What Happens to Solar Power When Batteries Are Full

My question is more than likely a good ole' rookie question, but I'm not too scared to ask. So, lets say we have a solar system, regardless if portable or fixed, but both have battery storage and solar panels charging them. Let's say the batteries are at 100% charge, and there are devices (let's say a computer, TV, etc . . ) that are directly connected to the system. If the batteries are at 100% charge but there is plenty of sunlight charging the panels, do the systems directly bypass the batters and provide power to your plugged-in devices? Alternatively, does it constantly take from the battery, and then the battery is constantly being recharged? If it is the latter, does this reduce battery life? Thanks, I hope the question is not too confusing to understand; I am a rookie at this. Sorry if this is a repost but I tried searching other post first. . . No luck for me.

The charge controller pushes current into batteries and/or inverter, depending on inverter load. If batteries are full, charge controller will only supply what inverter is demanding.

Well, the battery is a buffer, but there's no difference between "Solar supplies the load" and "Solar charges the batteries while at the same time the batteries supply the load".

Around here we call a solar power spill a sunny day. 8*)

underdog5004 said:

The charge controller pushes current into batteries and/or inverter, depending on inverter load. If batteries are full, charge controller will only supply what inverter is demanding.

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Underdog, thanks for replying to the Bulldogg . . . I just want to clarify: so you are saying, "Yes, the Solar panels will bypass the battery through the inverter; the inverter will supply the power 'things' that it is requesting to be supplied as long as that power is available to be supplied directly to it."

wpns said:

Well, the battery is a buffer, but there's no difference between "Solar supplies the load" and "Solar charges the batteries while at the same time the batteries supply the load".

Around here we call a solar power spill a sunny day. 8*)

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Please let me see if I get this right . . so if I have 800 watts being produced by the solar panels, but my batter charger only can take in 400 watts, the solar will charge it (if not at capacity) and the other 400 watts will be used to power any electronics connected need a portion of that remaining 400 watts? This stuff is exciting to me, just a kid in a Candy store I never been in. . . LOL ! ! !

Bulldogg said:

Please let me see if I get this right . . so if I have 800 watts being produced by the solar panels, but my batter charger only can take in 400 watts, the solar will charge it (if not at capacity) and the other 400 watts will be used to power any electronics connected need a portion of that remaining 400 watts? This stuff is exciting to me, just a kid in a Candy store I never been in. . . LOL ! ! !

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Let's say you have 800w potential power production from your panels. At the start, maybe your batteries are depleted from powering loads last night. So, the charge controller will push 800w of power into your batteries, less whatever loads are present on the inverter. Let's say you have 400w of loads on the inverter. So, 400w from panels are supplying the inverter, a d the other 400w are going into the batteries. As the batteries fill, eventually they take less power, ultimately 0w. As the "battery load" decreases (but inverter loads remain constant), less power will be pulled from the solar panels. Think of it like water, the power follows the path of least resistance, but as load requirements fall below potential power production, power production will be curtailed by the charge controller.

underdog5004 said:

Let's say you have 800w potential power production from your panels. At the start, maybe your batteries are depleted from powering loads last night. So, the charge controller will push 800w of power into your batteries, less whatever loads are present on the inverter. Let's say you have 400w of loads on the inverter. So, 400w from panels are supplying the inverter, a d the other 400w are going into the batteries. As the batteries fill, eventually they take less power, ultimately 0w. As the "battery load" decreases (but inverter loads remain constant), less power will be pulled from the solar panels. Think of it like water, the power follows the path of least resistance, but as load requirements fall below potential power production, power production will be curtailed by the charge controller.

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Underdog5004 - Thanks so much for the explanation.

Typical off-grid charts explains the situation:

Before the next solar production begins at about 6am, the batteries had a remaining SOC of 62%. In my situation it took from 6am to 12pm to charge the batteries from the remaining 63% to 100% SOC and in this time, the maximum possible power was retrieved from the PV modules.

After the batteries were completely charged, only the power which the house consumes is retrieved from the PV moudles (plus losses and idle/self consumption of the inverters). From about 3.30pm my solar was not always able to deliver enough power to the required load and the missing power was added by the battery, if the load get lower, the battery charges again (see middle chart).

From about 5:30pm to 7pm solar only helps a bit. And from 7pm through the night, the battery was the only power source. When solar production ends at about 7pm, I start into the night with about 90% SOC.

This cycle starts again on the next day.

Today my batteries were full by 2:30. So the inverter stopped pulling. So I turned on the window AC to pull more power to get that sweet sweet ROI a bit higher.

fmeili1 said:

Typical off-grid charts explains the situation:

Before the next solar production begins at about 6am, the batteries had a remaining SOC of 62%. In my situation it took from 6am to 12pm to charge the batteries from the remaining 63% to 100% SOC and in this time, the maximum possible power was retrieved from the PV modules.

After the batteries were completely charged, only the power which the house consumes is retrieved from the PV moudles (plus losses and idle/self consumption of the inverters). From about 3.30pm my solar was not always able to deliver enough power to the required load and the missing power was added by the battery, if the load get lower, the battery charges again (see middle chart).

From about 5:30pm to 7pm solar only helps a bit. And from 7pm through the night, the battery was the only power source. When solar production ends at about 7pm, I start into the night with about 90% SOC.

This cycle starts again on the next day.
View attachment 210946
View attachment 210948

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Thanks for the visuals and the answers by others; you have helped out a whole bunch. I'm just curious, and this is a total side issue on Exhibit 1. I notice your power usage, for the most part, appears under or around 1.5kw. During the day, it goes up, and then there are extreme but short spikes . . . Are those large devices like AC turning on?

Bulldogg said:

Thanks for the visuals and the answers by others; you have helped out a whole bunch. I'm just curious, and this is a total side issue on Exhibit 1. I notice your power usage, for the most part, appears under or around 1.5kw. During the day, it goes up, and then there are extreme but short spikes . . . Are those large devices like AC turning on?

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It's the 5 ton heat pump, sometimes the water heater and when my wife using the oven and /or cooktop.

Bulldogg said:

Please let me see if I get this right . . so if I have 800 watts being produced by the solar panels, but my batter charger only can take in 400 watts, the solar will charge it (if not at capacity) and the other 400 watts will be used to power any electronics connected need a portion of that remaining 400 watts? This stuff is exciting to me, just a kid in a Candy store I never been in. . . LOL ! ! !

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Now i am cautioning you here.

You state your battery charger can only output 400W...

Or do you mean the solar charge controller can only output 400W?

Or do you mean the battery bank is only needing 400W, at the moment, and the controller can output more, if demand is there?

Bulldogg said:

My question is more than likely a good ole' rookie question, but I'm not too scared to ask. So, lets say we have a solar system, regardless if portable or fixed, but both have battery storage and solar panels charging them. Let's say the batteries are at 100% charge, and there are devices (let's say a computer, TV, etc . . ) that are directly connected to the system. If the batteries are at 100% charge but there is plenty of sunlight charging the panels, do the systems directly bypass the batters and provide power to your plugged-in devices? Alternatively, does it constantly take from the battery, and then the battery is constantly being recharged? If it is the latter, does this reduce battery life? Thanks, I hope the question is not too confusing to understand; I am a rookie at this. Sorry if this is a repost but I tried searching other post first. . . No luck for me.

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That’s a great question… I had the same question here a short while ago… once it was explained to me by a very smart forum guy , it all made perfect sence … that leaves one free to explore newer questions……that’s the way ya grow….

Supervstech said:

Now i am cautioning you here.

You state your battery charger can only output 400W...

Or do you mean the solar charge controller can only output 400W?

Or do you mean the battery bank is only needing 400W, at the moment, and the controller can output more, if demand is there?

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Supervstech - I mean, "Or do you mean the battery bank is only needing 400W, at the moment, and the controller can output more, if demand is there?"

underdog5004 said:

The charge controller pushes current into batteries and/or inverter, depending on inverter load. If batteries are full, charge controller will only supply what inverter is demanding.

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Current is never pushed. It is always drawn, so the batteries are drawing the current from the MPPT up to the max it can deliver. The batteries are a load to the charger.

The inverter is a load to the batteries, so the inverter draws from the batteries.

It's simply a sum of sources and loads.

Current will be drawn from the MPPT until the DC bus reaches the MPPT's maximum set voltage - absorption or float depending on phase of charging.

If there is a load to the system, DC bus (battery) voltage will drop. If there is surplus solar available, more current is drawn from the MPPT to maintain the battery voltage, and voltage raises. In effect, the MPPT is directly powering the loads in this case.

plympton said:

Today my batteries were full by 2:30. So the inverter stopped pulling. So I turned on the window AC to pull more power to get that sweet sweet ROI a bit higher.

View attachment 210955

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Why did your inverter stop pulling when the batteries were full? Your inverter provides AC from DC based on your loads. Or did you mean your AiO with built-in MPPT stopped pulling power from the array?

sunshine_eggo said:

Why did your inverter stop pulling when the batteries were full? Your inverter provides AC from DC based on your loads. Or did you mean your AiO with built-in MPPT stopped pulling power from the array?

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Yes, of course. !

Think of your battery as a bucket and your solar array as a garden hose. Think of your loads as a pipe with a valve on it coming out of the bottom of the bucket. When the sun shines water flows from the garden hose into the bucket. When the bucket is full it overflows onto the ground and is wasted. When you turn on a load it draws from the bottom of the bucket. If more water is flowing in from the PV garden hose than is being drained by the load the bucket stays full and less is wasted by overflowing onto the ground. If more water flows out of the load pipe than flows in from the PV garden hose, the water level in the bucket drops.

Bulldogg said:

My question is more than likely a good ole' rookie question, but I'm not too scared to ask. So, lets say we have a solar system, regardless if portable or fixed, but both have battery storage and solar panels charging them. Let's say the batteries are at 100% charge, and there are devices (let's say a computer, TV, etc . . ) that are directly connected to the system. If the batteries are at 100% charge but there is plenty of sunlight charging the panels, do the systems directly bypass the batters and provide power to your plugged-in devices? Alternatively, does it constantly take from the battery, and then the battery is constantly being recharged? If it is the latter, does this reduce battery life? Thanks, I hope the question is not too confusing to understand; I am a rookie at this. Sorry if this is a repost but I tried searching other post first. . . No luck for me.

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First off the SCC job is to charge your battery. Depending on its settings it may entirely stop drawing solar power if it determines that the batteries are full. Depending on battery chemistry your charge controller goes through CC (constant current) and CV (constant voltage) points (there maybe other charge stages but I will not go into all that). If it should reach full it allows the DC common bus voltage to drop until it either floats at a fully charged resting battery voltage (often just a bit higher) or it continues down until it reaches a CC voltage restore point. During a float stage the PV through the SCC is still able to supply loads, up to its capacity, but if loads are more the battery supplies the difference. Loads supplied by the battery drop the batteries internal voltage.

Loads that are on the DC common bus as the battery and SCC/inverter are will take power first since they do not have a internal voltage that impedes this.

hugemoth said:

Think of your battery as a bucket and your solar array as a garden hose. Think of your loads as a pipe with a valve on it coming out of the bottom of the bucket. When the sun shines water flows from the garden hose into the bucket. When the bucket is full it overflows onto the ground and is wasted.

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As to what happens to the "overflow": It heats the panels, but doesn't damage them.

If the controller rejects the power by no longer loading the panel, the electron-hole pairs that would have provided current recombine in the semiconductor material, releasing the energy as heat and infrared light - and most of the infrared is absorbed and turned into heat.

If the controller rejects the power by shorting the array (as some do), some of the energy is released heating the wiring (mainly the current collecting conductors on the cells) and the remainder in electron-hole pair recombination, as above.

There's no damage because the amount of heating is the same as if the panel were not in an array but just sitting in the sun - a black thing under glass with at least one face exposed to circulating air. This doesn't get hot enough to damage the semiconductors. In the shorted case the current in the wiring is about the same as that of a fully loaded panel, which is well within the wiring's capabilities.

(Note that the damage from things like selective shading combined with protective diode failure occurs because the resulting heating/overload concentrates energy from a lot of cells into a few of them, resulting in high currents in and/or excessive back voltages across those cells.)