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Old 07-30-2020, 06:29 AM   #1
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Solar, battery, and power consumption questions

We have 400 w of Go-Power solar panels on our roof (FC 23FB) and two 6v AGM batteries. I'm still trying to get my head around how this will work in real-world situations.

I think the batteries are 100 amp hour (have to confirm with the dealer we purchased from). Assuming that's true, and noting that we can only use 50% of the capacity, that gives us 100 amp hours with no input from solar or shore power.

Say we use about 30 amp hours a day. That would give us 3 days on battery power alone.

But, we have 400 w of solar on the roof. So, my question is how do I calculate how quickly the solar panels will recharge the batteries—assuming it's sunny out (which, fortunately, it often is in Utah where we camp).

Also: I read an old post (2015) from someone here with solar panels saying it't not a good idea to be plugged in and have solar charging your batteries at the same time, because it will reduce the performance of the batteries. But isn't that what a charge controller is for? Doesn't make sense to me that a modern solar system couldn't accommodate this.
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Old 07-30-2020, 07:23 AM   #2
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I don’t think you have to worry about solar and being plugged in. As you said the charge controller takes care of that.
400 watts should in best case give you 20 amps per hour in full sun at the perfect angle. So maybe count on something like 15 to be conservative for 6 hours a day if you have full sun.
I don’t think under most conditions you will have any issues charging your two batteries to 100 percent with 400 watts of solar.
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Old 07-30-2020, 07:34 AM   #3
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Hold on here. If you have two 6v batteries in series and each one is 100 amp-hrs, your total capacity is just 100 AH @ 12vdc. You should not use more than 50AH although depending on the brand and specs, maybe a little more. You need to be pretty careful about use of stuff with only 50AH available.
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Old 07-30-2020, 07:51 AM   #4
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Originally Posted by lsbrodsky View Post
Hold on here. If you have two 6v batteries in series and each one is 100 amp-hrs, your total capacity is just 100 AH @ 12vdc. You should not use more than 50AH although depending on the brand and specs, maybe a little more. You need to be pretty careful about use of stuff with only 50AH available.
Larry
Ah, right. I forgot to do the conversion from 6v to 12v DC. I need to check with the dealer to see what the capacity of the batteries is. It very well may be more than 100 amp-hrs.

In any event, the good news is that it sounds like I should get 90 amps or so over 6 hours of charging from the solar panels, which should more than cover our daily usage.
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Old 07-30-2020, 08:12 AM   #5
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Ok, this will hopefully help.

Look at the left two columns, this is my solar yield from 400w zamp Obsidian in series/parallel. Connected to a Victron MPPT 100/30 charge controller going to 200Ah of Battleborn Lithium Ion batteries.

This was a few weeks ago in full sun in eastern Oregon parked in a field for a couple of nights.

My average discharge is 58Ah and i recall that on this trip it that was spot on, i was 56-58ah each day. Please note I have not been careful at all about usage, stereo with sat radio on all day, fridge, TV, charging everyones phones and laptops, wifi with 4G card running 24/7, streamed a soccer match... but minimal furnace usage, maybe 1hour.

I won't get into what all the colors in the bars mean except to say the white bit is the bulk charge where max power is being dumped back into the batteries and that the rest is just keeping the batteries topped up.

the super deep discharge in my data was a long time ago, so don't concern yourself about that.

I thought seeing some actual data might help.
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Old 07-30-2020, 08:21 AM   #6
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Originally Posted by lsbrodsky View Post
Hold on here. If you have two 6v batteries in series and each one is 100 amp-hrs, your total capacity is just 100 AH @ 12vdc. You should not use more than 50AH although depending on the brand and specs, maybe a little more. You need to be pretty careful about use of stuff with only 50AH available.
Larry
What Larry said... but I doubt your 6 volt AGMs are only rated at 100ah. If they are lifelines it would be more like 220. So 220 in series would be 220 so about 110 usable.

What brand AGMs are they?
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Old 07-30-2020, 09:29 AM   #7
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Hi

It's a pretty good bet that your batteries are around 200AH rated / 100AH usable. If they *are* 100AH rated, you got ripped off ... sorry about that.

First up, these are lead acid batteries. They get to about 85% charge pretty quickly. Getting to 100% charge can take a long time. How long depends on a bunch of things. Think in terms of at least a couple hours and maybe as long as 5 or 6 hours. Why get into this first? If you are only charged to 85% then your available amp hours are closer to 70 than 100.

But won't your voltage monitoring battery capacity gizmo *tell* you that it's only at 85%? Likely not. The gizmo probably will lie to you and say you are at 100% when you have only made it to <85%. That's just how voltage monitors work. To really *know*, you need a shunt based monitor. The Victron BMV-712 is one of many out there.

The one drain most of us can't get around is the fridge. They often (but not always) pull about 1A. That gets you to 24AH per day pretty much doing nothing. You may well (but probably don't) have as much as 1/4A of parasitic drain on top of that. If you did, that would add another 6AH, still doing nothing.

Toss in the water pump running from time to time, the vent fan over the stove at meal time. lights for a couple hours a day and you can *easily* be up around 40 to 50AH a day.

Note that we still have not mentioned the inverter or the furnace. Turn either one on and your batteries are going down in a hurry.

There is a *lot* of guesswork in all of the above. The *only* way to know what's going on with *your* trailer is to break out your multimeter and measure the drains. Without doing that, you are flying blind. Indeed a shunt based monitor will make this pretty easy ....

Solar wise, 400W is 28A at 14V. That assumes 100% efficiency and max sunlight. As mentioned above, 20A is doing pretty well in the real world. If you get that 20A for 5 hours a day, you have 100AH. The gotcha is the charge time past 85%. You may / may not get full charge in a real day / full sun.

How will you know if you have full charge? Your stock voltage based monitor will not tell you..... It also has a problem with temperature compensation .... By now you likely have guessed what the solution to that is

Bob
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Old 07-30-2020, 09:50 AM   #8
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I checked with the dealer and the batteries are 200 amp hours.
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Old 07-30-2020, 10:08 AM   #9
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You may / may not get full charge in a real day / full sun.

Yes, occasionally you may not get a full charge in a real day of "sun" when there is dense tree canopy, all-day fog and/or dark rain clouds . . . But on a typical day of sun with passing clouds and sparse tree canopy, your batteries will easily get fully charged. I had 400W of solar with a pair of 230AH golf cart batteries, which are the most difficult type to charge, and rarely did my batteries not fully charge in a day. 400W of solar and 200AH of batteries (100A usable) is the perfect balance of battery bank and solar array in my opinion. I'll wager you will have no problem keeping your batteries fully charged most days as long as you don't continuously park under a dense canopy of tree shade. There is no problem with having the solar charger operating while connected to shore power.
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Old 07-30-2020, 11:25 AM   #10
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If you have 400 watts solar and only 100amp 12v (2 x 6v) you are pretty much have more solar than you need, unless you live in super cloudy conditions. A rule of thumb is, and only a rule of thumb, calculate you will get half out, on average (average is different where you are) out of your panels. Thus: is you have 400 watts on your roof (not adjustable for optimum solar collection) your average on a sunny day would be approx 200 watts average. Again it is a rule of thumb and not for living in a rain forest or parked under trees. Another rule of thumb is the rule of 2 to 1. If you have 400 watts of solar then a good balance of battery would be 200amp of 12v (better to have 2 x 6v 200amp each). You can actually go the wrong way in that you have too much battery: example 400 watts of solar and 400amp 12v. This can get you in a situation where you might not be able to top off your 'lead acid' batteries enough times to keep them healthy.

The only way to accurately asses how much you are using and how to plan around that usage is to get a descent solar and battery management system. It does all the work for you and you know exactly the state of solar, batteries, historical data for planning. Call AM Solar located in Oregon and they can walk you through what your options are. Airstream and RV dealers have zero clue about this stuff.

I have 400w Renogy with two 6V Lifeline 300amp 6v (thus 300amp 12v)...breaking the 2:1 rule but never too far away from good sun. This is on a 26U AS. I run anything that can run off the inverter and have my converter charger turned off 100% of the time. Even with the heater running, Ham radios pulling tons of amps when transmitting, TVs on...I have never run into brining batteries t0o low and not fully charged back up in a safe amount of time. I am using a BlueSky Solar management systems with battery temperature monitorsand all usage and charging through a 'shunt'. AM Solar designed it. Four years and full timing. Love it. PS: Prices have come down a lot in four years. I would consider Lithium Iron Phospate batteries. You can actually lower your total battery amp total and also save a ton of weigh and far more flexible in discharging and re-charging.
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Old 07-30-2020, 11:47 AM   #11
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Thus: if you have 400 watts on your roof (not adjustable for optimum solar collection) your average on a sunny day would be approx 200 watts average.

Did you mean 400W=200A? In my experience, I typically get three times solar panel wattage on an average day of sunshine with lead acid batteries. Therefore 400W x 3 = 1200Wh of production. Divide that by an average 13.5V charging level equals 89Ah of battery charging. It is possible to get five to six times solar panel wattage, which would be closer to 180Ah if you had a large enough battery bank (Lithium), drawn down so it could absorb the 180Ah. Maybe one could get 200Ah of charging on a perfect day of solar with a large Lithium battery bank (400AH?) starting from a low state of charge with 400W of solar. In any case, the op will get way more than 200Wh of production on an average day of sunshine from 400W of solar.
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Old 07-30-2020, 11:50 AM   #12
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Thanks, everyone. To clarify, I do have 2x 6v 200 amp-hr batteries, not 2x 6v 100. So it sounds like with 400 w of solar I should be in good shape.
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Old 07-30-2020, 11:53 AM   #13
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Thanks, everyone. To clarify, I do have 2x 6v 200 amp-hr batteries, not 2x 6v 100. So it sounds like with 400 w of solar I should be in good shape.
BINGO! Just go use your Airstream and let the solar keep the batteries charged. The only time you will need to think about your batteries will be when there is no sunshine due to dense tree canopy or dark-dark rain clouds.
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Old 07-30-2020, 11:59 AM   #14
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Is a staycation feasible for you? Other posters found them to be enlightening. Batteries are notorious for underperforming, depending on their history (of abuse).
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Old 07-30-2020, 12:23 PM   #15
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Is a staycation feasible for you? Other posters found them to be enlightening. Batteries are notorious for underperforming, depending on their history (of abuse).
We're technically not supposed to have our trailer in our driveway for more than 24 hours before/after a trip because of HOA rules, but I have noticed that our neighbors have chosen to interpret that rule liberally.
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Old 07-30-2020, 01:08 PM   #16
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Originally Posted by switters View Post
We have 400 w of Go-Power solar panels on our roof (FC 23FB) and two 6v AGM batteries. I'm still trying to get my head around how this will work in real-world situations.

I think the batteries are 100 amp hour (have to confirm with the dealer we purchased from). Assuming that's true, and noting that we can only use 50% of the capacity, that gives us 100 amp hours with no input from solar or shore power.

Say we use about 30 amp hours a day. That would give us 3 days on battery power alone.

But, we have 400 w of solar on the roof. So, my question is how do I calculate how quickly the solar panels will recharge the batteries—assuming it's sunny out (which, fortunately, it often is in Utah where we camp).

Also: I read an old post (2015) from someone here with solar panels saying it't not a good idea to be plugged in and have solar charging your batteries at the same time, because it will reduce the performance of the batteries. But isn't that what a charge controller is for? Doesn't make sense to me that a modern solar system couldn't accommodate this.
I have a 2020 25' Flying Cloud and I am wondering if I have enough space on the roof to install 400 Watt of solar panels.
With the specified FC23' FB, please tell me how the panels are arranged. I do like to get that amount of solar panels on the roof myself.

Henk Bouhuijzen
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Old 07-30-2020, 01:48 PM   #17
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2015 FC25 rear bed with 4 100Watt Zamp Obsidian panels. Lots of space. Could add 2 90w slim panels easier but I don't need them.

https://www.airforums.com/forums/f44...ml#post2294947
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Old 07-30-2020, 05:23 PM   #18
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Originally Posted by HenkB View Post
I have a 2020 25' Flying Cloud and I am wondering if I have enough space on the roof to install 400 Watt of solar panels.
With the specified FC23' FB, please tell me how the panels are arranged. I do like to get that amount of solar panels on the roof myself.

Henk Bouhuijzen
Here's what my roof looks like.
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Old 07-31-2020, 01:35 PM   #19
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One thing I have noticed with my BlueSky solar charger and the shunt system is while charging on shore power the AHs used gets out of sync with the real world. It will shut down for low voltage and the BlueSky will show 30-40% remaining. After being back on full solar a while it gets itself back in sync.
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Old 08-01-2020, 07:29 AM   #20
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One thing I have noticed with my BlueSky solar charger and the shunt system is while charging on shore power the AHs used gets out of sync with the real world. It will shut down for low voltage and the BlueSky will show 30-40% remaining. After being back on full solar a while it gets itself back in sync.
guskmg
Hi

There is a calibration process on the Victron shunt systems. You need to adjust several parameters to get them to match your system. It's been a while, but I seem to recall a charge efficiency number, a "ignore current past this point" number, and a "this voltage means full" number. If any of them are out of whack vs your batteries, the system will not do as good a job as it could.

Bob
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