Question on supply for the outback flexmax80

Solar Expert Posts: 292 ✭✭
I have read the specs and I see the maximum voltage and panel array size, but what is the minimum? I will need more than 80 amps to generate 80 amps. Do I use a 25%, or 40% increase to deal with loss?

I am doing a system backwards from the normal way so that I can get a feel for the setup. My actual loads will be minimal in the beginning. My plans for what I had intended to do with an offgrid setup have recently changed and it looks like I will be city bound for the forseeable future.

What I plan on doing is using some Interstate L16 batteries with a rating of 280ah in a 48v setup having two strings of batteries for a total of 580ah. I am thinking the Flex 60 would not give me enough charge for the bank. That is why the 80 came into play.

So here is my question:
Going with the 580ah bank, and the Flex 80, What array amperage will I need to keep the charger happy? I have not figured out what panels I will go with yet.
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Re: Question on supply for the outback flexmax80

Generally, a good rule of thumb is around 5% to 13% of the Bank's amp*hour rating for charging current:
• 580 AH * 59 volts charging * 1/0.77 solar panel+controller derating * 0.05 rate of charge = 2,222 watts of solar panels
• 580 AH * 59 volts charging * 1/0.77 solar panel+controller derating * 0.13 rate of charge =5,777 watts of solar panels
The "optimum" solar array rating that a FM 80 would run:
• 80 amps * 59 volts * 1/0.77 derating = 6,130 Watts
You could go more as MPPT type charge controllers will simply limit their output current--but then they would hit the current limit more often on sunny/clear/cool days.

If you where looking at just doing a trickle/maintenance charge (keeping the batteries alive during storage), you could go as low as 1% rate of charge (roughly 440 watts of solar panels).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Question on supply for the outback flexmax80

I'd say you're doing it kind of sideways. You size the array to be able to supply the batteries' needs, then pick the controller that will handle it. Charge controller's don't really have a "minimum Amperage".

Something like this:

580 Amp hours @ 10% (Trojan's recommendation for L16's) = 58 amps of charge current.
58 * 57 Volts charging = 3306 Watts, less derating = 4300 Watt array

Now an FM60 will handle 58 Amps @ 57 Volts. But that's near its peak capacity. With the old MX60's we found they work best at around 75% capacity, so perhaps picking the FM80 is a good idea here.

That's a lot of power you've got there: potentially almost 14 kW hours!

BTW, Trojan has two L16 series; one is 320 Amp hours the other 390. So I'm assuming these aren't Trojan batteries. Nevertheless the 10% rule is good for all "tall case" batteries as they need extra current to keep the electrolyte from stratifying. Also remember that it is 10% net charge rate; you have to account for any loads that may be detracting from the charge.
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Thanks guys, that was the information I was looking for.
I calculated something incorrectly again. I didn't factor in a rate of charge. I will probably need to only run one string of batteries so I can cut the panel needs in half which will let me drop down to a smaller charger for a bank of 280ah.
Be back in a few days.
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

How many watts of panel would I need to supply enoigh juice for a 560ah battery bank ? 12v or 24v or 48v I am now undecided.
Re: Question on supply for the outback flexmax80

First, you need to pick the rate of charge... 5%, 10%, 13% etc...

5% is pretty much the minimum rate of charge I would suggest... AGM batteries will be OK at 5% rate of charge--Large/Tall flooded cells, that is about the bottom that you would want to try.

Also, flooded cell batteries have a higher self discharge rate. And as they get towards the end of their life, much of the solar array power will be making up for self discharge (especially if these are "fork lift" type batteries).

10% is a good number for flooded cell/tall flooded cell... You can go higher than 13%--it just becomes more expensive in solar array (flooded cell batteries tend to peak out around C/8 aka 12.5% rate of charge).

Next, choose your battery bank voltage... Base that on the peak power/current you will feel comfortable wiring up... Typically, my break points:
• <1,200 watts--12 volts is fine
• <2,400 watts--24 volts is fine
• >2,400 watts--look at 48 volts
Also, it depends on what you want to power... The Morning Star 300 watt TSW inverter is a really nice 12 volt device. Lots of features that no other 12 volt inverter has (remote inhibit, search mode, 600 watts for 10 minutes).

If you are going to power DC loads and send the wiring a fair amount of distance, 12 volts is really tough--You basically only have ~1.0 volts allowed drop before you hit voltage cutoff (11.5 volt min battery voltage, 1.0 volt wiring drop, 10.5 device cutoff). 24 and 48 volt banks are much easier to send the current any distance without spending a fortune in copper cabling.

Lastly, when you say 530 AH at 12/24/48 volts... that is 3 different numbers of batteries (2x 6 volt, 4x 6 volt, 8x 6 volt)... So I am not sure what number you are actually looking at (530AH at 48 vs 2,160 AH at 12 volts, etc.).

For a 530 AH battery bank, you are looking at ~10% rate of charge or ~53 amps... For a 2,160 AH @ 12 volts you would be looking at 216 Amps charging--or 3-4 60/80 amp solar charge controllers...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Iam just not getting the math of this. When I take the formula you provided and punch into my calculator I do not get the numbers you come up with. Is there any way you can take it one calculation at a time instead of e continous string. I understand that I am a complete idiot, but If I can get the first part down, maybe I could stop hitting my head on the wall for a while.
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80

The quantity of panels will vary with system Voltage as well as Amp hours. The basic math:

560 Amp hours charged at a target peak rate of 10% = 56 Amps.
56 Amps * 14.2 Volts (charging for a 12 Volt system) = 795 Watts, less derating (77%) = 1032 W array.
Double the Voltage to a 24 Volt system and you have to double the array size.
Double the Voltage again for a 48 Volt system and you need 4 times the 12 Volt system's array.

But understand that your total available Watt hours goes up too:

560 Amp hours @ 12 Volts = maximum 3360 Watt hours @ 50% DOD.
560 Amp hours @ 24 Volts = maximum 6720 Watt hours @ 50% DOD.
560 Amp hours @ 48 Volts = maximum 13440 Watt hours @ 50% DOD.

You should pick a system Voltage based on the maximum draw you expect at any one time. Generally, 12 Volt systems are best kept under 2 kW (1200 Watts is good), 24 Volts for twice that, et cetera. The size of the battery bank should be based on your daily Watt hour needs.

Using an MPPT type charge controller allows a lot of flexibility in array design and reduces concerns about the distance between the array and the charge controller. 30 feet is not unreasonable at all.
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

How do you get 1032 from 795? 795 times 1.30?
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80
Leper wrote: »
How do you get 1032 from 795? 795 times 1.30?

795 is 77% of 1032:

1032 * 0.77 = 794.64

or

795 / 0.77 = 1032.46

77% being the "standard" derating for panels (actual averaged output over "equivalent good sun hours" versus nameplate rating).
Re: Question on supply for the outback flexmax80

795 watts required / 0.77 panel+charge controller derating = 1,032 watts of panels

1/0.77 = 1.30

So, if you need 795 watts to charge your battery bank (for 5-13% rate of charge), then you need 1/0.77 or x1.30 that amount of power worth of solar panels (to account for all losses).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

All right. If I run a 12v system I would need 10x 135w panels to get 1350 watts of power and using products from the wind sun store those panels would cost me \$3600, but if I were to go to 24v I could use 10x 210 watt panels that would only be \$1400 more and I would double my available watt hours from the bank. \$.20 a watt savings by going 24v. Later on I could run the 12v panels in series to get them to 24v, if I had the surface area to accommodate all of the panel mounting area.
Re: Question on supply for the outback flexmax80

Watch the Vmp ratings of panels... Most modern "12 volt" panels have Vmp~17.5 volts... Older/smaller panels (for use without charge controllers) would have Vmp~15 volts or so.

Many "24 volt" panels really have Vmp~24 volts. And not ~35.0 volts (which would be needed for a 24 volt battery bank running on a PWM controller)...

Roughly, you need to match Vmp voltage of parallel panels/strings to within ~10%; And need to match Imp current to within ~10% for series connections.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80

Try to keep in mind that redesigning a system after it has been installed is usually a problem.

Some other considerations:
More than two panels in parallel requires a fuse per panel to prevent any possibly shorted panel from being back fed too much current by the still good panels.
On an MPPT controller it is best to run an array with a nominal Voltage only two times the system Voltage, so your 12 Volt system would use a 24 Volt array.
Those two items add up to wiring the ten 135 Watt, 12 Volt panels as five parallel strings (each with its own fuse) of two panels.

I really think you should work out a system according to your power requirements, rather than what gives the most Watt per dollar. If you buy a system that is too big you will have invested money in capacity you never use, and that's no bargain.
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Appreciate the comments. My usage goes way beyond what I can handle fully with solar. I am measuring usage on certain things that I plan on running from solar. I will be selling my big tv soon and replacing it with a smaller and much less power hungry one, as one example. I won't bore you with the details of everything else.

24v seems like a good balance. It is going to cost a bit more than I originally thought. That is ok. I have decided on the bank amp hour. That is my baseline. 560. Now I am figuring out the panel requirements, then I will find the right charger. Then he mounting/wiring/connections/sub panel/switches etc. I think that I can start acquiring panels and the installation items on a cost basis and get the batteries last.

That is my plan as of right now.
Re: Question on supply for the outback flexmax80

I would suggest selecting your inverter and battery bank first... Those will drive the solar panel capacity and charge controller... 560 AH at 24 volts?

And a real problem with buying panels a bit at a time--You need to match voltage/current as you connect them in series/parallel setup. And without knowing what solar charge controller actual array voltage/current, it can be a real pain to get the setup correct.

If you can get a "set" of panels at one time--it will make things easier.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

How often do solar panels change? If I buy one a month is it likely that I won't be able to get the same panel? On that note, if I did one system with only a 280ah bank panels and charger for it and built another like system, could they be combined for the advantage of the total amp hour. The batteries would only be a few months apart in age. If that is doable, I could get one of the systems together real quick.

Understand that if I end up with left over equipment at some point I am completely ok with that. I will find a use for it. I am a firm believer that two is one and one is none. I love redundancy.
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80
Leper wrote: »
Understand that if I end up with left over equipment at some point I am completely ok with that. I will find a use for it. I am a firm believer that two is one and one is none. I love redundancy.

I know what you mean.

How often panels change is anybody's guess. It is more a matter of will you be able to get panels with the same or near Vmp for parallel connections (highly likely) or the same/near Imp for serial connections. Usually you won't be adding a different panel into a string (this comes up when one gets damaged and needs replacement). Most likely you'd be adding in parallel. To that end most panels have the same or similar Vmp for their type ("12 Volt" or "24 Volt") but there are some odd balls out there.

You can always get an array/charge controller combination that is "maxed out" to begin with, and then if you want/need to up the battery capacity you just add a completely separate new array/controller.

Or you can add an entire new separate system if you really like redundancy. One with a small inverter for "light jobs" and another with a big (uses more standby current) inverter for heavier loads. There are lots of possibilities like this.

As for mixing batteries, the "older" they are the less likely you want to add new ones to the bank. Things that "age" batteries are time, depth of discharge, cycles, and abuse. Usually if the "old" ones are 1 year or less and still have good SG reading (somewhat problematic with AGM's which you can't read SG on) it isn't an issue. If, however, you've been cycling them down below 50% ... could be a problem.
Re: Question on supply for the outback flexmax80

I am not in the business--but at this point, it seems the economics of panels shift pretty dramatically in 9-12 months or so...

We had one person here that had a set of panels and it was cheaper to buy 20% larger panels and live with the ~20% loss (of the larger panels) than it was to buy the same panels that were not that old (if he could find any at all).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

You guys are great. This is great information. I will be using mppt controllers. I read the thread about the rogue controller and I was impressed. If I start with a 280ah bank at 12v the math works out that this would be a good controller to use. If I went 24v it would still be a good one to use. Ok, now I'm getting somewhere.
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Re: Question on supply for the outback flexmax80

Keep in mind the Rogue 3024 has limitations. It will only do 30 Amps at either 12 or 24 Volts. It will not do 48 Volts. It's input Voltage is limited to 34 Volts, which makes it incompatible with many panels.

It's great for small, 12 Volt system but I don't think I'd pick it for a 24 Volts system or if I was planning any system "size-up" changes in future.

But with a set of 225 Amp hour "golf cart" batteries and about 400 Watts of panel it does a great job for a system power a 300 Watt Morningstar SureSine.

Right, Tony?
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Hmmmm

The rogue controller has a max panel input of 864 watts for a24v bank. The kyocera panel kd210gl-Lpv is 210 watts, 26.6v 7.9a. 4 of these panels would get me to 840w which the rogue would handle, but is the 26.6v enough for the bank?
Re: Question on supply for the outback flexmax80
• Vmp=17.5 volts is good for a 12 volt bank
• 26.6v is enough for a 12 volt bank...(behind a MPPT charge controller; PWM would have ~35% losses with this Vmp on a 12 volt bank)
• You need Vmp~35 volts for a 24 volt bank
• Vmp~70 volts for a 48 volt bank
• or higher, if using MPPT charge controller
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Looking on the wind sun site, I don't see any 36v panels, so that means that I need to series two panels to get the voltage I need. Correct?
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80
Leper wrote: »
Looking on the wind sun site, I don't see any 36v panels, so that means that I need to series two panels to get the voltage I need. Correct?

Ah, no.
Panels have three Voltages.
The first is the "nominal" Voltage; 12 or 24
The second is the Vmp; usually 17.5 or 35
The third is the Voc; usually over 20 or over 40.
All three are important to know about when setting up a system.
Re: Question on supply for the outback flexmax80

And it depends on the charge controller too...

PWM Vmp needs to be very close to 17.5 volts (+/- 0.5 volts or so) to be efficient for a 12 volt battery bank.

MPPT Vmp needs Vmp>Vbatt-charging + wiring voltage drop + controller voltage drop + Vmp drop as panels heat up. Basically >~17.0 volts or so...

For PWM controller on a 24 volt battery bank:
• Vmp~35 volts for PWM controller on a 24 volt nominal battery bank.
For a MPPT controller on 24 volt battery bank:
• Vmp>= 35 volts
• Voc-cold < Vcontroller-input max
• Typically for a 45-80+ amp controller, Voc<150 VDC maximum.
• Vmp works out to be <~100 VDC for most people.
• Smaller controllers may have Vpanel max (Voc-cold) in the 35-75 volt range.
If you live in a warm region (i.e., Caribean), you can have Vmp a bit larger than 100 volts. Voc-cold increases the colder the panel get.

For MPPT controllers, most manufactures have a web page that lets you calculate the range of (like 35 volts < Vmp < 100 volts) for your min/max operating temperature ranges.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

Ah ha some of my confusion is going away now. Thank you for the explanation.
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80
Leper wrote: »
Ah ha some of my confusion is going away now. Thank you for the explanation.

Darn it, Bill; we're not explaining things right if he's getting less confused! We'll never land high-paying government jobs if people understand what we say!

Anecdote time: I had a "72 Volt" array on my MX60. In theory it should have handled it because the max V in is 150 Volts. In practice it shut down the controller in cold weather as Voc from the panels exceeded the max, even though it should have been under it. The key here is the cold weather which increases the panels' ability to produce Voltage.

As a rule-of-thumb array nominal Voltage shouldn't be more than 2X system Voltage on an MPPT controller; the efficiency drops off as the Voltage difference increases.
• Solar Expert Posts: 292 ✭✭
Re: Question on supply for the outback flexmax80

When I look at the "sun chart" the closest city to me that is listed if fort worth tx. Cold is really not an issue here except for about once every five years or so.
• Banned Posts: 17,615 ✭✭
Re: Question on supply for the outback flexmax80
Leper wrote: »
When I look at the "sun chart" the closest city to me that is listed if fort worth tx. Cold is really not an issue here except for about once every five years or so.

But heat can be. As cold makes output go up, so heat brings it down. You may have to de-rate your panels a little bit more for consistently high ambient temperatures.

Gosh there always has to be an issue, doesn't there?