VmP or VoC?

nschizzano · 3:44AM

Do I add up VMP or VOC for stringing a 400w array in Series?

I have a 135v - 450v window for the jackery explorer 5000. Jackery is confident if it goes over 450 in the cold, the unit will safetely manage the situation, be it regulate voltage or cut off.

I have 13 Solar Panels, I payed 100 dollars each. They are new Axitech. They were intended to be 400w each. But when I installed the last 6 from my stack, the panels appeared slightly different and turns out some were 440w.

Volts and amps all seem very close. Close enough to my knowledge.

Turns out they are all Bifacial too so thought that may be an added benefit.

I am not sure what numbers to add up. And if i can saftely use all 13.

I am in minneosta. If it gets too cold the jackery should safely cut off when the voltage goes over 450.

I have always added up VOC, the feller i got the panels from was a solar installer for 5 years and he said to add up VMP.

When I google search, it tells me to add up the VOC.

Here is the data:

Anyways I have: 6 Axitec AC-440TGB/108BB and 7 Axitec AC-400MBT/108V

Here is a Google overview and I beleive its accurate:

A single series string results in a total open-circuit voltage (Voc) of 498.47 V and a maximum power voltage (Vmp) of 410.12 V under standard testing conditions.Total Array Voltage CalculationIn a series circuit, voltage is additive, but the current is restricted to the lowest-rated panel.Axitec AC-440TGB/108BB: Voc = 39.83 V | Vmp = 32.24 VAxitec AC-400MBT/108V: Voc = 37.07 V | Vmp = 31.01 VOpen-Circuit Voltage Calculation (Voc):$6 × 39.83 V + 7 × 37.07 V = 238.98 V + 259.49 V = 498.47 VMaximum Power Voltage Calculation (Vmp):$6 × 32.24 V + 7 × 31.01 V = 193.44 V + 216.07 V = 410.12 V

Additionally my max Amp the jackery can handle is 15A, and these panels are rated below that around 13-14

seems like with the 440w panels it will still work in the mix.

They may produce more around 410-415 and thats okay, along with all being bifacial i should get a little extra juice.

Hope it works.

nschizzano · 4:08AM

Also, if my voltage keeps peaking 450v in the minneosta cold. I can always disconnect one or even two panels on the string very easily.

heavy_humster · 2:24PM

Short answer: for the upper voltage limit you add VoC, and you have to correct it upward for cold. Never size the ceiling off Vmp.

Why: Vmp is the operating voltage once the MPPT is actively tracking under load. VoC is open-circuit voltage, the highest the string can ever sit at, and it happens at the worst moment for you, a cold sunny morning before the controller has started pulling load (or any time the load drops out). The 450V on the Jackery is an absolute do-not-exceed input rating, so you check it against the worst case, which is cold VoC, not Vmp. So Google is right and your installer buddy is wrong on this one.

Here is the part that matters for Minnesota: VoC goes UP as it gets colder. Panels are rated at 25C (STC). Every degree below that adds voltage. For these Axitec mono panels the VoC temp coefficient is around -0.27 percent per C (check your datasheet, listed as Temp Coefficient of Voc or beta).

Your STC VoC is already 498V, that is over 450V before correcting for anything. Now correct for a cold Minneapolis morning:

Corrected VoC = 498 x [1 + 0.0027 x (25 - Tmin)]

At a realistic extreme of -30C:

498 x [1 + 0.0027 x 55] = 498 x 1.149 = about 572V

That is ~570-580V hitting a 450V input. That is not a little over, that is well into damage territory for the MPPT front end. I would not lean on the unit will manage it. A rated max input is a hard limit and it is there for a reason.

Also, disconnecting a panel or two by hand in the cold is not a safe plan, because peak VoC happens at dawn at the coldest temp, before you are even out there. The over-voltage comes first, the fix comes after.

What I would do: size the string so cold-corrected VoC stays under 450 with a little margin. Working backward at -30C (x1.149), your STC VoC budget is about 450 / 1.149 = 391V. Your panels average ~38.3V VoC each, so that is about 10 panels in series, not 13. Run 10 and keep the other 3 aside or on a separate setup.

Current side you are fine. Series does not add amps, so ~13-14A Imp is under the 15A limit. One caution: these are bifacial, and on a cold bright day with snow on the ground the rear-side gain plus snow reflection can push Isc up noticeably. Keep an eye that you do not creep over the 15A input on those exact cold sunny days.

Bottom line: add VoC, correct it for your coldest expected temperature, and that points to about 10 panels max on this unit. 13 in series will sit way over 450V on a cold morning.

VmP or VoC?

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