Now I know where Jimmy Buffet got the idea for his song "MATH SUCKS"!!!!!The equation doesn't balance.
Now I know where Jimmy Buffet got the idea for his song "MATH SUCKS"!!!!!The equation doesn't balance.
The part that is not evident in your analysis is the battery's resistance. There is still 500 watts of power being applied to the battery, the internal resistance of the battery causes the voltage to drop. Your explanation sort leads one to believe 440 watts (in the 12v example) just disappears, it doesn't, it converts to heat, it breaks down the sulfates, and if the voltage is high enough it starts breaking the water molecules. Most of those things aren't very good for batteries.Put the same solar panel directly on a 24 volt battery. The panel looks like a constant current source so the current is still 5 amps. This is only enough current to raise the battery voltage slightly. The panel now operates at 24 volts and 5 amps so the panel is now producing 24V*5A = 120 watts.
Put the same panel directly on a 12 volt battery. Still constant current at 5 amps, still only enough current to barely raise the battery voltage. The panel is once again at the battery voltage so the power produced is 12V*5A = 60 watts.
Walt is correct here. The panel will not produce 500W at 12V, or at any voltage below or above its Vmp. This is the entire point of having an MPPT. Look at the I-V curves for any panel and you will see how the power output changes at various voltages. The panel outputs a constant current and the power is determined by the voltage. Connected to a 12V lead battery, that isn’t full, the voltage will be initially limited and thus the power will be low. As the battery becomes more full, the voltage will continue to rise until the bad things you mentioned begin to happen.The part that is not evident in your analysis is the battery's resistance. There is still 500 watts of power being applied to the battery, the internal resistance of the battery causes the voltage to drop. Your explanation sort leads one to believe 440 watts (in the 12v example) just disappears, it doesn't, it converts to heat, it breaks down the sulfates, and if the voltage is high enough it starts breaking the water molecules.
For me the confusing part is what is limiting the voltage. The panel doesn't limit the voltage, the circuit and more specifically the resistance of the load attached to the panel limits the voltage, in this case the battery's SOC. (Assuming the panel is in perfect sunlight.) This makes sense to me, because there is nothing inherent in panel that could control the panel's output, i.e., the panel is basically dumb, it will keep producing as much amperage as it can and will drive the voltage up to get the load to take it. The controller then, tries to match the battery's need with the solar output for most efficient charging, raising or lowering voltage to provide amperage the battery can take.Connected to a 12V lead battery, that isn’t full, the voltage will be initially limited and thus the power will be low. As the battery becomes more full, the voltage will continue to rise until the bad things you mentioned begin to happen.
Panels can and do short, more often than many devices. Obviously, it must be fused near the battery, since controller are not normally wired through the main panel. You are correct in that it can't generate enough power on its own to burn the supply wires. As you say, in that way, it is self-regulating. But if there is a battery in the circuit ....... While the panel's voltage will increase, there is a maximum voltage and amperage it can attain. In that sense it is a self-regulated power source and does not need to be fused at the panel because an appropriately sized wire can be used, unlike a battery where the entire capacity of the battery can be discharged in fractions of a second, exceeding the ampacity of the wire.
Right, if for some reason the panel was wired directly to the battery or to the DC+ bus, then over current protection would be needed at the battery end of the wire but not the panel end. No OCP is needed between the panel and controller and the controller needs OCP at the battery end of the wire, not the controller end of the wire. There was some disagreement about this earlier in the thread.Panels can and do short, more often than many devices. Obviously, it must be fused near the battery, since controller are not normally wired through the main panel. You are correct in that it can't generate enough power on its own to burn the supply wires. As you say, in that way, it is self-regulating. But if there is a battery in the circuit ....
Hi Joe,Regarding the above link, we are talking about if fuse on the input side of the controller can protect against fire. My answered in the case above is not since there is excessive voltage not excessive current. This write up is from a 2013 magazine not from a technical review. Technology has improved and lessons learned.
The real problem is caused by a want-to-be solar panel installer with just enough knowledge to make trouble.
This is not normal case if installed properly.
I dismiss this writeup as a convincing proof for anything. You can comfortably ignore it.
^^ Yes.Right, if for some reason the panel was wired directly to the battery or to the DC+ bus, then over current protection would be needed at the battery end of the wire but not the panel end. No OCP is needed between the panel and controller and the controller needs OCP at the battery end of the wire, not the controller end of the wire. There was some disagreement about this earlier in the thread.
A minor point but ...... Battery impedance is brought up and is important. A battery of course has a very low impedance. The little more difficult concept is that the solar panel has a fairly high impedance....
Victron MPPT Connect app has a feature that provides a 30 day history of various data including - P Max - maximum power for the day. On 3 days the boat must have been healing in the absolute right direction to capture more than the 170W rated output.Interesting discussion - over the past year of running my panels, I've never seen them even reach their stated output - close a few times, but certainly never more...
dj
I think a lot depends on the panel. I have seen over 400W from my 335W Panasonics quite often with clouds. But my 200W Rich panels lag behind significantly. I believe I have only seen them reach their rating twice since installing them last year.Interesting discussion - over the past year of running my panels, I've never seen them even reach their stated output - close a few times, but certainly never more...
dj
The disagreement is still alive.No OCP is needed between the panel and controller and the controller needs OCP at the battery end of the wire, not the controller end of the wire. There was some disagreement about this earlier in the thread.