Total newbie trying to figure this out

Pcoch

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Jun 16, 2021
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I'm having trouble with my first time set up. I have a Coleman 120w panel, going to a 8.5a controller, to a 12v deep cycle marine battery. I'm using a 2000w inverter, and trying run an RV fridge off it. The fridge says it's 440w which seems high for such a small fridge, but even when running on gas, each time it tries to kick in I get a low voltage squeal from my inverter. Is this system to underpowered for the fridge? Plugging in my 150w laptop will also cause a low voltage warning in a matter of minutes.
 

Bob B

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Sounds like your battery is failing or not getting charged ..... You need to do some DVM readings of the battery voltage when you think it is full charged and then what it falls to when powering your devices.
 

Pcoch

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Sounds like your battery is failing or not getting charged ..... You need to do some DVM readings of the battery voltage when you think it is full charged and then what it falls to when powering your devices.
The inverter reads it 12 and so does my analog voltmeter. When the fridge kicks in it drops to 10-10.4 and then jumps back up to 11.7, it will keep doing this every minute or so until I unplug to fridge. The battery is less than a month old and only sat in my garage for a couple weeks before being put to use.
 

MichaelK

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A good rule of thumb is a lead-acid battery should not be drained at much more than 1/10th of C. If your battery is 100Ah at 12V, then 1/10 of C is 10Ah. At 12V that's a 120W max load to keep a happy battery. Since your frig is 440W, that is much higher than what your battery wants. So, yes, your system is too small.

Your low voltage numbers are an indicator that the load is too high, and that 120W panel is not keeping the battery fully charged. Is the battery laying flat on the roof of your RV? Flat, it might be putting out 60% of potential output. So, that 120W panel is likely only putting out 120W X 0.6X = 72W.

Let's do the math to design something workable. Getting back to 1/10C....
440W/12V = 36.6A X 10fold =366Ah battery.

Batteries like charging at 1/8C, so for a 366Ah battery you need... (366Ah/8) X 13V charging X 1.2FF = 714W. The 1.2FF is for a panel directly facing the sun. For a flat panel the FF is more like 1.6. So, (366Ah/8) X 13V charging X 1.6FF = 952W.

So, the answer is bigger batteries, and a lot more solar panels.
 

Bob B

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The inverter reads it 12 and so does my analog voltmeter. When the fridge kicks in it drops to 10-10.4 and then jumps back up to 11.7, it will keep doing this every minute or so until I unplug to fridge. The battery is less than a month old and only sat in my garage for a couple weeks before being put to use.
At 12V, the battery is nowhere near full charge .... actually it is at a very low SOC.
 

Rider

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Is the solar panel also 12VDC (nominal)? If so, the voltage may be too low for it to charge the battery. Typically, the solar panel should be 2-5 volts above the battery voltage to work. If a full SOC of the battery is 12.6V (thinking lead acid), the panel should be pushing 15-18 volts when conneted to the SCC and battery. if it's not, you're not really charging the battery.
 

Pcoch

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Jun 16, 2021
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A good rule of thumb is a lead-acid battery should not be drained at much more than 1/10th of C. If your battery is 100Ah at 12V, then 1/10 of C is 10Ah. At 12V that's a 120W max load to keep a happy battery. Since your frig is 440W, that is much higher than what your battery wants. So, yes, your system is too small.

Your low voltage numbers are an indicator that the load is too high, and that 120W panel is not keeping the battery fully charged. Is the battery laying flat on the roof of your RV? Flat, it might be putting out 60% of potential output. So, that 120W panel is likely only putting out 120W X 0.6X = 72W.

Let's do the math to design something workable. Getting back to 1/10C....
440W/12V = 36.6A X 10fold =366Ah battery.

Batteries like charging at 1/8C, so for a 366Ah battery you need... (366Ah/8) X 13V charging X 1.2FF = 714W. The 1.2FF is for a panel directly facing the sun. For a flat panel the FF is more like 1.6. So, (366Ah/8) X 13V charging X 1.6FF = 952W.

So, the answer is bigger batteries, and a lot more solar panels.
Thanks for breaking it down and crunching the numbers! I'm still figuring out what a volt amp and watt are so this was very helpful. Would a faster charge controller be a worthwhile investment as well? I see most folks on here recommending a 40a
 

MichaelK

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Would a faster charge controller be a worthwhile investment as well? I see most folks on here recommending a 40a
There are no faster or slower charge controllers. There are controllers with higher and lower amperage capacity. If you connect one 100W panel to a 20A charge controller it might put out 7.7A. If you connect one 100W panel to a 100A charge controller, it will produce the same 7.7A.

But, you could connect ten 100W panels, with all together putting out 77A. You can't do that with the 40A controller. That's what makes it faster. Note: these are theoretical numbers, which are not necessarily what you will see in the real-world.

The cheaper economy controllers talked about here max at at about 100V and 40A. No, I'm not recommending them any more. Focus on controller with higher amperage ratings and higher max voltages. You gain a lot by putting panels in series for higher voltage. I'd suggest starting out with Epever's Tracer 5415 as the bottom end.
 
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