On 2022/08/13 at 11:56 PM, jumper said:

what do you think about the MPPT operating range specified as 30-115V DC as per the sticker on the side? Do you think it's ok to exceed that?

Yes, I think it's safe to exceed 115 V. Even my 2016 PIP-4048MS (Axpert MKS 5K-48) says that, and I've examined the Solar Charge Controller firmware. It starts ramping down power starting at 130 V, linearly to zero at 145 V. There doesn't seem to be anything special about the 115 V figure, except that it's a little under the 120 V that some jurisdictions (used to?) call Extra Low Voltage. I think that they wanted to avoid having to get expensive certifications for non-ELV inverters, or something like that.

Of course, being a 3 kVA and 24 V model may make it different, but I suspect not.

However, even though 115-145 V may be safe, you should avoid Voc of over 130 V since the power will start derating there. If you're running say 140 V, then you'll only get 33% power, so it might be hard for the panels to draw enough power to get the working voltage below 130 V, especially if solar conditions are less than perfect.

But there is also anecdotal evidence that higher (>100 V) panel voltages cause the SCC to work harder and get hotter, running the fans more, and the hardware runs at a higher average temperature, shortening life. It's well known that the genuine Voltronic models only use slightly above average capacitors, not the highest life ones that cost a little more. And who knows what quality you will get in a clone.

If you do decide to keep to 2S, watch the performance in summer, especially in shady conditions. That combination might end up with too low a panel voltage; the SCC seems to need at least 5 V higher than the battery voltage to operate most efficiently.

10 hours ago, Andy07 said:

the spec sheet says they are 110 cell Mono, do you think this makes a difference?

Quote

Ok cool thank you, they do indeed have 5 columns 

It kind of makes a difference because solar panels generally run at about 0.5 V at the maximum power point, and about 0.6 V open circuit (so the notional internal diode is absorbing all the power). With only 55 cells, that means you can expect the highest voltage to be seen at 25°C to be about 55 x 0.6 = 33 V. But as @Scorp007 mentioned, all that really matters is the actual panel specifications. With these giant >500 W panels, those old rules of thumb don't apply so well any more.

In short, it makes a difference only for the purposes of rules of thumb.

Another rule of thumb that doesn't depend too much on panel specifications: a panel's Voc rating has to be under 45 V to be able to run 3S safely with a 145 V max PV model (though maybe not efficiently). That's because temperature coefficients are generally such that the increase in voltage at 0°C compared to 25°C is about 1.07x. 45 x 3 x 1.07 = 144.5 V, just squeaking under the "never exceed" limit. Even better if Voc is under 40.5 V, since 40.5 x 3 x 1.07 = 130 V, so that the SCC is never power derating (limiting).

19 minutes ago, Coulomb said:

It kind of makes a difference because solar panels generally run at about 0.5 V at the maximum power point, and about 0.6 V open circuit (so the notional internal diode is absorbing all the power). With only 55 cells, that means you can expect the highest voltage to be seen at 25°C to be about 55 x 0.6 = 33 V. But as @Scorp007 mentioned, all that really matters is the actual panel specifications. With these giant >500 W panels, those old rules of thumb don't apply so well any more.

In short, it makes a difference only for the purposes of rules of thumb.

Another rule of thumb that doesn't depend too much on panel specifications: a panel's Voc rating has to be under 45 V to be able to run 3S safely with a 145 V max PV model (though maybe not efficiently). That's because temperature coefficients are generally such that the increase in voltage at 0°C compared to 25°C is about 1.07x. 45 x 3 x 1.07 = 144.5 V, just squeaking under the "never exceed" limit. Even better if Voc is under 40.5 V, since 40.5 x 3 x 1.07 = 130 V, so that the SCC is never power derating (limiting).

Coulomb thanks for all the detail technical stuff. I think the debate is partly 2S or 3S. I can just confirm running a clone at 36.5-37V under winter/summer conditions with 24V battery works fine. I have mismatch panels with the same volts but current is over double from the one to the other.  They are quite happily married on the Axpert MPPT or the Epever external. In both cases they provide the same current to the batteries. Yes my Axpert clone does under perform in the PV and AC charging side. I just accepted it.

I checked today and while the Epever has been charging at the full design 20A the temp on the big heat sink was stable at 39 deg.

1 hour ago, Coulomb said:

However, even though 115-145 V may be safe, you should avoid Voc of over 130 V since the power will start derating there. If you're running say 140 V, then you'll only get 33% power, so it might be hard for the panels to draw enough power to get the working voltage below 130 V, especially if solar conditions are less than perfect.

Thanks for all the info Coulomb, I've had to switch to 2S with my older panels as they have a Voc of 45V and I can get 10cm of snow in Winter and +40C in Summer and absolutely no shade, ...ever 🤣

22 hours ago, Scorp007 said:

I can just confirm running a clone at 36.5-37V under winter/summer conditions with 24V battery works fine.

Duh! I forgot that this is a 24 V system. So 2S should work fine. It might have been marginal with a 48 V battery.

Thanks for all the info, with 2 panels I’m getting about 900w, keen to add the third to get this a bit higher. Voltages are all definitely well within range so I don’t see any issues. 

I connected the third panel and so far so good! No issues, I can only think that there was a faulty component or dry capacitors but all is good. Thanks for all the help