Sizing a solar array

[davelondon]

I'm planning a vehicle mounted system using a 48V LiFePO4 battery, the Victron Multi RS Solar and three of these solar panels in series. I'd like to understand in more detail how to compare these spec sheets to ensure that these solar panels are appropriate for the MPPT controller. I'd also like to understand which are hard limits and which can be exceeded without safety implications. I'll list them then go through the MPPT specs one-by-one:

From the MPPT controller specs:
Maximum DC voltage 450 V
Start-up voltage 120 V
MPPT operating voltage range 80 – 450 V
Max operational PV input current 18 A
Max PV Short circuit current reverse polarity protection 20 A
Maximum DC charging power 4000 W
Max PV short circuit current (Isc PV) 30 A

From the solar panel specs [values for 3 panels in series in square brackets]:
Rated power in watts (Pmax): 700 W [2100 W]
Open circuit voltage (Voc): 47.17 V [141.51 V]
Short circuit current (Isc): 18.53 A
Maximum power voltage (Vmpp): 39.64 V [118.92 V]
Maximum power current (Impp): 17.68 A

Maximum DC voltage 450 V
I understand that the "Maximum DC voltage: 450 V" relates to the "Open circuit voltage (Voc)", so 141V is well within the capabilities of the MPPT controller. I also understand that this is a hard limit and must not be exceeded under any circumstances, because this is the maximum voltage that the MPPT controller can switch off. I've also read that the Voc figure can be larger for low temperatures, so perhaps better that the solar panel Voc figure is under 80% of the MPPT maximum voltage?

Start-up voltage 120 V
I have no idea what this is.

MPPT operating voltage range 80 – 450 V
The lower bound of 80V - should this be compared with the Voc or the Vmpp? We're well above it with 3 panels, but in the case of a 2 panel array?

Max operational PV input current 18 A
Should this be compared with Isc or Impp? If it's a hard limit, and Isc, then we're above the limit... This means that two strings of these panels couldn't be used in parallel?

Max PV Short circuit current reverse polarity protection 20 A
I have no idea what this is.

Maximum DC charging power 4000 W
We're well below this, but I understand this is a soft limit, and having over 4000W of solar will just cap the output at 4000W.

Max PV short circuit current (Isc PV) 30 A
We're well below this at Isc = 18.53A. Is this a hard limit?

 

[sunshine_eggo]

I'm planning a vehicle mounted system using a 48V LiFePO4 battery, the Victron Multi RS Solar and three of these solar panels in series. I'd like to understand in more detail how to compare these spec sheets to ensure that these solar panels are appropriate for the MPPT controller. I'd also like to understand which are hard limits and which can be exceeded without safety implications. I'll list them then go through the MPPT specs one-by-one:

From the MPPT controller specs:
Maximum DC voltage 450 V
Start-up voltage 120 V
MPPT operating voltage range 80 – 450 V
Max operational PV input current 18 A
Max PV Short circuit current reverse polarity protection 20 A
Maximum DC charging power 4000 W
Max PV short circuit current (Isc PV) 30 A

You are spending way more money than you need. A simple 250/70 is more than you need.

From the solar panel specs [values for 3 panels in series in square brackets]:
Rated power in watts (Pmax): 700 W [2100 W]
Open circuit voltage (Voc): 47.17 V [141.51 V]
Short circuit current (Isc): 18.53 A
Maximum power voltage (Vmpp): 39.64 V [118.92 V]
Maximum power current (Impp): 17.68 A

Yep

Maximum DC voltage 450 V
I understand that the "Maximum DC voltage: 450 V" relates to the "Open circuit voltage (Voc)", so 141V is well within the capabilities of the MPPT controller. I also understand that this is a hard limit and must not be exceeded under any circumstances, because this is the maximum voltage that the MPPT controller can switch off. I've also read that the Voc figure can be larger for low temperatures, so perhaps better that the solar panel Voc figure is under 80% of the MPPT maximum voltage?

Start-up voltage 120 V
I have no idea what this is.

The open circuit voltage of the array must be at or above 120V, or the MPPT will not start charging.

MPPT operating voltage range 80 – 450 V
The lower bound of 80V - should this be compared with the Voc or the Vmpp? We're well above it with 3 panels, but in the case of a 2 panel array?

Vmp.

Max operational PV input current 18 A
Should this be compared with Isc or Impp? If it's a hard limit, and Isc, then we're above the limit... This means that two strings of these panels couldn't be used in parallel?

Imp. It's generally regarded as a hard limit with Victron.

Max PV Short circuit current reverse polarity protection 20 A
I have no idea what this is.

Isc.

Maximum DC charging power 4000 W
We're well below this, but I understand this is a soft limit, and having over 4000W of solar will just cap the output at 4000W.

Yep.

Max PV short circuit current (Isc PV) 30 A
We're well below this at Isc = 18.53A. Is this a hard limit?

Generally regarded as a hard limit.

Again, you literally have no need for a 450V unit. You don't even need a 250/70, but that's the smallest they offer in 250V.

 

[davelondon]

Thanks!

Again, you literally have no need for a 450V unit. You don't even need a 250/70, but that's the smallest they offer in 250V.

It’s a Multi RS Solar so as well as being an MPPT controller it’s an inverter etc.

 

[sunshine_eggo]

Are you in the EU?

 

[davelondon]

Are you in the EU?

I’m in the UK