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Old 05-23-2020, 10:05 AM   #1
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How can solar panels ONLY charge the battery?

I've heard many people claim that solar panels charge the battery ONLY and do not supply electrical loads in the RV. How is this possible?

A battery can only be in one state at a time - it is either charging or discharging. It can't be doing both at the same time. Whether it is charging or discharging depends upon the voltage of the bus it is connected to. If the voltage on the bus the battery is connected to is higher than the battery's internal voltage, then the battery is charging. If the voltage on the bus the battery is connected to is lower than internal battery voltage, the battery is discharging. Do I have that correct so far?

The charging controller connected to the solar panels charges the batteries by supplying a voltage (anywhere from 13.2 vdc to ~15.5 vdc) which is higher than the standard internal voltage of the battery (12.7 volts or less). But the charging controller is not connected to the batteries alone. It is connected to the entire DC power bus of the RV. Unless the solar charging controller is connected to the battery alone, and the controller and battery are disconnected from the rest of the RV DC system, I don't see how it is possible for the solar panels to ONLY charge the battery and not be supplying all the DC electrical loads at the same time.

Am I missing something?
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Old 05-23-2020, 10:55 AM   #2
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At the risk of getting my hair parted by the experts, I'll wade in . . .
Without getting into all the technically details on the electro-chemical reaction aspects of "charging" and "discharging", I think you're stuck on semantics.
The way I conceptualize it is as a hose, bucket and spigot where the charging source is a water hose, the batteries are a bucket, and the bucket has a spigot (in the case of FLA batteries, that spigot is halfway down the side of the bucket, for lithium, it's almost at the bottom of the bucket).
In the case of solar, the electrical charge coming off the panels is like a water hose fulling the bucket (batteries). The electrical current/draw required by all the appliances/devices hooked to your 12V system dictate how "open" you need to turn the spigot - the bigger the draw, the more you need to open the spigot and the more water rushes out of the bucket. In other words, your batteries are charging, represented by the "water coming through the hose" and they are discharging, represented by the "water leaving through the spigot".
If the amount of water coming into the bucket through the hose is more than the amount of water leaving through the spigot, your bucket is going to fill with water - you have a net charging situation, which will continue until your bucket is full and then the hose shuts off (yes, there are variances with bulk, absorb, float charges, etc., but this is the simplistic view . . . the solar controller will shut off until there's more room in the bucket for more water). If the flow through the hose is less than the outflow through the spigot, your bucket is gradually losing water - you have a net discharge situation, which will continue until the water level drops below the level of the spigot (e.g., 50% discharge for FLA), at which point "low battery" alarm/indication tells you it's time to fire up the genny! Theoretically, the amount of water from the hose could be exactly equal to the amount of water leaving through the spigot.
Now, to your question, is the water coming out of the spigot coming from the bucket? Or is it coming from the hose feeding the bucket? Does it matter?
I think the point of the common refrain of "solar doesn't power stuff, it just puts electricity into the batteries" is a shorthand way of saying you need to match your battery bank to your "water hose" and your "spigot." It does you no good to have a firehose feeding a 16 oz bucket with a 3" spigot (or, in terms of solar, a 1500W solar panel array to feed a loaded Class A with 4 TVs, microwave, and a bevy of other electrical consumption if you only have a 50 Ah battery . . . )
Hope that "Fisher-Price" version helps, but I'm happy to let the experts weigh in . . .
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Old 05-23-2020, 11:40 AM   #3
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Originally Posted by NavyLCDR View Post
I've heard many people claim that solar panels charge the battery ONLY and do not supply electrical loads in the RV. How is this possible?

A battery can only be in one state at a time - it is either charging or discharging. It can't be doing both at the same time. Whether it is charging or discharging depends upon the voltage of the bus it is connected to. If the voltage on the bus the battery is connected to is higher than the battery's internal voltage, then the battery is charging. If the voltage on the bus the battery is connected to is lower than internal battery voltage, the battery is discharging. Do I have that correct so far?

The charging controller connected to the solar panels charges the batteries by supplying a voltage (anywhere from 13.2 vdc to ~15.5 vdc) which is higher than the standard internal voltage of the battery (12.7 volts or less). But the charging controller is not connected to the batteries alone. It is connected to the entire DC power bus of the RV. Unless the solar charging controller is connected to the battery alone, and the controller and battery are disconnected from the rest of the RV DC system, I don't see how it is possible for the solar panels to ONLY charge the battery and not be supplying all the DC electrical loads at the same time.

Am I missing something?
You need to find a hobby. FISHING, GEOCATCHING, ETC. Why would anyone care about 12volts going anywhere.
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Old 05-23-2020, 12:08 PM   #4
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"I've heard many people claim that solar panels charge the battery ONLY and do not supply electrical loads in the RV. How is this possible?"

Yes, solar will supply power to the various accessories. I can disconnect my batteries and run the lights, exhaust fan. stereo etc.
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Old 05-23-2020, 12:28 PM   #5
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The point of solar for us is solely to charge the batteries, so we disconnect the entire load from the batteries via the master disconnect switch and dedicate every amp from the solar panels for recharging the batteries. This is why you see people wire the charge controller directly to the batteries.

Unless you have a lot of solar, there's not going to be a substantial amount of current available to run too many items anyways, batteries are designed for high amp loads so it makes more sense to charge the batteries via solar and then let them carry the load once charged.
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Old 05-23-2020, 01:00 PM   #6
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OK, as an Electrical Engineer by education, this response and the others thus far, pretty much hit the proverbial nail on the head. BTW, I like the bucket and water source analogy. It’s something I would probably utilize as an explanation to non-techies. Good on you!

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Old 05-23-2020, 01:49 PM   #7
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charging

First of all, and as simply as one can think about it, you do NOT draw your power to appliances or plugs FROM the panels. You draw from the inverter, which either draws from shore power, generator power or from the batteries. The panels only charge the batteries. They do not hook into the inverter or any other point of your system except your batteries. They are ONLY hooked up to the batteries. If the sun is out, they will charge up to full, and then change to trickle (based on the charge controller) and switch back up to full charging based on if you start drawing a lot of power from the batteries (assuming you aren't plugged into shore power.
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Old 05-23-2020, 02:10 PM   #8
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^ WRONG

First off your inverter does not supply 12 volts, your converter does.
My solar system is connected to the converter output. So it can run my 12 volt accessories with the battery disconnected when the sun is shining.
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Old 05-23-2020, 02:18 PM   #9
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^ WRONG

First off your inverter does not supply 12 volts, your converter does.
My solar system is connected to the converter output. So it can run my 12 volt accessories with the battery disconnected when the sun is shining.
Just out of curiosity, if you are boondocking, why would you disconnect your battery from the solar? Donít you want to charge the batteries for night use, when the sun is not shining?
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Old 05-23-2020, 02:25 PM   #10
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NOT WRONG! The appliances and plugs he mentions use 120 volt power, and it comes from the inverter. The INVERTER uses 12 volt power from the battery and/or inverter and converters it to 120 volt. But I know what you meant. Lol
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Old 05-23-2020, 02:32 PM   #11
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Does this mean...?

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Originally Posted by 01tundra View Post
The point of solar for us is solely to charge the batteries, so we disconnect the entire load from the batteries via the master disconnect switch and dedicate every amp from the solar panels for recharging the batteries. This is why you see people wire the charge controller directly to the batteries.

Unless you have a lot of solar, there's not going to be a substantial amount of current available to run too many items anyways, batteries are designed for high amp loads so it makes more sense to charge the batteries via solar and then let them carry the load once charged.
Does this mean that you occupy the trailer with the propane, CO, and smoke alarms disconnected? Is that a good idea?
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Old 05-23-2020, 02:38 PM   #12
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Just out of curiosity, if you are boondocking, why would you disconnect your battery from the solar? Don’t you want to charge the batteries for night use, when the sun is not shining?
I do not disconnect the batteries. I am saying that the solar panels not only charge batteries but can and will supply 12 volts to accessories in the trailer with out batteries connected.
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Old 05-23-2020, 02:44 PM   #13
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First of all, and as simply as one can think about it, you do NOT draw your power to appliances or plugs FROM the panels. You draw from the inverter, which either draws from shore power, generator power or from the batteries. The panels only charge the batteries. They do not hook into the inverter or any other point of your system except your batteries. They are ONLY hooked up to the batteries. If the sun is out, they will charge up to full, and then change to trickle (based on the charge controller) and switch back up to full charging based on if you start drawing a lot of power from the batteries (assuming you aren't plugged into shore power.
Might want to look at your wiring diagram, there. Because I will bet that you have two wires from the solar panel charging controller to the batteries. Then I will bet that you have two wires from the batteries to the input of the inverter (more than likely through a disconnect switch). Now, pull the terminals off the batteries. Leave everything else connected. What do you end up with? Your solar panel charging controller connected directly to the input of your inverter.

Now, if things are as you claim they are, when you are boondocking and want to charge the batteries with the solar panels, you are without electricity in the RV. Because your solar panels are connected only to the batteries and not connected to any other part of the electrical system (which can be accomplished with the battery disconnect switch). Then when you want to have electricity, you must disconnect your batteries from the solar panels and connect the battery to your inverter - because if what you state is true, you cannot have your solar panels connected to the electrical system. And that may very well be the way you operate. But most people I know do not disconnect the batteries from their RV, even during the day when they are charging with the solar panels.

And this is the whole reason for my asking.
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Old 05-23-2020, 02:48 PM   #14
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It's really simple. Solar panels produce energy that's fed into the 12v system. If they produce more than what is being consumed the excess goes to the batteries until they're charged at which point the controller shuts down the incoming power.

Since solar panels produce energyall the time there is sun on them, and power is often consumed in short intervals, batteries spend most of their daylight hours charging.

If one has 300 watts of solar panels that's potentially 300 watt hours or in 8 hours 2.4 Kwh. If less than 2.4Kwh is used from the batteries in a 24 hour period, the batteries get fully charged during the next day's sun.

There are inefficiencies in Lead Acid batteries so you'll actually need to use less than the 2.4 Kwh but even in cool weather with lots of furnace use my max power use runs under 1.2 Kwh. (100 ah from 2 Battleborns)

I charge them with a 160 watt suitcase and during Summer months it does very well.
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Old 05-23-2020, 03:24 PM   #15
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...when I respond to these questions about 'solar', and make a statement that 'solar is not powering anything in your RV directly...', I am simply referring to the fact that the solar is not going directly to a fuse panel or electrical panel, like 120v power does - solar power is only going 'DIRECTLY' to your Batteries, to charge them as they are needed to be charged.

If you want to get technical, about the issue of whether a battery can be 'at use' at the same time it is 'being charged', then that's something of a different matter. Some will say that this is 'really' your solar power indirectly powering your 12v systems, but it is not WIRED directly to them - in essence, without your battery bank, solar would not be a good source or 'consistent' power - that's what your batteries are for.
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Old 05-23-2020, 04:27 PM   #16
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In a nutshell, yes, the batteries are connected to your RV’s 12 volt system, 120 volt inverter is as well. (Via the batteries) Also connected to your batteries, is the charge controller from the solar, as well as your built in RV charger. With no electrical hooked into the RV, the main charger sits there and does nothing.

Assume full charge on your batteries. If it is daylight, and solar is capable of putting out 10 amps, you run the fans, say they are drawing 8 amps, (2 amps may also be available to be used for the batteries if needed) In this case, then all the current is really coming from the solar panels. Same idea for the inverter. Depends on the current draw.

Increase that loading to 15 amps, solar will supply about 10, battery supplies about 5. There are some losses and minor differences, but that is what happens. So, basically during daylight hours, solar will be in play to save the draws from the battery. If you have a larger solar setup, then most everything up to the solar size limits or available sunlight will be fed from the solar controller, thus trying to keep the battery full until dark.

When you connect to electrical, the main charger will run most everything on the 12 volt side, while the charge controller for the solar will probably not be doing much, unless it sees a larger demand than the main converter can handle.

Another way of looking at the garden hose idea posted earlier.
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Old 05-23-2020, 04:40 PM   #17
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...when I respond to these questions about 'solar', and make a statement that 'solar is not powering anything in your RV directly...', I am simply referring to the fact that the solar is not going directly to a fuse panel or electrical panel, like 120v power does - solar power is only going 'DIRECTLY' to your Batteries, to charge them as they are needed to be charged.

If you want to get technical, about the issue of whether a battery can be 'at use' at the same time it is 'being charged', then that's something of a different matter. Some will say that this is 'really' your solar power indirectly powering your 12v systems, but it is not WIRED directly to them - in essence, without your battery bank, solar would not be a good source or 'consistent' power - that's what your batteries are for.
However, in reality, the solar power does go directly to the reverse polarity protection fuses on the converter, as long as your batteries are also connected to the converter. The power from the solar panels does not go through the battery to get to the converter. The power from the solar panels goes directly to the converter, and distributed throughout the RV through the battery fuse. The battery is connected in parallel in the circuit, not in series. Follow the line from the solar panel below, to the terminal block on the battery and back up into the converter. If you pull the terminal block off the battery with removing any other wires, you have a direct connection to the converter.

But, I guess if people want to still think and claim that their solar panels go directly to their batteries, not the converter, and then it's the battery that goes to the converter, all hooked up together, then so be it. But any basic level electrician will have a different opinion.

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Old 05-23-2020, 04:47 PM   #18
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Now back to our regular program.
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Old 05-23-2020, 04:59 PM   #19
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Old 05-23-2020, 06:18 PM   #20
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Wow . . . and I thought my Post #2 was overboard . . .

Just remember, all those electrons are irrelevant to toasting marshmallows!!!
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