Two many solar panel amps?

[TwoBlocked]

Thanks everyone. My limited experience with these stand alone installations is that when there is a light load, charged batteries, and the sun is shining, the voltage from the panel(s) is higher than the charging voltage which is higher than the load voltage (like 18V, 13.5V, 12.0). But when the batteries are low, and/or the load is heavy, and/or the sun isn't shining, they are all at the same voltage (like 11.8V 11.8V, 11.8V).

These panels are all about 150W, 20V max, 7.5A max. At my first visit, they just needed the radio tower working right away, and I had another time critical project also going on. Soooo... not much time to read a manual and went by hearsay about why the third panel was not connected. Replacing the controller did get things working, but can't say for certain the controller was bad, although not having the negative connected to ground may have been a problem (yes, it's connected now). Later, I posed my question to you fine folks, read the manual, and convinced the installer (who is more a boss than a co-worker...) to read the manual. He changed his mind about adding the additional panel (which I had already done as the batteries could not keep up overnight).

Sooo... got a break to go check things out when the sun was actually shining on the panels. A pretty obvious problem with panel placement as you can see from the picture. A DC clamp meter verified that the shaded panel was only putting out 1/4 the amps as the other panels. At the time the panels were 12.0V, battery 12.0V and load 12.0V. I forget the exact amps going to and from each of these, but the batteries were charging a little and the radio load was light.

Don't know if the operators or we will reposition the panel(s). Might need another panel. Not sure if a real calculation was done. On my suggestion, the operator put the batteries on a generator/charger overnight so he won't get a auto-call in the middle of the night. We'll see how things are this morning.

Thanks again, folks!

 

[winnie]

I think that @TwoBlocked issue has been resolved, but I wanted to go back to clatify the current source vs voltage source point.

First remember that both of these concepts are ideals, not reality. Real power sources have voltage vs current curves.

Most of the time we deal with 'stiff voltage sources'. An ideal voltage source is one which maintains a constant output voltage no matter what current gets drawn. The load can control the power by changing the current drawn, but can do nothing to change the voltage. You turn off the power by opening the circuit, zero current flows, the switch terminals have to block the constant voltage of the source. The current drawn is set by the load. 0A or 1000A, the voltage remains the same.

Of course a real voltage source has internal resistance. As the load increases the voltage drops, eventually to zero. But even a moderately sized lead acid battery can supply hundreds of amps to a short circuit. We say that a stiff voltage source has low internal resistance.

Now think about an ideal current source. This provides a constant current in all cases. The terminal voltage changes as necessary to maintain the current. The load still controls the power delivered, but by changing the 'voltage it draws'.

Think about how you would use a switch to control the power output of a current source...you can't open the circuit! If you try to open the circuit then the voltage will climb until the switch arcs over, because the current has to remain constant. If you want to shut off the constant current supply, you short circuit the output. The constant current keeps flowing with zero voltage.

'Stiff current sources' have high internal resistance. Note that this description doesn't mean that there really is a resistor inside dissipating power, just that the output current doesn't change when the output voltage changes.

Solar panels act as current sources over a limited voltage range. From zero volts up to a threshold set by the number of cells, the output current remains relatively constant and set by the light level. If the current the panel wants to provide is greater than what the load 'needs', then one of two things will happen. Either the panel output voltage will rise until the load takes the current anyway, or the system voltage will rise high enough that the panel will reach its 'compliance limit'.

A charge controller circuit might depend upon the natural output current of the solar panel to set the current to the battery. In this case too many panels means too much current flowing.

Jon

 

[ggunn]

I think that @TwoBlocked issue has been resolved, but I wanted to go back to clatify the current source vs voltage source point.

First remember that both of these concepts are ideals, not reality. Real power sources have voltage vs current curves.

Most of the time we deal with 'stiff voltage sources'. An ideal voltage source is one which maintains a constant output voltage no matter what current gets drawn. The load can control the power by changing the current drawn, but can do nothing to change the voltage. You turn off the power by opening the circuit, zero current flows, the switch terminals have to block the constant voltage of the source. The current drawn is set by the load. 0A or 1000A, the voltage remains the same.

Of course a real voltage source has internal resistance. As the load increases the voltage drops, eventually to zero. But even a moderately sized lead acid battery can supply hundreds of amps to a short circuit. We say that a stiff voltage source has low internal resistance.

Now think about an ideal current source. This provides a constant current in all cases. The terminal voltage changes as necessary to maintain the current. The load still controls the power delivered, but by changing the 'voltage it draws'.

Think about how you would use a switch to control the power output of a current source...you can't open the circuit! If you try to open the circuit then the voltage will climb until the switch arcs over, because the current has to remain constant. If you want to shut off the constant current supply, you short circuit the output. The constant current keeps flowing with zero voltage.

'Stiff current sources' have high internal resistance. Note that this description doesn't mean that there really is a resistor inside dissipating power, just that the output current doesn't change when the output voltage changes.

Solar panels act as current sources over a limited voltage range. From zero volts up to a threshold set by the number of cells, the output current remains relatively constant and set by the light level. If the current the panel wants to provide is greater than what the load 'needs', then one of two things will happen. Either the panel output voltage will rise until the load takes the current anyway, or the system voltage will rise high enough that the panel will reach its 'compliance limit'.

A charge controller circuit might depend upon the natural output current of the solar panel to set the current to the battery. In this case too many panels means too much current flowing.

Jon

That is pretty much what I said.

 

[electrofelon]

Direct charging of a battery from a solar panel, without the use of a controller that can adjust the voltage, is fairly common and works pretty well. Batteries and PV are pretty good "match" for this use. We use "12v" panels for this, which have a mppt voltage of around 17 volts typically. Where you really notice the disadvantage of powering a load directly off a current source is when you try running a pump or a resistive load. You can of course size things to work well under a given set of conditions, but add variable Sun into the mix and you're not going to be a happy camper.