Watching sweep Vmp and Pmax on a SunSaver MPPT...

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RandomJoe
RandomJoe Solar Expert Posts: 472 ✭✭✭
in General Solar Power Topics #1
So I have a new toy. I wanted to get some data from my 12V system, and decided to get the SunSaver MPPT since I can plug it into the monitoring computer and get all kinds of nifty values. It showed up yesterday and I've been watching the numbers as the sun rises this morning.

The SS MPPT provides a few data points that Outback doesn't, which have been interesting to watch: Sweep Voc, Sweep Vmp and Sweep Pmax. But while watching them I had a couple questions - just curiosity - that perhaps others will know the answers to...

First, how often does an MPPT controller sweep? These sweep values seem to change quite frequently, far more often than I thought the controller would do so. That could just be because the sun is still rising so the voltage changes frequently - perhaps at midday it doesn't sweep as often?

Second, Vmp surprises me a bit. I figured as the sun rose it would simply steadily rise, along with Pmax, but it doesn't. Pmax is rising steadily, but Vmp will occasionally go DOWN several tenths. I suppose that may have something to do with temperature - normally more sun means higher Vmp, but it also means the panel heats up which would reduce Vmp? Still, the Pmax is always *higher* even with the drop in Vmp, just seems a little counterintuitive!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Watching sweep Vmp and Pmax on a SunSaver MPPT...

    How often does an MPPT controller sweep? Depends on the controller. The old MX60's you were left without a doubt as it says "sweeping" when it's doing it. Newer controllers continuously re-evaluate their parameters.

    Vmp is not actually a number for an MPPT controller, as it picks whichever V & A it thinks is best at the moment for the SOC of the batteries. Panels are chiefly current producers, not Voltage sources, so the Voltage from a panel is not a linear function of insolation.

    I can watch mine all day long and never see the same numbers twice. :p
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  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    #3
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    Re: Watching sweep Vmp and Pmax on a SunSaver MPPT...

    My curiosity on the sweep frequency is I had always figured you'd be able to "notice" it happening - as in, there'd be a brief period where it was either not charging or (in the case of finding Pmax) charging more than needed. I *have* seen my FM-80 do that on occasion, you'll see the PV voltage slowly fall, and the output current go up - up - WAY up until it find the peak, then back down again as it drops the voltage below Vmp, and then it settles down and gets back to normal. It takes a few seconds.

    But with the Morningstar it seems to do the sweep faster than I can see, or maybe the sweep values aren't actually from a sweep it's just whatever ideal numbers it's found as it perturbs the voltage up/down a bit over time. I can scan the data as fast as I want, and I'll just see the values change to new ones but the charge current never dips/rises.

    Just makes me curious, I like knowing what's going on inside! :p

    Now that it's midday I see I probably need to check things over on this system. I'm only getting to 85W (after washing off the panel), specs are 135W STC / 95 PTC and I've always gotten at least PTC on the other panels. This system has only ever had a simple PWM controller so I never really knew what it was doing. Perhaps I went too thin on the lead-in wire from the panel, I didn't put quite as much care into this system as the big one! :roll:
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