RV Question about solar panels, controllers, and batteries

So I have asked this in an RV forum and did not get the response I was hoping for. So I thought I would post it here.


I keep hearing that you need to disconnect your panels from your controller if you are going to remove your batteries. I understand that if you do not you will have live wires in the battery bay after you remove them. But I keep seeing the myth that your solar controller will burn up if you do that because it will convert the energy from the solar panels into heat and cook itself. So if you can help me understand something, I would greatly appreciate it.


Myth or Fact: If you do not disable or disconnect the solar panels from the solar controller, you risk burning up your controller if a battery is not also connected to the controller.


I can see the logic in the myth and don't necessarily doubt that this could happen with some controllers.


But, what is the difference in the scenario where your batteries are disconnected and unable to absorb the engery and the batteries being 100% SOC and unable to absorb the energy?


If the controller somehow manages the panel side of the circuit and opens it somehow when the batteries are full, why doesn't it fail the panel circuit open when batteries are not present to protect itself? Or does this already happen in some fashion and the myth remains a myth?


I am hopeful someone can explain the how and why here. I can and will test it sometime myself but am hopeful someone else has done this or has specific knowledge.


My intention when I have the trailer out next is to measure voltage on the controller to negative battery terminal while it is disconnected in the sun and see if voltage is flowing into the batteries when the controller is powered off since I think it is powered from the batteries and not the solar (is this generally true?). That should tell me whether the controller is disconnecting the panels when powered off. I will also pull the controller out and try to measure heat on the unit / heatsink.

For particulars about my setup:
Factory installed Go Power 570w kit with non-bluetooth PWM 30 amp controller and 3x190w panels and 1500w inverter with transfer switch. Everything there is factory and connected to 2x100ah GoPower LFP batteries which are also non-bluetooth. I do have a Victron smartshunt with bluetooth.

If anyone has any input to any of this, or has already done this type of thing, please please let me know .

Thanks all

Depends on the controller. Some have very specific warning.

I believe the issue is at higher current flowing through an MPPT and the controller may not have time to adjust for a battery disconnect. Will it automatically always destroy the controller, NO. If the batteries are floating there should not be an issue.

It is a FACT that it is possible and has happened sometimes, but most of the time, no, no issues will occur. The danger is to the controller itself and not your lack of handling live wires.
I advise ALWAYS disconnect the solar panel first before disconnecting the battery (or anything else), everyone advises this, and this is good practice to be sure you are not the 1 that learns the hard way.

Forget about trying to measure the controller output. Unless you are doing a scientific experiment, just rely on the parameters of the controller to tell you what the max output will be. If you have major issues, then maybe you can test, but without a battery present your measurements will be off anyway.
The controller should have the info you need, even if you have to do internet search.
The controller will vary its output based on the state of charge of the battery - a dead battery gets lots of voltage and a full battery gets nearly none.

The solar panel will output more voltage than you think (though they usually say it on the sticker). A 12 volt 100w panel will put out 18.x volts. The controller fixes it and sends the right voltage to the battery. The same solar panel might put out higher voltage which might be too much for the controller to handle without a battery present. IF for some strange reason the panel spikes and that is when you dont have a batt, then you will be buying a new controller. Dont be that guy.
You can run panels in series and get 38 volts going to a controller that changes it to charge a 12 v battery too. Depends on the controller.

I go as far as to put a switch (DC breaker) on the positive lead coming off the panel, so I can easily disconnect the panel and reconnect safely.

What is the difference between batteries being disconnected and being full in this case? What prevents the solar power from being lost as heat if it cannot be absorbed by the full battery?

@0truck0 - I am not trying to be mean, but there is not a lot in your post that is super helpful here. I am approaching this is a more scientific manner to try to understand the why and the how, but not the what necessarily. The actual what, not what everything thinks, is ultimately what I am trying to prove or disprove. But to feel good about the result, I need to try to understand the why. I do appreciate your input.

It is not the matter of being able to absorb the PV energy as much as it screws the logic of the controller that uses the battery voltage to set proper charging conditions as it powers itself from the battery. Disconnecting the battery removes battery voltage, being fully charged does not.

However many can survive battery disconnection and re-connection while PV is available.

Ok, so in your opinion, there is risk to burning up the controller then, it is a logic issue. And the logic issue can likely be solved by a power cycle I would think. Would you expect the controller to lose power if you disconnect the batteries or can the controller also be powered by the PV? I get this may differ between controllers. I am trying to figure out why there are differences, and if vendors are just being cheap and putting folks at risk, I would like to understand why and how they might do that (money I am sure...). Thanks for your insight.

datdude said:

What is the difference between batteries being disconnected and being full in this case? What prevents the solar power from being lost as heat if it cannot be absorbed by the full battery?

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If the battery is full and very little power is flowing the controller has already adjusted to restrict power to near zero. The solar power is not moving anywhere.

As for disconnected.... my controller will not power up until the battery is connected. Manual is very specific to connect to battery before connecting to the panels. Am I going to risk $700 to test every possibility? No.

Best to read and follow the operating instructions for your controller. Don't get electrical information from FB.

But the PV panels are still generating power from the sun, even when the batteries are full, right? As long as the sun is shining the PV panels produce power I think. Those are definitely more questions than answers . Where does that power go it if cannot flow into the batteries? I am here asking, not facebook. ?? Facebook, Reddit, RV Forums all spread misinformation. I am trying to better understand all of the technicals so I can defend my thoughts when I tell them stuff like this is a myth if it is a myth. My controller is NOT $700 lol. Neither are the majority of them out there in RVs. Most are similar to mine, PWM 30a controllers.

ETA: I have found a new google search and am back to reading on my own as well. I really don't expect or even want to be spoon fed the information if I can figure out how to find it myself.

Ok, so it seems if the batteries are full, the controller should sent the power back to the PV and the PV will lost that to heat. Since they are relativelty ineffecient, the inflow of power doesn't appear to hurt them. This causes an increase in voltage during this. I wonder if the same happens when the batteries are removed from the controller then, if there is some default configuration of the solar controller when no power is fed via the batteries. Goes back to the idea I need to see if power is flowing down the output of the controller when disconnected from the battery. Still reading to make sure I understand all of that.

ETA: I probably mischaracterized the power flow. I think the power doesn't return to the PV, it just never actually leaves and is lost as heat in the PV. Sorry I am so dumb on this stuff. But I'm trying to get smarter .

PV panels produce nothing without a load. No current = no power.

datdude said:

But the PV panels are still generating power from the sun, even when the batteries are full, right? As long as the sun is shining the PV panels produce power I think. ...

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PV panels without load only produce voltage. We call that Voc (Volt open cell). This voltage amount is higher than Vmp (Volts max production) which is the panels voltage when it is creating power under an amperage draw. The amperage rating of the panels is a similar thing Isc (Current short circuit) is an amperage rating the panel can reach if you short circuit it (connect the pos and neg leads together). Imp (current max production) is the current the panels will deliver at Vmp and is how the panels rated watts is derived W=VA Example a 100w panel has a Vmp of 18v and a Imp of 5.55a this equals 100w.

Panels sitting in the sun without load are not much different than any other object sitting in the sun. They do have potential (voltage) and could produce power if loaded.

datdude said:

If you do not disable or disconnect the solar panels from the solar controller, you risk burning up your controller if a battery is not also connected to the controller.

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This may happen with low cost PWM controllers like the unit provided with your kit. More advanced controllers are usually protected against panel disconnect. In fact the Victron range of controllers happily operate without battery connection. ( once initially powered with a battery to determine operating voltage).
Your PWM controller needs a battery connection to enable its circuits to determine system volts. By disconnecting the battery with solar connected it may set a 24 volt charge profile, not good for your system on reconnecting.
A PWM controller is , in simple terms, an electronic switch that connects the panels to battery.
Using PWM controllers with lithium batteries could cause reduce battery servicelife due to less than ideal charging. A chage to a MPPT controller would give better charging profile and more power from the panels. Victron smart 100/50 recomended.

datdude said:

need to see if power is flowing down the output of the controller when disconnected from the battery.

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Without a circuit no power can flow. The PWM controller is a switching circuit that connects panel to battery, powered and controlled by the battery.

datdude said:

PV panels are still generating power from the sun, even when the batteries are full, right?

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No, not really. Solar panels produce current that can flow through a circuit, positive to negative terminal on the panel. If this circuit is broken, the PWM will effectively do this when the battery is full, there is no current and thus no power.

datdude said:

think the power doesn't return to the PV, it just never actually leaves and is lost as heat

Click to expand...

The solar cell is a current generator when in sunlight. If there is no circuit, positive to negative, no current, no power. Power equals volts x current.

Thanks all for the comments. It is not helping with the original question since it does sound like it depends on the controller lol. It sounds like some might be damaged from this and some will definitely not be damaged. It can mess with the logic in the controller and cause it to make unwanted changes if they try to autodetect settings.

And I REALLY appreciate the extra help in explaining things. It gives me more to google and search on the forums.

BTW, if anyone has some sites they like for learning this stuff they can send I would also appreciate that. It is hard when you don't have a lot of technical knowledge to determine how good the information a site has really is. Especially if the site is tied to someone trying to sell you something.

datdude said:

Thanks all for the comments. It is not helping with the original question since it does sound like it depends on the controller lol. It sounds like some might be damaged from this and some will definitely not be damaged. It can mess with the logic in the controller and cause it to make unwanted changes if they try to autodetect settings.

And I REALLY appreciate the extra help in explaining things. It gives me more to google and search on the forums.

Click to expand...

Your original question was answered. If you wish to learn more about solar the Forum here has many resources you can look through as well as simply asking.

Incidentally it is always a good idea to read the manual that comes with any equipment you buy. It should tell you proper procedure for operating it. For instance if your PWM SCC says you must hook up battery first than PV than that is what you do.

I said that because of the statement that some controllers can handle it and others may not be handling it properly. IE, there is not a clear answer other than "it depends."

i have read the manual - they are very light on any technical details.

You got the answer that you asked. If you want specifics on your particular model and not the general reason why, then need to contact manufacturer or research the details further.

datdude said:

So I have asked this in an RV forum and did not get the response I was hoping for. So I thought I would post it here.


I keep hearing that you need to disconnect your panels from your controller if you are going to remove your batteries. I understand that if you do not you will have live wires in the battery bay after you remove them. But I keep seeing the myth that your solar controller will burn up if you do that because it will convert the energy from the solar panels into heat and cook itself. So if you can help me understand something, I would greatly appreciate it.

Click to expand...

Nope, this would be a bad myth and one I never heard. Controllers are more like a light switch here, they can turn off the power, they can reduce the power also (dimmer switch)

datdude said:

Myth or Fact: If you do not disable or disconnect the solar panels from the solar controller, you risk burning up your controller if a battery is not also connected to the controller.

Click to expand...

Never heard this one, seems very out there. The controller will be able to handle the normal power from the panel. The risk is errant spikes as mentioned above. The battery is a safety sponge in the system.

datdude said:

I can see the logic in the myth and don't necessarily doubt that this could happen with some controllers.


But, what is the difference in the scenario where your batteries are disconnected and unable to absorb the engery and the batteries being 100% SOC and unable to absorb the energy?

Click to expand...

I cant see the connection here. A full battery is nothing at all like an absent battery, you are way confused on the basics if that is the question, again, not to be mean... When batteries are at 100% then the controller will either turn off the "light switch" and let no current flow, or will "float" charge with a reduced voltage. You can charge a battery forever at a reduced voltage and not harm or over charged the battery. There are several charging profiles that your controller will go through as it charges then maintains the battery.

datdude said:

If the controller somehow manages the panel side of the circuit and opens it somehow when the batteries are full, why doesn't it fail the panel circuit open when batteries are not present to protect itself? Or does this already happen in some fashion and the myth remains a myth?

Click to expand...

WIth a missing battery or a missing panel only there is no fault, no open circuit as u suggest. Some controllers will power themselves from a panel only, some will not turn on unless there is a battery. If there is no battery then the controller knows this and will not send volts out, and will not try, and cannot even bc there is nothing there, the open circuit on battery side...
The question is worded poorly and is almost an invalid question bc of this. IDT you understand enough to ask, not to be mean... The controller manages everything, it does not "somehow" manage. Some controllers are not designed to take over-voltage spikes without a battery to dump the spike into, instruction tell you to disconnect panel first before battery. Some controllers are designed such.

datdude said:

I am hopeful someone can explain the how and why here. I can and will test it sometime myself but am hopeful someone else has done this or has specific knowledge.

Click to expand...

good luck. My controller is not designed to work with only solar panel. I have many times disconnected the battery with panel still connected and not had any issues. All by accident and I do better now and never have this happen anymore bc of the potential risk and per the manufacturers instructions. For detailed reasons I cannot provide the info, manufacturer can if they are willing and speak english; they are not and do not.
If you test a panel getting 100% full sun under perfect conditions, yours 100watts, then you will get likely 18.x volts, close to what the sticker says on the back of pansl What does this mean? It means you have a working panel. Tilt the panel and you get the same voltage! The key is that a tilted panel will give less amps at the same voltage. Test away, you will learn little. The controller is what is changing the voltage and amps and you cannot really test this without just using a monitor, and need batteries of different SOC to see what controller does at each SOC.

datdude said:

My intention when I have the trailer out next is to measure voltage on the controller to negative battery terminal while it is disconnected in the sun and see if voltage is flowing into the batteries when the controller is powered off since I think it is powered from the batteries and not the solar (is this generally true?). That should tell me whether the controller is disconnecting the panels when powered off. I will also pull the controller out and try to measure heat on the unit / heatsink.

Click to expand...

As mentioned already, it does not work like this. Controller is smart. Controller will ONLY send the amount of voltage to the battery as necessary and needed by the battery. The controller will monitor the battery many many many times per second to determine SOC. Measuring current from a controller to a dead battery will show full volts (14.6 probably) and to a charged battery less voltage (13.x). These parameters can usually be programmed in by the user via something like an MT50 or other device, or high-end models will be controllable right at the controller's interface. Some controllers have few or no adjustments.

datdude said:

For particulars about my setup:
Factory installed Go Power 570w kit with non-bluetooth PWM 30 amp controller and 3x190w panels and 1500w inverter with transfer switch. Everything there is factory and connected to 2x100ah GoPower LFP batteries which are also non-bluetooth. I do have a Victron smartshunt with bluetooth.

Click to expand...

PWM is a little better in low light and might turn on sooner than MPPT, but a PWM factory option usually signals cost-cutting by the manufacturer as PWM is cheaper than MPPT. Bluetooth has no significance to your question.

datdude said:

If anyone has any input to any of this, or has already done this type of thing, please please let me

know .
.

Click to expand...

Done what? Unhooked the battery first? Yes, many have inadvertently done this. Some ruined their controllers, some did not

datdude said:

Thanks all

Click to expand...

Some controllers are made to not have a battery, most need the battery. If you want us to tell you that you can ignore the instructions and use your panels with no battery then wait, someone will eventually tell you that you can. If you follow that one then have a spare controller on hand.

I cant help further. This is not a complicated subject. Disconnect your solar panels first, and reconnect your solar panels last when working on your system.

Panels do nothing when not hooked up to a source, they are about as useless as a light bulb sitting on top of your circuit breaker box - does nothing, makes no light, has no light.... There is no good analogy for this but there ya go

Unclear exactly what you hope to gain in knowledge. You have a small panel and a small battery and limited knowledge.... keep reading through different forum subjects and you will eventually learn more from that.

0truck0 said:

You got the answer that you asked. If you want specifics on your particular model and not the general reason why, then need to contact manufacturer or research the details further.



Nope, this would be a bad myth and one I never heard. Controllers are more like a light switch here, they can turn off the power, they can reduce the power also (dimmer switch)

Never heard this one, seems very out there. The controller will be able to handle the normal power from the panel. The risk is errant spikes as mentioned above. The battery is a safety sponge in the system.


I cant see the connection here. A full battery is nothing at all like an absent battery, you are way confused on the basics if that is the question, again, not to be mean... When batteries are at 100% then the controller will either turn off the "light switch" and let no current flow, or will "float" charge with a reduced voltage. You can charge a battery forever at a reduced voltage and not harm or over charged the battery. There are several charging profiles that your controller will go through as it charges then maintains the battery.

WIth a missing battery or a missing panel only there is no fault, no open circuit as u suggest. Some controllers will power themselves from a panel only, some will not turn on unless there is a battery. If there is no battery then the controller knows this and will not send volts out, and will not try, and cannot even bc there is nothing there, the open circuit on battery side...
The question is worded poorly and is almost an invalid question bc of this. IDT you understand enough to ask, not to be mean... The controller manages everything, it does not "somehow" manage. Some controllers are not designed to take over-voltage spikes without a battery to dump the spike into, instruction tell you to disconnect panel first before battery. Some controllers are designed such.

good luck. My controller is not designed to work with only solar panel. I have many times disconnected the battery with panel still connected and not had any issues. All by accident and I do better now and never have this happen anymore bc of the potential risk and per the manufacturers instructions. For detailed reasons I cannot provide the info, manufacturer can if they are willing and speak english; they are not and do not.
If you test a panel getting 100% full sun under perfect conditions, yours 100watts, then you will get likely 18.x volts, close to what the sticker says on the back of pansl What does this mean? It means you have a working panel. Tilt the panel and you get the same voltage! The key is that a tilted panel will give less amps at the same voltage. Test away, you will learn little. The controller is what is changing the voltage and amps and you cannot really test this without just using a monitor, and need batteries of different SOC to see what controller does at each SOC.

As mentioned already, it does not work like this. Controller is smart. Controller will ONLY send the amount of voltage to the battery as necessary and needed by the battery. The controller will monitor the battery many many many times per second to determine SOC. Measuring current from a controller to a dead battery will show full volts (14.6 probably) and to a charged battery less voltage (13.x). These parameters can usually be programmed in by the user via something like an MT50 or other device, or high-end models will be controllable right at the controller's interface. Some controllers have few or no adjustments.

PWM is a little better in low light and might turn on sooner than MPPT, but a PWM factory option usually signals cost-cutting by the manufacturer as PWM is cheaper than MPPT. Bluetooth has no significance to your question.

Done what? Unhooked the battery first? Yes, many have inadvertently done this. Some ruined their controllers, some did not

Some controllers are made to not have a battery, most need the battery. If you want us to tell you that you can ignore the instructions and use your panels with no battery then wait, someone will eventually tell you that you can. If you follow that one then have a spare controller on hand.

I cant help further. This is not a complicated subject. Disconnect your solar panels first, and reconnect your solar panels last when working on your system.

Panels do nothing when not hooked up to a source, they are about as useless as a light bulb sitting on top of your circuit breaker box - does nothing, makes no light, has no light.... There is no good analogy for this but there ya go

Unclear exactly what you hope to gain in knowledge. You have a small panel and a small battery and limited knowledge.... keep reading through different forum subjects and you will eventually learn more from that.

Click to expand...

But if the solar panels create power whether or not it is needed (they have no indicator of load need), they create power 100% of the time they are in the sun. If that power cannot flow into the battery, it MUST be dissipated as heat somewhere, whether that it in the controller, the path, or more likely within the PV. If PV is just left alone with nothing plugged in, the panel itself will be hotter than it would if it was able to deliver its power somewhere. PV constantly creates voltage in sunlight and must send that energy somewhere.

ETA: this could also be dissipated in the heatsink on the controller. Some have rather large heatsinks. They must shed some power into that or they wouldn't need it (unless it is somehow needed for the PWM feature and is not present on MPPT or something.

And for everyone's benefit, everything I say should be considered as much a question as a statement lol.