What happens to excess solar panel current when the batteries are full?

[Oscarpop]

Do solar panels just simply keep charging when there is sun, or do they have less ability to put charge into the battery when the battery is charged?

I always figured the controller regulated the amount of charge that it allowed to put into the batteries ?

 

[johnalison]

In anticipation of people who actually know what they are talking about, I believe that the controller is there to limit the voltage available since the panel will produce something like 20v. With a very small panel, this may never be reached, but above a certain size it becomes necessary. When the batteries are charged, then no current is taken from the panel, leaving it in the same position as an open circuit, which does no harm.

 

[Twister_Ken]

The panels are turned off, so as not to drain all the energy from the sun?

 

[pmagowan]

The panels are turned off, so as not to drain all the energy from the sun?

If this analysis is true then my conclusion is that we have too many solar panels in the UK at present. The sun seems well drained!

 

[TonyBuckley]

It's a bit like wind turbines. These have been proven to reduce the rotational speed of the planet as they provide resistance to the spin.

Solar panels suck up the sun's energy and if not used to charge then simply wasted energy.

You weigh less at the poles than you do at the equator.

One of these 'facts' is true.

 

[VicS]

The panels are turned off, so as not to drain all the energy from the sun?

Of course they are not, The controller switches to "reflect mode" and the energy is reflected back towards the sun at wavelengths outside the visible spectrum, which is why you cannot see it.

 

[danielefua]

In anticipation of people who actually know what they are talking about, I believe that the controller is there to limit the voltage available since the panel will produce something like 20v. With a very small panel, this may never be reached, but above a certain size it becomes necessary. When the batteries are charged, then no current is taken from the panel, leaving it in the same position as an open circuit, which does no harm.

Correct reply! Contrarily to alternators where, without a load, the voltage can "runaway" and become very high and detrimental to the circuitry (the diodes), solar cell output voltage has got its physical limit that can never be exceeded. Unless the overall panel is poorly built and arcing is allowed (quite unlikely), open circuit solar panels will just reach the nominal voltage and stay there without any current flow. The surface of the panel exposed to sunlight will continue to be heated by the sun almost as in the normal charging status, at most by an increase of 10-15% (its power conversion efficiency) which usually goes into current generation.

Daniel

 

[30boat]

Of course they are not, The controller switches to "reflect mode" and the energy is reflected back towards the sun at wavelengths outside the visible spectrum, which is why you cannot see it.

I'm amazed at the collective wisdom here. So much to learn so little time.

 

[ianj99]

If this analysis is true then my conclusion is that we have too many solar panels in the UK at present. The sun seems well drained!

Obviously this is correct because since the season ended and boaties stopped using their solar panels, the sun has been working again.

 

[Foolish Muse]

"Do solar panels just simply keep charging when there is sun, or do they have less ability to put charge into the battery when the battery is charged?"

Keep in mind that solar panels do not push charge into a battery. Rather, the battery sucks the charge in. As the battery gets charged, it sucks slower and slower. This is why when using a generator, people rarely charge their batteries to 100% while at sea; because it takes a lot longer to charge from 90% to 95% than it takes to charge from 75% to 90%. Most people only charge to 85% if using a generator while at sea. Of course solar panels do not take any fuel or effort, so you just leave them charging all the time.
So when a battery is fully charged, it just stops sucking completely.

 

[TQA]

So when a battery is fully charged, it just stops sucking completely.

Hmm if this were true it would be impossible to overcharge a battery.

Next batter please.

BTW Just in case anyone finds the above misleading and is tempted to leave a battery on charge forever it is very definitely possible to overcharge lead acid batteries. The batteries will gas producing oxygen and hydrogen with risk of explosion. In extreme cases the lead plates will deform and bursting of the case is possible. The above assumes the battery to be vented. Life will be more interesting with a non vented battery.
Readers DO NOT TRY THIS AT HOME.

 

[pvb]

Obviously this is correct because since the season ended and boaties stopped using their solar panels, the sun has been working again.

Is this about global warming???

 

[Foolish Muse]

Hmm if this were true it would be impossible to overcharge a battery. The batteries will gas producing oxygen and hydrogen with risk of explosion.

The OP is talking about solar panels with a charging regulator here, not an uncontrolled alternator from a car. He can leave his system in place in the bright sunshine forever without worry. Even if he just had a diode in place, he would have no worries about explosion. No need to frighten the audience.

 

[Binman]

+1, my panel has been on for over 2 months with a diode, so won't be troubled by over charging, been to ill to get out to it at the moment. My boat that is.

 

[Billjratt]

Batteries don't suck. Electricity is 'pushed' from the source (whichever it is) Volts are the name given to something called electromotive force!
The energy transferred varies with the 'potential difference' between the source and the load - so when the battery voltage is down a bit, (11volts) and the charging device is active, (trying to reach 14.4V) lots of current can be pushed down the 3.4volt "slope". As the battery voltage rises to full charge, the voltages tend to equalise (less 'slope' ) so less push = less current. A regulated source will realise when to stop charging, but unregulated solarcells will not, as the opencircuit voltage is usually around 20v - unhealthy in the Mediterranean sun! . To answer the OP, the basic regulator for solar cells contains a 'dump resistor' to which spare energy is diverted once the battery is sensed as full, as the output from the cell array will continue to flow as long as the Sun shines, unlike an alternator which gets turned down by it's regulator, so doesn't need a dump device.

 

[William_H]

There are various types of regulators for solar panels charging batteries.
The simple arrangement of panel directly to the battery means that you have 20+ volts pushing current into the battery. The current flow drags down the panel voltage due to internal resistance of the panel. So panel voltage equals battery voltage.
The prolem is that if the battery becomes fully charged (large panel small battery) the panel keeps pushing the current into the battery to destroy the battery by boiling. The max potential voltage of the panel of 20+ volts keeps pushing current in no matter battery charge state.

Now an MPPT type controller actually manufactures a voltage suitbale for charging like 14.2 volts. This means that when this 14.2 volts is presented to the battery terminals a current will flow depending on the inherent charge of the battery (say 13 volts.) The curent in depending on difference between the charge voltage and the internal voltage. So as the battery charges so the current falls so saving the battery from destruction. MPPT types often use the stepped charge voltage regime like a charger or alternator controller.
This because the curreent into a half charged battery will be much less than that into a flat battery. Current in continually tapers off so the last part of battery charge is slow to acheive.
Now a simpler controller might have a series transistor regulator (like a 7812) this increases the series resistance so that regardless of current the output of the controller is reduced to 14.2 volts. Problem here is that the transistor gets hot dissipating some of the excess panel voltage. This needs a big heat sink.
A simpler cheaper option is to simply turn off the current completely to the battery when the voltage rises to apparently fully charged. (14.2 volts) After the current is turned off the batterry voltage is measured again and if it is then less than desired the controller turns on again. This can be done at 100 of times per second. The ratio of time on to time off controlls the actual charge. On all the time for a flat battery off almost all the time for a charged battery. In terms of efficiency for a flat battery or small panel this is not much different to direct panel to battery except that if battery does become fully charged it is protected.
A variation on this is to switch the solar panel power to a dump resistor when in the off cycle.
These types are called pulse width modulated controllers.

Finally another type has the panel directly connected to the battery full time. But a transistor is set up with voltage sensing so that if the battery terminal voltage rises above 14.2 volts ie battery is fully charged the transistor starts to conduct switching to a resistor to dissipate the heat. Or just dissipating the heat in the transistor. This latter type is not often seen commercially.
good luck olewill

 

[Niander]

What would happen if the suns voltage became too low?

 

[jordanbasset]

What would happen if the suns voltage became too low?

You would probably have to move away from Scotland

 

[JumbleDuck]

To answer the OP, the basic regulator for solar cells contains a 'dump resistor' to which spare energy is diverted once the battery is sensed as full, as the output from the cell array will continue to flow as long as the Sun shines, unlike an alternator which gets turned down by it's regulator, so doesn't need a dump device.

A disconnected solar panel won't produce any current, regardless of the sun.

 

[danielefua]

There are various types of regulators for solar panels charging batteries.
The simple arrangement of panel directly to the battery means that you have 20+ volts pushing current into the battery. The current flow drags down the panel voltage due to internal resistance of the panel. So panel voltage equals battery voltage.
The prolem is that if the battery becomes fully charged (large panel small battery) the panel keeps pushing the current into the battery to destroy the battery by boiling. The max potential voltage of the panel of 20+ volts keeps pushing current in no matter battery charge state.

Now an MPPT type controller actually manufactures a voltage suitbale for charging like 14.2 volts. This means that when this 14.2 volts is presented to the battery terminals a current will flow depending on the inherent charge of the battery (say 13 volts.) The curent in depending on difference between the charge voltage and the internal voltage. So as the battery charges so the current falls so saving the battery from destruction. MPPT types often use the stepped charge voltage regime like a charger or alternator controller.
This because the curreent into a half charged battery will be much less than that into a flat battery. Current in continually tapers off so the last part of battery charge is slow to acheive.
Now a simpler controller might have a series transistor regulator (like a 7812) this increases the series resistance so that regardless of current the output of the controller is reduced to 14.2 volts. Problem here is that the transistor gets hot dissipating some of the excess panel voltage. This needs a big heat sink.
A simpler cheaper option is to simply turn off the current completely to the battery when the voltage rises to apparently fully charged. (14.2 volts) After the current is turned off the batterry voltage is measured again and if it is then less than desired the controller turns on again. This can be done at 100 of times per second. The ratio of time on to time off controlls the actual charge. On all the time for a flat battery off almost all the time for a charged battery. In terms of efficiency for a flat battery or small panel this is not much different to direct panel to battery except that if battery does become fully charged it is protected.
A variation on this is to switch the solar panel power to a dump resistor when in the off cycle.
These types are called pulse width modulated controllers.

Finally another type has the panel directly connected to the battery full time. But a transistor is set up with voltage sensing so that if the battery terminal voltage rises above 14.2 volts ie battery is fully charged the transistor starts to conduct switching to a resistor to dissipate the heat. Or just dissipating the heat in the transistor. This latter type is not often seen commercially.
good luck olewill

Thank you olewill, this is the most sensible reply in the whole thread. I did not know about the existence of solar panel regulators with the dump resistor because they do not make much sense but... never mind.

In principle, dump resistors are needed only with permanent magnet alternators; in all other cases there are better ways to solve the problem of controlling the charge.

Daniel