Wattage from Solar Panel Fluctuates?

PorkChopsMmm · August 2010

I have a very strange scenario and when I search for it on the forums or the net I only find results that don't apply. My situation is this... I am testing a solar solution at my home for a near term installation at our offgrid cabin. As such I have access to AC power here but won't in the future. I am trying to get all of the kinks worked out before taking it up to the cabin.

Here is my setup:
Xantrex Prosine 2.5 2500 watt inverter charger
Outback MX60 MPPT Charge Controller
8 55 amp hour Optima Yellowtop Gel batteries
1 240 watt China solar panel with specs below

Rated Power (Max) 240W
Tolerance +- 5%
Voltage at PMax (Vmp) 30.0V
Current at Pmax (Imp) 8.0 A
Open-Circuit Voltage (Voc) 36.0 V
Short-Circuit Current (Isc) 8.96 A
Nominal Operating Cell Temp (NOCT) 45 +- 2 degrees Celcius
Maximum System Voltage 600VDC
Maximum Series Fuse Rating 15A
Operating Temperature -40 to + 85 degrees Celcius
Fire Resistance Class C
Cell Technology Poly-Si

My problem is this: I just noticed that when the batteries are 100% charged or even, say, 80% charged if there is no load to the inverter the wattage reported from the solar panel on the Outback MX60 drops to a low level. For example, I was just downstairs and the panel was reporting 60 watts being generated. I plugged in a light and it instantly shot from 60 watts to 140 watts (the panel is kind of in the shade so 140 watts is ok from 240, again, just testing). I plugged the light and it dropped back down to 60 watts. I have never heard of this before.

The inverter is linked to the battery bank via 4 gauge wire, the batteries are connected in an array with copper buss bars in "Method 3" from the UK website, and the inverter is hooked up to the same copper buss bars as the charge controller with 1 gauge wire. The solar panel is linked to the charge controller via 4 gauge wire also.

What could be causing this? The solar panel has routinely produce up to 195 watts and from the charge controller I can see it is charging the batteries. I recently set my Xantrex inverter to go into standby mode for any loads less than 17 watts, could this be having an effect? I thought the inverter and charge controller were independent of each other. Would having the Charge Controlr and Inverter connected to the same positive and negative buss bars of the battery bank, and physically close together, cause this weird behavior? Attached are pictures with load and without, taken seconds apart.

I called Outback and the tech didn't seem to know much. She mentioned line loss but couldn't explain why it would fluctuate with load. Thanks in advance!

With load:

Without load:

Vic · August 2010

Re: Wattage from Solar Panel Fluctuates?

Hi Pork..

If your system is not tied to the grid, and there is little or no load on the inverter, then, the Charge Controller will keep the batteries in equilibrum. If the batteries are fully charged, the only power supplied by the CC is just to keep the battery voltage constant while providing enough power (current) to keep the inverter running. When there IS a load on the inverter, then the CC will supply more current to keep the battery voltage constant (at its setpoint), up to the lilmit of the solar panel's ability to supply the current.

Once the battery is fully charged, there is no need for any additional power until there is additional demand placed on the inverter (or any other load connected to the battery).

Hope that I am interpreting your wonderment correctly. Vic

PorkChopsMmm · August 2010

Re: Wattage from Solar Panel Fluctuates?

Vic wrote: »

Hi Pork..

If your system is not tied to the grid, and there is little or no load on the inverter, then, the Charge Controller will keep the batteries in equilibrum. If the batteries are fully charged, the only power supplied by the CC is just to keep the battery voltage constant while providing enough power (current) to keep the inverter running. When there IS a load on the inverter, then the CC will supply more current to keep the battery voltage constant (at its setpoint), up to the lilmit of the solar panel's ability to supply the current.

Once the battery is fully charged, there is no need for any additional power until there is additional demand placed on the inverter (or any other load connected to the battery).

Hope that I am interpreting your wonderment correctly. Vic

Vic, thanks, this is what I was hoping. The tech out Outback couldn't confirm if that was happening or not. It makes sense that if the batteries are full that the CC throttles itself. I must have been mistaken when I saw it throttling back and the batteries were between 60-70%

mike95490 · August 2010

Re: Wattage from Solar Panel Fluctuates?

PorkChopsMmm wrote: »

She mentioned line loss but couldn't explain why it would fluctuate with load.

Line Loss, ALWAYS fluctuates with load. No load, no current flow, no loss in your 0.5 ohm wires. More current drawn, more loss in the line (0.5 ohm)

Larger problem, the one panel you have, 240W, can't really be expected to deliver more than 80% (192w) to the controller. You have a really large controller, and a really small PV array, and will loose a lot to the internal overhead losses. Something like the Morningstar MPPT 15A controller will deliver MORE power to the batteries.

PorkChopsMmm · August 2010

Re: Wattage from Solar Panel Fluctuates?

mike90045 wrote: »

Line Loss, ALWAYS fluctuates with load. No load, no current flow, no loss in your 0.5 ohm wires. More current drawn, more loss in the line (0.5 ohm)

Larger problem, the one panel you have, 240W, can't really be expected to deliver more than 80% (192w) to the controller. You have a really large controller, and a really small PV array, and will loose a lot to the internal overhead losses. Something like the Morningstar MPPT 15A controller will deliver MORE power to the batteries.

Mike, thanks for the heads up. I have 2 more 240 watt panels going up tonight to add to the existing 1 240 watt panel. I was just trying to make sure everything worked before I hoisted the other two up to the roof.

Jakachira · August 2010

Re: Wattage from Solar Panel Fluctuates?

As Vic said, your battery is almost full, and the 60watts its for floating . However, if you put a load on your battery, the regulator will quickly try to replace the power your load is consuming. If your panel is producing 190watts per hour and the battery is fully charged, then any load with power consumption of less than 190Watts will be run comfortably with your battery not discharging. So, your regulator is fine. Remember your inverter is a loadon its own on the battery (mine consume < 0.6amps when nothing is connected to it)

blackswan555 · August 2010

Re: Wattage from Solar Panel Fluctuates?

As everyone says, If the batteries/ load don`t want it, the FM does not "collect" it from the panels,
If it is reasonably predictable, It is a good time to run any none time critical loads , eg water pumping, washing mc etc, If you can use any "spare" without it going into batteries, you can save up to 20% ish from bat in > out losses, If not used for anything else, FM aux can be used /programed to switch relays etc (max 200ma 12v)

Have a good one
Tim

ps I would strongly advise battery temp sensors on all charge sources,

PorkChopsMmm · August 2010

Re: Wattage from Solar Panel Fluctuates?

Everyone, thank you for the responses. Today I could see the charge controller responding in the ways that you indicated.

Wattage from Solar Panel Fluctuates?

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