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Author Topic: Battery Bank Sizing  (Read 4689 times)

butchiiii

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Battery Bank Sizing
« on: April 28, 2018, 10:00:21 PM »
Hello, we are looking into solar for our next upgrade. Question what size of battery bank would
I need if we have approx 2000 watts of panels? Not sure what the rule of thumb is. I'm currently
trying to figure out how much power we use over the course of a day. Thanks for the input.
Butch & Joanna
Ram 2015 Reg Cab 1500
2015 Keystone Passport 2890RL

solarman

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Re: Battery Bank Sizing
« Reply #1 on: April 28, 2018, 11:12:42 PM »
Hello, we are looking into solar for our next upgrade. Question what size of battery bank would
I need if we have approx 2000 watts of panels? Not sure what the rule of thumb is. I'm currently
trying to figure out how much power we use over the course of a day. Thanks for the input.



2000 Watts on an RV.. that's going to take a lot of room...!

anyway, before you even think you need 2000 watts, first get a good estimate of your daily power needs.

I suggest you setup a spreadsheet and list all the devices you intend to power and for how long.

daily use ( over 24 hours ) is measured in watt hours,  so if you need to say power 5 lights for 6 hrs and they are rated at 5 W
each then the total is 5*6*5 = 150 Whr

a gas fridge is typically 18 W and could be 75% duty cycle so 18 W * 24hrs * 0.75 = 324 Whr
a fantastic fan on #2 setting uses about 24 W.. so if you run it overnight say 10 hrs then 24*10 = 240 Whr

do this for each appliance and total it up. once you have that then I can help you with panel sizing and battery selection.

rules of thumb are just guesswork and give you a compromised system that will be too small or too large and cost more
than a correctly designed system.

I do this for a living, so I know.  :)



KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

Isaac-1

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Re: Battery Bank Sizing
« Reply #2 on: April 29, 2018, 01:30:39 AM »
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.
2002 Safari Trek 2830

Kevin Means

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Re: Battery Bank Sizing
« Reply #3 on: April 29, 2018, 02:14:11 AM »
Your plan to know how much power you typically consume in a 24 hour period is a good one. However, I doubt 2000 watts of panels would physically fit on a 33 foot RV's roof, unless they were installed above the AC units on some type of rack. Trying to wedge that many panels in between AC unit(s), antennas, roof vents etc. would likely result in shading, which would render a panel, and any others connected in series, useless. Do you have a real need for that much power, or are you just "going big?"

If you scour the web, you'll find some who suggest 100 watts of solar for every 100 amp hours of battery capacity. After nearly two decades of relying heavily on solar, I don't find that recommendation to be very realistic. It's just not enough solar to fully recharge lead/acid batteries that get used like most RVers use them. It may look doable on paper, because the output of solar panels is measured in lab tests under ideal conditions, but in my experience, it's not enough solar.

I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do. Do they want to replenish some, or all of the power they use? Clearly, your goal is to replenish all of the power you use. That was our goal too, and we achieve it with a lot less than 2000 watts of solar, even though our coach is one of the more power hungry RVs.

As a frame of reference, we've got a 960 watt array and eight Group 31 AGM house batteries, with a total capacity of 840 amp hours, and we almost never have to run our generator. A lead/acid battery bank with 2000 amp hours of capacity (to match a 2000 watt array) would require 19 similar batteries. At nearly $300.00 a piece, that's gonna be a pricey (and heavy) battery bank. Do you even have enough room in your RV for that many batteries?

Of course you don't "need" to have a battery bank with enough capacity to store all the power your panels generate, but it's kind of an expensive waste to buy solar panels and then not use the power they produce. Having said that, there's something to be said for having extra solar power - for a stretch of cloudy days, but a 2000 watt array would be serious overkill for most RVs... IMO. If you installed a lithium battery bank, you could get away with having a lot less battery capacity, and it would take up a lot less space, but the cost would be astronomical.

Kev
2011 Winnebago Tour 42QD
Towing a Jeep Rubicon Unlimited LJ
RVI Brake 2, TST 507 TPMS, 960 watts of solar, SolaRVector tilt
Lakeside, California

Gary RV_Wizard

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Re: Battery Bank Sizing
« Reply #4 on: April 29, 2018, 11:13:09 AM »
The battery bank size has little to do with the panel wattage. Instead, you need to determine your daily energy consumption and size the battery bank to hold enough energy (amp-hours) to meet your needs when the solar is not actively generating more power, or at least not as much as you are using.  If the sun shone directly overhead 24/7, you would only need a tiny battery to even out the varying power from the panels, but in most places you need to operate off battery for 12-20 hours/day.

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days. You have to estimate the average charging rate from the panels to determine if they will produce enough power for the batteries to store and later dispense as needed.
Gary
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jje1960

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Re: Battery Bank Sizing
« Reply #5 on: April 29, 2018, 11:35:21 AM »
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Wondering the same thing.... Unless your finger stuck on the keyboard and you really mean 200W....  This is 2000 Watts on a 40' Newmar Coach with tag axle...
« Last Edit: April 29, 2018, 11:42:05 AM by jje1960 »
Jim
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Back2PA

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Re: Battery Bank Sizing
« Reply #6 on: April 29, 2018, 11:52:26 AM »
This is 2000 Watts on a 40' Newmar Coach with tag axle...

Wow.
Scott
2014 Montana High Country 343RL (37')
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Alpena Jeff

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Re: Battery Bank Sizing
« Reply #7 on: April 29, 2018, 12:04:02 PM »
Yikes! How much battery does he have?
I'd have to go back to work!
Jeff & Judy
2016 Newmar Ventana
2018 GMC Canyon All Terrain toad
Blue Ox Aventa LX - RVi3 - EEZRV
Retired to "the lake" in north Michigan

Back2PA

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Re: Battery Bank Sizing
« Reply #8 on: April 29, 2018, 12:17:54 PM »
I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Yup, I put the max I could on mine but with kitchen and bath skylight/vents, shower skylight, fridge vent, kitchen hood vent, satellite and batwing the max was 800. As it was the installer removed the rain cover from the kitchen Fantastic Fan due to shading. I do kinda wish I'd looked harder at the higher density residential panels. Mighta been able to approach 1000.
Scott
2014 Montana High Country 343RL (37')
2011 SD F-250 Crewcab LB 4x4, 6.2 Gas, 10K gross
Eezrv TPMS
Fulltimer

Kevin Means

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Re: Battery Bank Sizing
« Reply #9 on: April 29, 2018, 12:59:11 PM »
The battery bank size has little to do with the panel wattage. Instead, you need to determine your daily energy consumption and size the battery bank to hold enough energy (amp-hours) to meet your needs when the solar is not actively generating more power, or at least not as much as you are using.  If the sun shone directly overhead 24/7, you would only need a tiny battery to even out the varying power from the panels, but in most places you need to operate off battery for 12-20 hours/day.

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days. You have to estimate the average charging rate from the panels to determine if they will produce enough power for the batteries to store and later dispense as needed.
In essence, that's correct, but unfortunately, it's one of those, "The devil's in the details" things. A battery bank that wil get you through a 24 hour period could end up being at a 50% or 90% SOC, depending on several factors. If one were only going to boondock occasionally, a lead/acid battery bank that only occasionally ended up at 50% SOC wouldn't suffer too much, but as I learned the hard way, frequent boondocking, and repeatedly discharging the batteries to such a low SOC will kill them prematurely.

When sizing a lead/acid battery bank for frequent boondocking, I'd recommend one with significantly more AH capacity than one that's only used for occasional boondocking. Yes, both may meet your needs and get you through the night, but the larger capacity battery bank will live longer and give you a little padding. We're frequent boondockers and found that we had to increase our AH capacity by about 30% to keep our batteries from being at about 55% each morning.

If you're a frequent boondocker, and you can keep your lead/acid batteries from discharging below 70% (I use 75%) and they're fully recharged soon thereafter, your batteries will live a lot longer. Once you've determined how much AH capacity you'll need to achieve that, you can size the array you'll need to fully recharge them IF that's what you want your solar array to do.

Kev
2011 Winnebago Tour 42QD
Towing a Jeep Rubicon Unlimited LJ
RVI Brake 2, TST 507 TPMS, 960 watts of solar, SolaRVector tilt
Lakeside, California

solarman

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Re: Battery Bank Sizing
« Reply #10 on: April 29, 2018, 01:10:05 PM »

I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do. Do they want to replenish some, or all of the power they use? Clearly, your goal is to replenish all of the power you use. That was our goal too, and we achieve it with a lot less than 2000 watts of solar, even though our coach is one of the more power hungry RVs.


guesswork, pure and simple


Quote

As a frame of reference, we've got a 960 watt array and eight Group 31 AGM house batteries, with a total capacity of 840 amp hours, and we almost never have to run our generator. A lead/acid battery bank with 2000 amp hours of capacity (to match a 2000 watt array) would require 19 similar batteries. At nearly $300.00 a piece, that's gonna be a pricey (and heavy) battery bank. Do you even have enough room in your RV for that many batteries?


this is the very mistake i'm trying to help the OP avoid, eight 31 agm's ? no,no,no.. after 40 years experience as an MSEE and 130 solar installs, some over 20KW, i've never seen such a bad solution. I not trying to be a dick here, but I am qualified to make these statements.
KZ MXT20 480 W solar
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solarman

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Re: Battery Bank Sizing
« Reply #11 on: April 29, 2018, 01:17:34 PM »
The battery bank size has little to do with the panel wattage.


hogwash, battery size and panel watts are dependent on each other

Quote

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days.


correct, also where do you think the energy is stored for cloudy days ?
KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

Alpena Jeff

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Re: Battery Bank Sizing
« Reply #12 on: April 29, 2018, 01:23:11 PM »



this is the very mistake i'm trying to help the OP avoid, eight 31 agm's ? no,no,no.. after 40 years experience as an MSEE and 130 solar installs, some over 20KW, i've never seen such a bad solution. I not trying to be a dick here, but I am qualified to make these statements.
Please explain this statement.
Jeff & Judy
2016 Newmar Ventana
2018 GMC Canyon All Terrain toad
Blue Ox Aventa LX - RVi3 - EEZRV
Retired to "the lake" in north Michigan

butchiiii

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Re: Battery Bank Sizing
« Reply #13 on: April 29, 2018, 01:24:52 PM »
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Hi Issac,
Next upgrade will be a 40ft 5th wheel. I was looking at 335 watt panels that are 77"x 39".
I think that there would probably be enough space to put 6 panels. This is all in the planning
stages. I just trying to get all of my home work done 1st.
Butch & Joanna
Ram 2015 Reg Cab 1500
2015 Keystone Passport 2890RL

butchiiii

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Re: Battery Bank Sizing
« Reply #14 on: April 29, 2018, 01:26:37 PM »
Hi everyone,
Just got back from Sin city. Give me some time to digest your answers. I appreciate
them and will probably have more questions.
Thanks
Butch & Joanna
Ram 2015 Reg Cab 1500
2015 Keystone Passport 2890RL

VallAndMo

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Re: Battery Bank Sizing
« Reply #15 on: April 29, 2018, 02:42:27 PM »
Hi Kev,

I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do.

Great advice. I was working with 100Ah/100W but will be sure to take your adjusted rule of thumb in consideration.

Thanks for posting it.

Cheers,
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VallAndMo

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Re: Battery Bank Sizing
« Reply #16 on: April 29, 2018, 02:50:47 PM »
Hello Solarman,
guesswork, pure and simple

I do not agree. I think rules of thumb, like the one Kev posted, are very useful in the early stages of designing a system, for example before one is able to measure his exact power requirements and is more interested in getting a "ballpark figure" for budget/planning purposes (exactly my case right now).

Of course, as soon as one *can* measure power needs (and before investing in an expensive system), he/she should do it and adjust the initial planning/design accordingly.

Cheers,
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   Vall.

Kevin Means

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Re: Battery Bank Sizing
« Reply #17 on: April 29, 2018, 03:02:13 PM »
Solarman, initially, there was indeed a fair amount of "guesswork" as I tried to size our battery bank and solar array to meet our needs. So much so that I hired a guy named Bob Sheerer (AKA HandyBob) to help me. A Killowatt meter was used to determine our A/C usage, and a Trimetric RV 2030 was installed to measure our typical DC consumption. In the end, our system works very well.

As long as we have good sunlight, we go to bed with about a 90% SOC and our house batteries are never below 75% SOC in the morning. During the winter months, our batteries are fully charged by about 1:00 PM if we tilt the panels, 3:30 PM if we don't.

We've got a 2800 watt inverter (85% efficient) that runs 24 hours a day to power our 22 CF res fridge, and those two devices alone consume between 180 and 200 amps per day (depending on how often the compressor runs.) The coach has two macerator toilets, a 40" LED TV that runs maybe two hours a night, LED lighting throughout and we charge our phones overnight. The microwave oven and TVs have A/C cutoff switches to eliminate parasitic draws when they're not being used, and we typically use only our catalytic LP heater to heat the coach vs. the Aqua Hot furnace.

I think we were pretty thorough in determining our actual usage, but I'm open to constuctive criticism. Our goals were to prevent our batteries from discharging below 75% SOC overnight, and to reduce generator run time as much as possible. In the end, we had to increase our battery bank's AH capacity by about 30%, and install 960 watts of solar to achieve that. With those goals in mind, what would you have done differently to determine actual usage when sizing a solar array and battery bank?

It's been my experience that most RVers don't monitor their battery's SOC very well, usually because they have no way of doing so. They just want their solar panels to recharge their batteries, and they don't want to have to think about it. Getting them to measure their actual usage, for the purpose of determining how much solar and AH capacity they'll need, can be a lesson in futility.

Kev
2011 Winnebago Tour 42QD
Towing a Jeep Rubicon Unlimited LJ
RVI Brake 2, TST 507 TPMS, 960 watts of solar, SolaRVector tilt
Lakeside, California

John From Detroit

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Re: Battery Bank Sizing
« Reply #18 on: April 29, 2018, 04:26:47 PM »
I can tell you the minimum battery recommendations for an INVERTER but for solar panels???

Well the maximum recommended charge rate is 30% C/20 that is for every 100 amp hours (At teh 20 hour rate) of battery.. 30 amps. 2,000 watts is what. 166 amps.

Of course you'd never get that much but that means you need around 600 amp hours of battery. 3 pair of GC-2 should do it

FOr the inverter one pair per 1,000 watts is recommended.

Now, as to finding room on an RV roof fore 2KW of panels. Not there yet but panel effiency is improving what used to take a humungous panel now takes one that is only muntug and soon will take one that is only mung (Relative word length applies here) as they improve the effiency of the cells.

But I don't think 2KW will fit on my house.....yet....
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Isaac-1

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Re: Battery Bank Sizing
« Reply #19 on: April 29, 2018, 06:41:01 PM »
Panel efficiency really has not improved that much in the last 10-15 years, though there has been some improvement in closer cell spacing on the panels.  I forget the exact efficiency improvement, though i did some research on it last year and found since the year 2000 cell efficiency has only went up 4 or 5 percent on commonly available commercial panels.
2002 Safari Trek 2830

solarman

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Re: Battery Bank Sizing
« Reply #20 on: April 29, 2018, 07:22:43 PM »
Please explain this statement.


Simply put, the main issues arise from cable,connection and internal impedance, if we eliminate cabling and terminal contact
variances ( very difficult for DIY as you don't have the equipment, but achievable ) we are left with internal impedance ( apparent resistance of the battery ). this is quite variable, certainly more so with AGM

Now, a battery with a higher impedance will, over time, result in less charge current to that battery, thus resulting in a lower average
 SOC ( state of charge ), this has the effect of aging the battery and increasing its internal impedance. over many cycles the effect becomes more pronounced. when the battery pack is idle ( minimal load ) other batteries in the string of higher SOC will discharge into the lower SOC battery and self equalize the voltages, but, this effectively makes the whole set equal to the worst battery in the pack.

this just gets worse as we add more batteries to the string, the net result is shorter lifespan and capacity due to all the unequal current flows.

with FLA these effects can be mitigated somewhat by periodically charging with an equalizing charge for an extended period, but as we know, equalizing causes more plate corrosion and reduces life.. !! we only equalize to reduce sulphation and electrolyte stratification. with agm, you can't equalize, so you have to hold at float for at least 24 hours to get the SOC to balance somewhat.


other issues with parallel FLA include cell shorts, imagine what happens when one cell shorts in a parallel bank.. the other batteries discharge into the failed battery and that's going to give you a very bad hair day with fire, smoke and hot acid !!!

now, with eight agm's having lower and more variable internal resistance, the owner has no way in hell of ever balancing the charge currents, I know this from experience actually measuring charge and discharge currents on parallel strings, it just ain't gonna happen..
so again,  the net effect is much shorter life and reduced capacity. all at a 200% premium over FLA.


KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

butchiiii

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Re: Battery Bank Sizing
« Reply #21 on: April 29, 2018, 07:29:57 PM »
Thank you for the replies.  Question if the panels I get put out a VMPP  of 36 volts I wouldn't
need to run the in series (theoretically) correct? Could they each be independently run to
the charge controller? Won't that reduce the shading problem to only affect whichever panel
is being shaded.  Also the panels seem to have 3 bypass diodes so it seems that partial shading
only takes out some of the panel not all of it.  Does this make any sense or am I thinking about
it incorrectly.
Here are panels that I was looking at.

http://www.amerescosolar.com/sites/default/files/Hyundai%20305W%20Solar%20Panel_0.pdf

https://www.solar-electric.com/lib/wind-sun/AXITEC_AXI_power_72c_310W-330W.pdf
Butch & Joanna
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2015 Keystone Passport 2890RL

Isaac-1

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Re: Battery Bank Sizing
« Reply #22 on: April 29, 2018, 07:49:21 PM »
Solarman, if not 8 Group 31 AGM's how would you solve the battery problem within the constraints of a typical RV battery bay and budget? 

As I see it 8 6V GC2 AGM's in series parallel might help, but you are still faced with similar issues, just on a smaller scale at 4 parallel banks, 4V or 2V industrial deep cycles might be nice, but then you get sticker shock on the price, and may face other physical limitations.  Flooded deep cycles might work, but often there is a limitation on access when squeezing in more batteries than a coach originally came equipped with...
2002 Safari Trek 2830

Frank B

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Re: Battery Bank Sizing
« Reply #23 on: April 29, 2018, 08:00:08 PM »
Butchiiii:


Sounds like your thinking is much like mine.  Here is a link to my project: http://www.rvforum.net/SMF_forum/index.php/topic,106669.0.html


I used panels made from 5" ingots rather than the more standard 6" ones, which made the panels closer to 32" wide.  This still allows me to walk on the roof of our 30' TT.


AFAIK, yes, shading will only take out the shaded panel if they are all wired in parallel, and if they have bypass diodes (mine do), then it only takes out the sections of the panel that are shaded.  Whether 36v is optimal or not may well depend on the controller you use (my panels are rated at 39Vmpp).  But you are probably in the ballpark.


I deliberately put more panel on the roof than I needed to account for periodic shading, and cloudy days.  Panel cost for me was relatively minor compared to the total cost of the project, with shipping and controller being the 'big ticket' items.  4 panels or 6 really didn't make that much difference, so I went for 6.  I have zero regrets.  My system is panel heavy, bit I don't mind.  My batteries are charged by 1 PM the next day most of the time.  If I am wasting sunlight, well, sunlight is still free. :-)  I also have room for growth in battery capacity if I ever go to lithium.  And I still get 15 to 25 amps in the rain.


My 1200 watt system is overkill for us.  This winter in Arizona and SoCal we were without shore power almost exclusively for 3 months, but we did not spare ourselves any conveniences.  Microwave, waffle iron, electric toaster, hot air corn popper, computers and laser printer -- none of them taxed the system at all.  I even ran the fridge one afternoon on 110 from the inverter because where we were was SO quiet that the noise of the flame in the fridge bothered me. :-)  Like others have suggested, 2kw of solar may be far more than you will use.


Frank.
« Last Edit: April 29, 2018, 08:09:04 PM by Frank B »
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solarman

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Re: Battery Bank Sizing
« Reply #24 on: April 29, 2018, 08:04:51 PM »
Solarman, initially, there was indeed a fair amount of "guesswork" as I tried to size our battery bank and solar array to meet our needs. So much so that I hired a guy named Bob Sheerer (AKA HandyBob) to help me. A Killowatt meter was used to determine our A/C usage, and a Trimetric RV 2030 was installed to measure our typical DC consumption. In the end, our system works very well.


handybob.. omg, i'm sorry you had to do that.

Quote

As long as we have good sunlight, we go to bed with about a 90% SOC and our house batteries are never below 75% SOC in the morning. During the winter months, our batteries are fully charged by about 1:00 PM if we tilt the panels, 3:30 PM if we don't.


for off grid we use 5 days autonomy, rv probably better to design for 3 to 4 days..
system should be designed to use no more than 15% capacity each day to allow for cloudy days.
this assumes FLA. for agm you can go lower, so one more day with same capacity or reduce capacity

Quote

We've got a 2800 watt inverter (85% efficient) that runs 24 hours a day to power our 22 CF res fridge, and those two devices alone consume between 180 and 200 amps per day (depending on how often the compressor runs.) The coach has two macerator toilets, a 40" LED TV that runs maybe two hours a night, LED lighting throughout and we charge our phones overnight. The microwave oven and TVs have A/C cutoff switches to eliminate parasitic draws when they're not being used, and we typically use only our catalytic LP heater to heat the coach vs. the Aqua Hot furnace.


that's good, however, handybob did not teach you that amps per day is wrong, consumption is always in watt hours.


Quote

I think we were pretty thorough in determining our actual usage, but I'm open to constuctive criticism. Our goals were to prevent our batteries from discharging below 75% SOC overnight, and to reduce generator run time as much as possible. In the end, we had to increase our battery bank's AH capacity by about 30%, and install 960 watts of solar to achieve that. With those goals in mind, what would you have done differently to determine actual usage when sizing a solar array and battery bank?


you did quite well, however your first mistake was to stick with 12 volts and parallel so many batteries when there is a simpler and lower cost solution. secondly for 960 w and 12 V you will need an 80 amp mppt controller and that's expensive.
moving to 24 or even 48 volts would halve the controller and cabling costs and you would not need to parallel.



Quote

It's been my experience that most RVers don't monitor their battery's SOC very well, usually because they have no way of doing so. They just want their solar panels to recharge their batteries, and they don't want to have to think about it. Getting them to measure their actual usage, for the purpose of determining how much solar and AH capacity they'll need, can be a lesson in futility.

Kev


quite true, it is very easy to make costly mistakes with solar, i have an uphill battle on my hands right now.. LOL

KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

solarman

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Re: Battery Bank Sizing
« Reply #25 on: April 29, 2018, 08:29:47 PM »
Thank you for the replies.  Question if the panels I get put out a VMPP  of 36 volts I wouldn't
need to run the in series (theoretically) correct? Could they each be independently run to
the charge controller? Won't that reduce the shading problem to only affect whichever panel
is being shaded.  Also the panels seem to have 3 bypass diodes so it seems that partial shading
only takes out some of the panel not all of it.  Does this make any sense or am I thinking about
it incorrectly.
Here are panels that I was looking at.

http://www.amerescosolar.com/sites/default/files/Hyundai%20305W%20Solar%20Panel_0.pdf

https://www.solar-electric.com/lib/wind-sun/AXITEC_AXI_power_72c_310W-330W.pdf

both those panels are good..

let's say you had room for 6 at 300 watts each for 1800 W
then you could connect them as a 2s3p array, that is, 2 in series and three sets in parallel
and feed a 150 Volt MPPT controller into a 24 or 48 Volt battery.
for 24 V you would need a 1800/24=75 amp controller
for 48 V you would need a 1800/48=37 amp controller

in practice you will not get 1800 w so a lower cost 35 amp controller would suffice for 48V

1800 W is unsuitable for 12 Volts as you would need multiple expensive controllers, heavy gauge wiring etc..



KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

solarman

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Re: Battery Bank Sizing
« Reply #26 on: April 29, 2018, 09:15:13 PM »
Solarman, if not 8 Group 31 AGM's how would you solve the battery problem within the constraints of a typical RV battery bay and budget? 

As I see it 8 6V GC2 AGM's in series parallel might help, but you are still faced with similar issues, just on a smaller scale at 4 parallel banks, 4V or 2V industrial deep cycles might be nice, but then you get sticker shock on the price, and may face other physical limitations.  Flooded deep cycles might work, but often there is a limitation on access when squeezing in more batteries than a coach originally came equipped with...


the stated cost is $300 each.. so eight of them is $2400 and he has a parallel setup that will need to be replaced sooner than he would like. also, i assume he paid for heavy interconnect cabling as he is stuck at 12 volts

from his statement his capacity is 840 Amp/hr at 12V
so the capacity of this bank is  12*840 = 10080 watt hours

his 960 watt panels is outside normal design practice and should really be 24 or 48 Volts, i'll explain in
the tutorial i'm going to post shortly in a new thread

with 10080 Whr  then at 24 Volts would be 10080 / 24 = 420 A/hr
and at 48 Volts would be 10080 / 48 = 210 A/hr

if we went real cheap and used 6 Volt golf cart batteries then a good fit is 48 volts
so he could have used eight of those at $85 each for a total of  85*8 = $680

ok, so this is FLA and not AGM, so lets do the math for agm

again going cheap we could use Duracell agm at $180 ea so that's 180*8 = $1440

the lower spec batteries will most likely last longer than the higher spec'd units
as they will not suffer any of the parallel issues.


the biggest problem i see is the dumb asses that build coaches, they buy and build on a commercial chassis and because
it's 12 volts, they automatically assume that solar is going to be 12v too.. and it's not the case.


« Last Edit: April 29, 2018, 09:21:37 PM by solarman »
KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

butchiiii

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  • A day RVing is better than a Great day at work!
Re: Battery Bank Sizing
« Reply #27 on: April 29, 2018, 09:18:58 PM »
both those panels are good..

let's say you had room for 6 at 300 watts each for 1800 W
then you could connect them as a 2s3p array, that is, 2 in series and three sets in parallel
and feed a 150 Volt MPPT controller into a 24 or 48 Volt battery.
for 24 V you would need a 1800/24=75 amp controller
for 48 V you would need a 1800/48=37 amp controller

in practice you will not get 1800 w so a lower cost 35 amp controller would suffice for 48V

1800 W is unsuitable for 12 Volts as you would need multiple expensive controllers, heavy gauge wiring etc..


Solarman that makes sense. At the battery output to feed the RV what is used to step the power back down to 12v?
Butch & Joanna
Ram 2015 Reg Cab 1500
2015 Keystone Passport 2890RL

solarman

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Re: Battery Bank Sizing
« Reply #28 on: April 29, 2018, 09:32:59 PM »

Solarman that makes sense. At the battery output to feed the RV what is used to step the power back down to 12v?


you need a 48V to 12V buck converter, they are used in the golf cart world and cost about $50 for a 360 Watt unit
that may be sufficient for your needs, larger units are available.
you will also need a means to charge the battery from the mains 120 V and also an inverter to power 120 V appliances,
this is available as a combined inverter/charger/transfer switch, I have one in my own rv.

you throw away or disconnect the POS WFCO 12V converter in the RV




KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

Isaac-1

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Re: Battery Bank Sizing
« Reply #29 on: April 29, 2018, 11:14:16 PM »
Solarman that DC-DC converter may be fine for keeping the control electronics working on the water heater, fridge, etc, but how do you hand cranking the generator from the house bank if it is running at 48VDC?
2002 Safari Trek 2830