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Old 08-05-2022, 09:12 PM   #1
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Some solar/battery issues

So I decided to do a little test of my current solar power. I have a 190 watt panel permanently attached on the roof, a 100 ah lithium battery and a Victron Smart Solar 100/30 converter in my tongue box. My fridge is the notorious Dometic RM8551 three-way. Am just now learning how to use the Victron AP on my phone. Need practice.

Must I get a solar friendly fridge? Of course I know the power needs of the 8551 could never under even the strongest sun-power be satisfied with my current solar situation. The point is just to test and see what happens. It is sunny and in the nineties outside. Maybe I find a path here to not using propane boondocking in the desert. To start, the one lithium is totally healthy. That Victron phone AP assures a robust 13.53 volts.

I turned on the fridge, - battery power, 5 dots. No alarms. After fifteen minutes on battery no warning red blinking lights on the fridge, and still plenty of voltage in the battery. But in 15 minutes it had dropped to 13.3 volts. Yes the 8551 was sucking power from it… seems my solar panel is working hard to keep up but - clearly a losing battle. I shut down the fridge.

What happened next? After five minutes now with no load the Smart Solar AP indicated a significant rebound for the lithium battery. That was fun. If I was Will Prouse I would take these statistics and use them to interpolate/conjure just how many minutes it would take before the battery gets down to eleven volts. Will it be - another battery, or double the solar, or just using the Engel?

Much to think about here so, decided instead to remove the screen from the Maxi Fan and give the unit a thorough cleaning. Another first for me, after nine years.
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Old 08-05-2022, 10:24 PM   #2
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So many variables to consider. Do you have an amp meter to know how much current that fridge is pulling on a typical summer day? Ditto, other medium to high current draw appliances.

If you camp where you get sun all day long and during summer/high angle sun months, you can probably just add more PV. If you camp during the lower angle sun months aka shoulder months and you have much sun each day you will probably be OK with doubling your PV and probably adding more battery capacity (another 100Ah LFP in parallel).

Camp with less sun, more clouds and/or November to March then you're going to need considerably more PV and battery capacity.

But, at least you have started real world testing!
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Old 08-05-2022, 10:44 PM   #3
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...
What happened next? After five minutes now with no load the Smart Solar AP indicated a significant rebound for the lithium battery.
...
Much to think about here so, decided instead to remove the screen from the Maxi Fan and give the unit a thorough cleaning. Another first for me, after nine years.
There is a clue in the fast rebound. Perhaps something like the old "surface charge" - the voltage in the 13 volt range is fleeting and the battery isn't working hard until it drops below 13 volts. Subsequently, the charge recovers quickly since you didn't really use a significant % of the battery capacity.


9 years before cleaning the screen? I know my wife wouldn't put up with that!
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Old 08-06-2022, 07:54 AM   #4
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In lifepo4 the discharge rate is non linear and goes like this:



So the first bit 14~13v is the matter of like 5% capacity but going from 13 to 12 is a more substantial amount. Also pulling any load will cause the voltage to drop while under load. Hard to get an exact read on real capacity and usage without a shunt like the victron smartshunt
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Old 08-06-2022, 09:23 AM   #5
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If you use the app for the 712, you can watch the current in & out of the battery at the same time you watch voltage. Check the current before you turn on the refrigerator (it will likely be a positive amount) and after you turn on the refrigerator - it will likely be negative. The difference will be close to what it draws.

You can also check the Consumed Amp Hours. Note what it shows when you start. It will probably increase while the refrigerator is on, showing what the refrigerator is using minus what the solar panel is producing.

While your single panel will more than likely not be able to keep up with the refrigerator on 12V, if you can use the Victron app to see how many amp hours are being used, giving you an idea of how long you can run it. You do have to consider how long it will take your solar panel to put back what you took out of the battery.
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Old 08-06-2022, 09:24 AM   #6
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In lifepo4 the discharge rate is non linear and goes like this:

So the first bit 14~13v is the matter of like 5% capacity but going from 13 to 12 is a more substantial amount. Also pulling any load will cause the voltage to drop while under load. Hard to get an exact read on real capacity and usage without a shunt like the victron smartshunt

And conversely, makers of drills and other home gadgets use this non-linear behavior to claim that their batteries are "20 Volts", when in the real world they are equivalent to an 18 Volt battery. Hype and marketing...
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Old 08-07-2022, 02:03 PM   #7
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make sure your battery is fully charged, turn the fridge on, let it run 2 days, then look at the 'History', it will tell you the total solar output for the last bunch of days, you specifically want to see the previous day. this will include the power it took to recharge the battery from the previous night plus the amount needed to keep the battery charged while the fridge is running, hence give you the total watt*hours.

here's one of those graphs from my typical total power usage while camping...

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Old 08-07-2022, 02:32 PM   #8
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Thanks for the report Myron. Our 8555 continues to chug along and in spring and fall camping on propane or on AC does an adequate job. However it will not do much more than 45degrees F cooling from Ambient. We have an Engel cooler also. We have transitioned to using the Engel for meat and “critical perishables. The 8555 we use for things that can do ok if the temp goes up beyond 40F. I am good with this operation for now. The Engel draw is such that our two flooded 6 volts can keep up on DC and of course are no problem when we’re on AC. I think if the 8555 conks out, we will replace it with a second Engel. I’ll figure that out when the time comes. The Engel cooler we have draws between .8 and 2.8 amp
Hours depemding on whether it’s cooling or just maintaining temp. Set on about “1.75” out of 5 it will run about 30-35 percent of the time and maintain 34F. We’ve had it several years now. We like it a lot. I can rotate my beer from the 8555 where I stock it at night and get it pretty cold by morning then put some in the Engel for afternoon consumption. If it gets real hot where we are camped, I go to straight 100 proof whiskey for my evening libation. Tastes great, less filling and does the job. Two votes for Engel.
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Old 08-07-2022, 02:43 PM   #9
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Another thing I really like about my Norcold compressor fridge, it makes great ice. that 100 proof whiskey on that 100 degree day would be sitting on a giant ice cube if it was mine (I keep three regular trays of ice and one giant cube tray in the freezer... as each regular tray is used up, it gets refilled with purified spring water and put on the bottom of the stack... usually frozen solid by next day).



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Old 08-07-2022, 03:31 PM   #10
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Now you’re talking. And you can always put a cube or a cold beer bottle on your
Forehead.
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Old 08-09-2022, 01:54 AM   #11
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Now you’re talking. And you can always put a cube or a cold beer bottle on your
Forehead.
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these days more and more of my craft beers come in cans, and I'm actually OK with that, since cans are a lot easier to recycle and transport and dispose of, they take less space in the fridge, etc etc.

that all said, I'd rather put the cold beer in a mug for consumption
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Old 08-09-2022, 07:55 AM   #12
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I bought some real good craft beer from the Toppling Giant brewery in Decorah, Iowa. It was in cans and I couldn’t tell any difference from the same beer from the tap. Though it’s not always condoned by beer lovers I like my beer pretty cold if I can get it.
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Old 08-13-2022, 08:57 AM   #13
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Ah, the things you learn after you dig a little. It puzzled me why my MPPT display unit wasn’t showing stuff about my solar/lithium battery configuration I saw you guys were talking about.
RE Jon: “…If you use the app for the 712, you can watch the current in & out of the battery at the same time you watch voltage …You can also check the Consumed Amp Hours. Note what it shows when you start… showing what the refrigerator is using minus what the solar panel is producing..”
I thought mine was the Victron smart solar 712, or 702, like yours. Actually, turns out it ain’t. My "smart solar monitor thing" is an earlier, less fancy version.

I got it when I got my (old) Morningstar Sunsaver Duo SSD-25RM controller. When I later upgraded to the Victron MPPT 100/30 the display still worked fine with it, so gave it no further thought… until reading this thread. Then saw no battery level pictogram on it, no mention of 712 or 702 on its face and, had no shunt. The thing reads battery levels just fine, just like the display on my Xantrex inverter, and using the Victron AP on my smart phone. I get readings of Amp hrs, and… phone software claims it’s the latest upgrade. Yet, some info seemed missing.

Question is, is it worth spending another $200+ for the 702, or the 712 version battery monitor, which includes getting that shunt?
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Old 08-13-2022, 09:24 AM   #14
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Ah, the things you learn after you dig a little. It puzzled me why my MPPT display unit wasn’t showing stuff about my solar/lithium battery configuration I saw you guys were talking about.
RE Jon: “…If you use the app for the 712, you can watch the current in & out of the battery at the same time you watch voltage …You can also check the Consumed Amp Hours. Note what it shows when you start… showing what the refrigerator is using minus what the solar panel is producing..”
I thought mine was the Victron smart solar 712, or 702, like yours. Actually, turns out it ain’t. My "smart solar monitor thing" is an earlier, less fancy version.

I got it when I got my (old) Morningstar Sunsaver Duo SSD-25RM controller. When I later upgraded to the Victron MPPT 100/30 the display still worked fine with it, so gave it no further thought… until reading this thread. Then saw no battery level pictogram on it, no mention of 712 or 702 on its face and, had no shunt. The thing reads battery levels just fine, just like the display on my Xantrex inverter, and using the Victron AP on my smart phone. I get readings of Amp hrs, and… phone software claims it’s the latest upgrade. Yet, some info seemed missing.

Question is, is it worth spending another $200+ for the 702, or the 712 version battery monitor, which includes getting that shunt?

The Victron BWM-7xx things are battery monitors, not solar controllers, and they all use a shunt to measure the current. Early versions did not have Bluetooth.
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Old 08-13-2022, 05:38 PM   #15
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OK Dave, John, Jon, the beers are on me.
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