# How estimate the electrical load of a photovoltaic module?

Registered Users Posts: 1
Hello, I'm newbie in this topic, but I want to know more about it. My problem is, I want to measure the power and voltage values ​​of a set of modules connected in series (2 PV modules, only this).

I have only 2 PV modules:

SUN EARTH
PV-MODULE 235 Wp
POLYCRYSTALLINE TPB 156x156-60-P

ELECTRICAL DATA (STC)
Max. power (Pmpp): 235 W
Max. voltage (Umpp): 29,2 V
Max. current (Impp): 8,05 A
Open circuit voltage (Uoc): 36,7 V
Short circuit current (Isc): 8,47 A

So, in serie this data is:
Max. power (Pmpp): 470 W
Max. voltage (Umpp): 58,4 V
Max. current (Impp): 8,05 A
Open circuit voltage (Uoc): 73,4 V
Short circuit current (Isc): 8,47 A

But I want to measure the data of these two PV modules over a long period of one day or one week.

I have an equipment to measure data from each PV module separately as one I will make a kind of shadow and the other not.

So my doubt or my question is about if the two PV modules are connected in series and I want that the system is connected to an electric charge during the same period, what kind of electric load I use.

Where I can found that Rload min and max reference? I find that Rmin is Voc/4Isc and Rmax is 4Voc/Isc, but I want to know why?

Thanks ..

Re: How estimate the electrical load of a photovoltaic module?

Welcome to the forum Cosuroca!

Backing up a moment... In general, if you are trying to understand/measure/model solar panels, you need to both understand the panels (current, voltage, and power curves) and the loads.

Solar panels are (for the most part) "Current Sources". That means that the panel (between a dead short and Vmp -- voltage maximum power) the current is proportional to the amount of sun hitting the panels. So, the output voltage of the panels is very dependent on the loads themselves.

If you put a simple resistive load on solar panels with variable sun, you are not going to learn very much. If you have programmable electronic loads, setting them for constant voltage can give you interesting results (say you are charging a 48 volt battery, set the voltage to ~58 volts and monitor the current vs sun light vs shading).

But, you will quickly figure out that these Vmp~30 volt panels do not play well with 12/24/48 volt lead acid battery banks.

For battery banks, you need a Vmp voltage (standard test conditions):
12 volt bank ~ 17.5 to 19 volts Vmp
24 volt bank ~ 35 to 38 volts Vmp
48 volt bank ~ 70-76 volts Vmp

Your Vmp 29.2 volt panels do not "fit" charging a battery bank with a PWM (pulse width modulation) charge controller (basically an On/Off switch).

The Vmp @ 30 volts is too high for a 12 volt battery bank (almost 50% loss of panel wattage). And Vmp 30 volts is too low to charge a 24 volt battery bank (needs ~30 volts minimum from array).

Similar for Vmp-array = 60 volts is too low for a 48 volt battery bank (which does need a minimum of 60 VDC to charge fully/quickly).

The last 48 volt array issue being that Vmp is at ~25C (standard test conditions). When you run a panel in hot weather under full sun, your Vmp can drop by 20% (+/-)--So your Vmp-array of 60 volts actually becomes Vmp-array at 48 volts on a hot day. Which is way to low to even begin to charge a 48 volt battery bank.

With these type of panels, we recommend using a MPPT type charge controller. These controllers use a buck mode (typically) switching power supply to take high voltage/low current and efficiently down convert to low voltage/high current needed to recharge the battery bank.

A MPPT controller does a quick measurement of the Array's current and voltage (from zero to maximum amps, and builds a table of P=V*I of the array--Looking for Pmax at the moment of time/under that amount of sun). It them "sets" the Vmp-array (say 48 volts for two panels in series under hot sun) and efficiently down converts to the ~14.5 volts needed to charge the 12 volt bank (or ~29 volts needed to charge a 24 volt bank).

Note that charge controllers operate in several different modes:

Bulk: Maximum output current (from ~10.5 volts to ~14.4 volts)
Absorb: Set voltage mode (at ~14.5 volts volts, as battery charges, current drops)
Float: Ste voltage mode (at ~13.5 volts, keep battery charged, supply current to DC loads as needed)

So--You can see there are several modes where you can get "maximum" Power from the solar array--That is Bulk mode (battery draws all power), or the other two modes if the loads equal or exceed solar array output power (draw bank voltage below set point voltage).

As you can see--This is not an "easy" environment to simulate "on the cheap" using power resistors. You either should pick a charge controller (PWM or MPPT) and battery bank that will represent your "standard" PV off grid power system setup. And you will need some way to draw a controlled amount of power/current from the DC Battery bank to make sure that the MPPT (or even PWM) charge controller are in a "known state".

Does this help (sorry--English is my only language, and I am not very good at it. That is why I became an engineer--so I would not have to talk with people. :p)?

Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset