TLDR: Probably a combination of the SCC frequency and Inverter frequency
...There are no other loads currently. Just 124 watts for the lights.
That doesn't make sense to me with this:
The voltage seems consistent, the load fluctuates a lot more when the SCC is connected.
So how is the load fluctuating if it's constant?
So... thinking out loud....
Why do LED lights flicker?
AC power is a sine wave; it's going on/off 50 or 60 times per second depending on your supplier's frequency (e.g., in the US it's 60 Hz).
Powering an incandescent bulb that's no issue as its heat that makes the light and the heat rides through the on/off cycles.
LEDs have no similar persistence, they seem to be consistently on because that frequency is typically fast enough the eye can't perceive the difference.
LEDs change brightness based on changing power (i.e., brighter if voltage/current goes up). A change in power with frequency can cause them to
pulsate, which is different from flickering (a noticeable off state).
the overhead shed lights flicker whenever the sun is out.
LEDs connected to DC power don't flicker. So, if your LEDs are flickering or pulsating I'm going to go out on a limb and say they're AC-powered. Switching them to DC should eliminate any flickering.
- If they're flickering, something is happening to the AC frequency such that it's either reduced or varying to where you can notice it. That would indicate some sort of inverter issue. Probably not the case as other stuff would fail.
- If they're pulsating, something is happening to the AC power and that could be a few things.
If you have a cell phone camera I'd say capture video of it working and not-working and compare the frames. If you have a friend with an oscilloscope, they can definitely tell you what's going on.
They work fin[e] when I disconnect the charge controller or the sun is down.
Solar Charge Controllers (SCC) take the power from the panel and direct it into the battery. Like any battery charger, they charge the battery by supplying current at a voltage
higher than the battery voltage. There are also typically multiple charge stages, but we can ignore that for now.
A Pulse Width Modulated (PWM) SCC operates by flipping on-off at a variable frequency as needed to properly charge the battery. So, the charging voltage on such a system is changing between 14.1 and 10.5 V depending on the charge in the battery. A good quality PWM should minimize the output fluctuation with a capacitor.
Inverters are generally pretty good about keeping the frequency constant. Typically as the source (battery) voltage drops they up the current to keep the output power constant; but the AC voltage does fall in accordance with input voltage. Good quality inverters with big capacitors probably wouldn't notice rapid changes in the input from a PWM.
If the SCC voltage is varying between 14.1 and 10.5, then that's the voltage the inverter sees. So the output voltage is probably going up and down too. A digital meter won't see any sort of fluctuation as it is averaging multiple cycles rather individual waveforms. Applying an additional load might also make them stop appearing to flicker as it'll bring the inverter voltage down.
But, I could see where the frequency output of the inverter could have a harmonic frequency with the SCC that caused the LEDs to appear to pulsate. Probably nothing to worry about electrically. I'm not sure if switching your SCC to an MPPT would make a difference or not, I seem to recall someone saying MPPTs used PWM on the output too.
Possibly a capacitor between the battery and the SCC (or inverter) to stabilize the input voltage? One of the EEs could probably tell you for sure (
@BiduleOhm?)