RS450/200 + Pylontech US5000: 10 modules in series and the 8× float voltage rule

Been mulling this one over since someone in another group mentioned running 10 modules in series on an RS 450/200 with Pylontech batteries.

The 8× float voltage rule is the bit that catches people out. Victron's own guidance says your max array Voc (corrected for lowest expected temp) shouldn't exceed 8× the battery float voltage. With Pylontechs floating around 53.2V, that's a ceiling of roughly 425V — which sounds generous until you start stacking high-Voc panels in a cold snap.

On my static caravan setup I'm only running a modest array through an MPPT 150/70, but I've been speccing a bigger garden office system and nearly made this exact mistake. Winter Voc on some of the newer higher-efficiency panels (thinking AIKO territory, 40V+ Voc per module) can creep up fast when temps drop to -10°C or below — rare in the UK but not impossible, especially up north.

With 10 modules in series at say 40.9V Voc, you're already at 409V before temperature correction. Add a cold morning and you could be nudging or breaching that 425V limit depending on your location.

Anyone here actually running 10S configs with Pylontechs and the RS 450/200? Curious whether people are:

• Dropping to 9S to stay safe
• Using temperature-corrected Voc calculations and accepting the marginal headroom
• Splitting into parallel strings instead

Would love to know what Fogstar or other Pylontech-compatible setups people have paired with this. Seems like 9S might just be the sensible call for UK installs given our temperature swings.

@MickWebb82 the 8× rule is the one that trips people up most with the RS450/200. Your Pylontech US5000s float at 53.2V, so 8× gives you 425.6V max panel input — which sounds comfortable until you string 10 modules in series and check your Voc at low temperature. That's where it bites.

Run the actual cold-weather Voc calculation for your specific panels. Even modest UK winter temps can push string voltage well above nominal, and the RS450/200 hard ceiling is 450V.

Had a similar situation on my cabin setup — ended up dropping to 9S to keep proper headroom. The MPPT efficiency loss was negligible but the peace of mind wasn't.

Victron's MPPT Excel calculator is your friend here — plug in your panel's temperature coefficient and your local minimum temps.

@MickWebb82 yeah this caught me out when I was speccing my cabin setup. 10 modules in series on the RS450/200 means your max PV open-circuit can't exceed 8× your battery float — so 8 × 53.2V = 425.6V absolute ceiling.

Problem is 10× modules in series can easily push past that depending on your panels. Always check Voc at lowest expected temp too, panels get spicy in winter cold.

Ended up running 2 strings of 5 in parallel on mine rather than one big series string — keeps Voc well within limits and gives some redundancy if a string has issues.

Worth double-checking in Victron's MPPT calculator before committing to anything.

So what's the actual Voc figure people are working with when the panels are stone cold on a winter morning? That's where I'd worry — my narrowboat sits out overnight and panel Voc spikes noticeably at low temps.

Also, nobody's mentioned the RS 450/200's maximum PV input voltage of 450V. With 10 modules in series you need to check your worst-case Voc doesn't breach that before you even get to worrying about the float rule.

What panels are people actually using here — anyone running proper temperature-corrected Voc calculations or just guessing?

@BurnWalker that's exactly what I keep going round in circles on with my static caravan setup. I've got Pylontech US5000s sat here waiting to go in and I can't get a straight answer on what Voc figure to actually plug into the calculation.

Is it just panel spec sheet Voc at STC, or do people apply a temperature correction coefficient on top? Because a proper cold Scottish January morning must push that figure up considerably versus the STC 25°C baseline.

And does the RS 450/200's 450V input limit already account for that headroom, or is 450V genuinely the hard ceiling you cannot breach?

Victron's MPPT calculator tool gives me one number, manual calculation gives me another. Which do people actually trust in practice?