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

Q&A and troubleshooting
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1

I am designing an off-grid system for Mallorca using:

  • 4 × SmartSolar MPPT RS 450/200
  • 28 × Pylontech US5000 (CAN-BMS, DVCC enabled)
  • AIKO Neostar 3P+ A480 modules
  • Module Voc = 40.9 V, Voc temperature coefficient = -0.22 %/°C
  • 10 modules in series per string

The minimum ambient temperature at the installation site is about +2 °C. A temperature below 0 °C would be extremely unusual.

Therefore:

  • String Voc at 25 °C = 409 V
  • String Voc at +2 °C ≈ 430 V
  • This is below the absolute 450 V limit.

However, the RS450/200 manual states:

“The maximum open circuit voltage of the PV array must be less than 8 times the minimum battery voltage when at float.”

With the Victron/Pylontech recommended float voltage of about 51 V, this would limit the array to approximately 408 V, which would rule out many otherwise valid 48 V system designs.

My questions are:

  1. Is the 8× float voltage requirement a hard operating limit or a conservative design recommendation?
  2. Does it also apply when the battery is managed by the Pylontech CAN-BMS with DVCC?
  3. Would this 10-module string (≈430 V Voc at +2 °C) be considered a supported configuration, or should I reduce the string to 9 modules?

Can the MPPT RS actually trigger Error 29 solely because the PV Voc exceeds 8 × float voltage, even though the PV voltage remains below the absolute 450 V limit?

Thank you.

2

Use this tool, it will answer most of your questions.

3

Hi @Heidelbike

The 8x float voltage rule is a hard limit, and a constraint of the internal DC to DC converter.

It must be adhered to. DVCC does not change the situation.

The limit itself comes into effect when the actual PV and battery voltage reaches these points. As the battery fills, the MPPT reduces its output by raising the PV voltage (and correspondingly lowering the PV current). The limit to this reduction is the 8X float limit, at that point it can’t raise PV voltage any further, and if this isn’t at the VoC point where PV current is zero, then power will continue to flow into the battery and it will be overcharged.

Then the MPPT will shut down with an alarm.

A PV array at Voc is a normal and expected operating condition, not some rare case. Further as PV voltage rises due to low temperatures, a normal system in a cool climate is more likely to experience high PV voltages than just when in a Voc condition.

I know these RS design restrictions can be frustrating when trying to optimise, but they must be adhered to for a reliable system.