I understand there are many inputs like daily WH load, hours of peak sun, reserve days, etc. I also know there are inefficiency factors to add to the calculation.

    I'm trying to get a complete understanding of this myself, rather than be spoon fed, if you know what I mean.

    Thanks for the help.

    Sure. Here's the basic way of doing it:

    Battery bank is based on average Watt hours used daily and Depth Of Discharge. The latter is usually 25% as that works best with most systems.

    So you have AC Watt hours / inverter efficiency = DC Watt hours + inverter consumption (and any other DC loads) = total DC Watt hours. DC Watt hours / nominal system Voltage = Amp hours used. Multiply by 4 for 25% DOD = minimum battery bank size (20 hour rating). Round up to nearest available capacity.

    That capacity * 0.10 = peak charge current desired. (Under some circumstances you may go lower, under others you may want to go higher.) Multiply peak current * nominal system Voltage to get usable Watts. Divide by derating factor of panels & controller (usually 0.77) to get array size for an MPPT controller. (You can also multiply the desired peak current by the 'ideal' Vmp for the system Voltage to get array size on a PWM controller.)

    You can check that array size against your AC Watt hours by this formula: array Watts * hours of equivalent good sun * over-all derating factor (usually 0.52). It should be slightly higher.

    Remember that the power output is for sunny days. If you have a lot of not-so-sunny days you'll want to increase the peak charge current. You should not go over 15% for flooded cells (always check with manufacturer on this). AGM's can take more current in and out.

    The 25% DOD factor gives you two days average use before you hit the maximum (for most batteries) 50% State Of Charge. On day three you start the generator. Building a system with more capacity than that can be done but it usually isn't economical (large capacity doing nothing much of the time).

    Simple, isn't it?
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    Simple, isn't it?

    Well it took me a few minutes to read through and make sure I understood it. But actually it is simple, or ya did a very good job of explaining it. :D You could write that up in a sticky and I think it would be helpful.

    And right off I learned enough to ask another question...
    Remember that the power output is for sunny days. If you have a lot of not-so-sunny days you'll want to increase the peak charge current....

    So if I had a 380AH battery bank, my charge current should be 38A. But on cloudy days I might want to increase that to 43A. Why?

    Is this something that can be set up automatically in a good controller like the Classic?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...
    catamount wrote: »
    Well it took me a few minutes to read through and make sure I understood it. But actually it is simple, or ya did a very good job of explaining it. :D You could write that up in a sticky and I think it would be helpful.

    And right off I learned enough to ask another question...



    So if I had a 380AH battery bank, my charge current should be 38A. But on cloudy days I might want to increase that to 43A. Why?

    Is this something that can be set up automatically in a good controller like the Classic?

    Ah, not quite. You'd still want the 38 Amps on cloudy days the trouble is you wouldn't get it from the same array that can produce it on sunny days. As such if you have a lot of cloudy days you plan the array to be larger than it needs to be on sunny days in hopes that it won't be too small on the cloudy ones.

    You can "over panel" most of the high end MPPT controllers and then set a current limit that's lower than the max. Example 38 Amps @ 24 Volts would normally be a 1200 Watt array. If you make it a 2400 Watt array that would be capable of 76 Amps on a sunny day, but you set the output limit to 38 Amps and then it will not go over that when the sun shines and hopefully it will still put out that current when the clouds take over.

    But of course PV output falls drastically under less than ideal sun conditions so you can't ever say for sure that you will have that much power on a cloudy day. How much cloud? It may reduce insolation 10%, or 90%. So you can up the array to what is a practical limit and accept that it won't be good enough every day but may just give you a little edge on those marginal days.
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Was hoping you could check my numbers. I thought I would need a lot more panel watts than what I came up with. Did I go wrong somewhere?

    Bold is my numbers.

    AC Watt hours 5630 / inverter efficiency 93% = DC Watt hours 6054 + inverter consumption 23 = total DC Watt hours 6077.

    DC Watt hours 6077 / nominal system Voltage 48 = Amp hours used 126. Multiply by 4 for 25% DOD = minimum battery bank size 504.

    Round up to nearest available capacity. I'll say my batteries are 380AH so my capacity will be 760AH with 16 batteries.

    That capacity 760 * 0.10 = peak charge current desired 76A. (Under some circumstances you may go lower, under others you may want to go higher.)

    Multiply peak current 76A * nominal system Voltage (panels are 18.9V) to get usable Watts = 1436 watts. Divide by derating factor of panels & controller (usually 0.77) to get array size for an MPPT controller = 1865W. (You can also multiply the desired peak current by the 'ideal' Vmp for the system Voltage to get array size on a PWM controller.)

    You can check that array size against your AC Watt hours by this formula: array 1865W Watts * 4.2 hours of equivalent good sun * over-all derating factor (usually 0.52) = 4073W . It should be slightly higher.

    My sanity check (4073) comes in significantly higher than AC Watt hours if I use 4.2 hours of sun, but is only a little bit higher if I use the ESH of 2 hours.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    How did you get 23 Watt hours for inverter consumption? I think that may be the Watts it uses, not the Watt hours it will consume over the time it is on.

    Nominal system Voltage is not Vmp of the panels, it is the nominal Voltage of the battery bank: 12, 24, or 48. In this case 48. So you multiply 76 Amps * 48 Volts and get 3648 Watts, not 1436. Divide by 0.77 = 4378 Watt array. That's where you went wrong.

    That's quite a leap from 504 Amp hours needed to a 760 Amp hour battery bank. They make 6 Volt's in smaller capacity. You could tailor that down to around 520 with these: http://www.solar-electric.com/cr260am6vode.html

    That would shrink the array size considerably: 52 Amps * 48 Volts / 0.77 = 3242 Watts.

    3242 Watts * 4 hours * 0.52 = 6.7 kW hours AC, well above the usage estimate of 5.6 kW hours.
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Yup I screwed up the inverter consumption, should have been 23W*24H.

    Your corrected array wattage is a lot closer to what I was expecting.

    I made a leap with the battery choice because I used the type that I had on hand for this exercise. But you're of course correct that it probably wouldn't be the best way to go.

    Thanks for the schoolin'....
  • catamount
    catamount Registered Users Posts: 12
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Ok, new question.

    I'm building a microcontroller-based battery monitor, using simple voltage dividers and hall effect current sensors. I haven't got the current sensors yet, but my question is more theoretical...

    How does an advanced battery monitor know the state of charge at any given time? Not whether the battery is charging or discharging, but rather how many AH are left?

    All the fancy battery monitors seem to advertise that feature.

    The only thing I can think is that the battery monitor senses when the bank goes on float, and that combined with user entered AH capacity will let the monitor know that if capacity is (ex) 380AH, and it knows that 40AH have been discharged from it since the bank went on float, you could say the bank is discharged 40/380= 10%.

    Is that the only way, or am I missing something obvious?

    Thanks for any replies.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    You can learn a lot from the Smart Gauge guys:

    http://www.smartgauge.co.uk/technical1.html

    Their monitor works by monitoring voltage levels only (no current flow measurements).

    Your Hall Effect transducers... Will they give you enough accuracy (usually DC accuracy is not so good over time) or should you use a precision current shunt?



    wind-sun_2269_13629585Deltec 100 amp, 100 millivolt current shunt


    trans_1x1.gif
    wind-sun_2269_13640931Deltec 500 amp, 50 millivolt current shunt




    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Can You Stack Inverters to Change Battery Input Voltage? And Other Questions...

    Battery monitors are programmed with the battery capacity and efficiency. Then they count the Amps hours in and Amp hours out and apply that data to the provided information and calculate state of charge. It is not 100% accurate, and as time goes by and the real battery capacity diminishes it becomes less so.