# Raising the Vmp Voltage of Solar Panels to match the charge controller.

Registered Users Posts: 6 ✭✭
edited June 2016 #1

I am facing a slight issue, I have a UPS/Solar 1500 Watt inverter which works in both the solar and UPS mode. Manual says that Vmp voltage of the panels should be between 30-32 Volts for best charging of the 24Volts battery bank. I have recently installed 8 solar panels of 250 Watt each with specifications says that Vmp voltage of each panel to be 31 Volts.

I have observed the output for last two days under sunny conditions, Vmp voltage of solar panels turns out to be in range 24Volts to 27 Volts maximum which is not as per the specification of Inverter.

I intend to install 8 more panels into the system, my question is what if I buy a 180Watt panels with specifications of Vmp=24Volts and add two of them in series only to raise the Vmp voltage of system to around 30-32 volts. Will it work?

Summery is

8 panels of 250 Watts with Vmp of 31 volts as per specification already installed in parallel but data gives the Vmp between 25-27 volts,

If two panels of 180 Watt with Vmp of 24 Volts if attached in series with the rest of the system should pull the Vmp voltage somewhat above 30 Volts.

Rest of the six panels of 180 Watt can further be attached in parallel with the system.

Can I make these sorts of tweaks in system.

Tagged:

Short answer... For your setup (as is), you need Vmp-array ~ 35-39 volts or so to charge your 24 volt battery bank (assume lead acid batteries)... The panels you have right now are Vmp~30 volts and, as you can see, as they get "hot" (even in cool climates under a hot sun), the Vmp-array falls to as low as ~Vmp-array-hot ~24 volts.. Not high enough voltage to charge your 24 volt battery bank (which needs ~29+ volts for a cool battery bank to charge fully/quickly).

So--Your options, get Vmp~18 volt (2x in series panels) or a Vmp~36 volt single solar panel, and add those as parallel strings (create a Vmp-array~36 volts)..

Or, use your Vmp~30 volt panels, put them in 2x series for Vmp-array~60 volts, and purchase a MPPT (maximum power point tracking) solar charge controller (rated for your voltage and current) to charge your battery bank directly.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 6 ✭✭

Thanks for a quick reply, as this was really very helpful that I buy solar panels with Vmp 18 Volts and put both of them in series to produce Vmp 36 volts.

Above is a better idea but I have question, will my system with Vmp of 31 Volts and that of Vmp 36 volts setup (two panels of 150 Watt with Vmp of 18 Volts in series) will work under the parallel configuration.

I mean if I connect 31Volts Vmp in parallel to 36 Volts Vmp. Can I do this thing

• Solar Expert Posts: 3,741 ✭✭✭✭
Sandtiger said:
will my system with Vmp of 31 Volts and that of Vmp 36 volts setup (two panels of 150 Watt with Vmp of 18 Volts in series) will work under the parallel configuration.

I mean if I connect 31Volts Vmp in parallel to 36 Volts Vmp. Can I do this thing

It will not work well... their voltages are too far apart.  The MPPT controller won't know which power point to use.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
edited June 2016 #5
And you really need Vmp-array > 35 volts for either pwm or mppt solar charge controllers for a 24 volt battery bank.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 6 ✭✭

Thanks you for your reply as was really very helpful as I have realized that I will have to use the panels in series to increase the Vmp and then further use maximum power point tracking (MPPT) to take care of Vmp.

I have another question, I want to retain my stock charge controller and add MPPT to the system such that MPPT now provides DC output to the my stock charge controller without having any functions of three stage charging or converting the dc output to AC. Only thing, I want is that it checks the output of solar, modifies the Vmp voltage and gives over a new modified value of 30 or 32 Vmp over to my stock.

Reason for this is

I feel that my stock charge controller is best what the market can offer and all what I want from it. it has dual functions like during day it acts as solar charge controller and during night it turns into uninterrupted power supply (UPS) with capacity of 1500Watts.  My battery bank is only 2*160AH lead acid batteries in series. During day, inverter uses solar power to power the load and charge the batteries and during night it uses utility line to power the load and charge the batteries as we have very frequent and unexpected power break downs here. We have nothing like sending power back to the grid so this serves all what I can wish for with only exception that it does not provides mppt function.

Do you think, market offers a product as simple as this without any advanced functions or do you recommend a simple dc converter or regulator to achieve this or any recommendation.

• Solar Expert Posts: 3,741 ✭✭✭✭
Sandtiger said:
I have realized that I will have to use the panels in series to increase the Vmp and then further use maximum power point tracking (MPPT) to take care of Vmp.
If you put panels in series, their current (Imp) must be the same.  Even if Imp is the same for your 250 watt and your 150 watt panels, how are you planning to put two 150 watt panels in series with eight 250 watt panels?

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 6 ✭✭
edited June 2016 #8

Only reason I asked this question was that my 250 Watt panels gives me an average Vmp between 25Volts to 29 Volts maximum so issue was raising the Vmp voltage slightly or maybe by a 15 % whereas manual for this panel quotes a Vmp voltage of 31 Volts.

By buying a 150Watt panels with manual saying that Vmp of each to be 18Volts which could have added in series to make it a 36Volts Vmp and in practice this could have turned out to be around 33Volts.

In parallel Voltages remains same whereas current adds up so this would have implied that if I added two 150 Watt panels in series and would have made a string of 5 sets then I could have increased the voltage for the entire system.

If my current Vmp for 8 * 250 Watt panels were Vmp=27 Volts

If Vmp of new 6 * 150 Watt string were Vmp=33 Volts

Then Averaging things

1.         250 Watt    Vmp=27

2.       250 Watt    Vmp=27

3.       250 Watt    Vmp=27

4.       250 Watt    Vmp=27

5.       250 Watt    Vmp=27

6.       250 Watt    Vmp=27

7.       250 Watt    Vmp=27

8.       250 Watt    Vmp=27

9.       150 Watt* 2    Vmp=33

10.   150 Watt*2    Vmp=33

11.   150 Watt *2   Vmp=33

12.   150 Watt  *2  Vmp=33

13.   150 Watt   *2 Vmp=33

14.   150 Watt   *2 Vmp=33

Average Vmp= (33*6 + 27 * 8)/14=(198 +216)/14 =29.57 Volts.

That was what was going into my mind, obviously I would have lost some performance in terms of Watts per panels.

Average voltage across 14 different panels "does not matter". It is the specific Imp should match to within 10% (high to low spec.) for each panel, if the panels are in series.

And the Vmp of the panel (and string of panels) match to within 10% (or better) if they are put in parallel.

And, to a degree, it matters if the charge controller is PMW or MPPT type.

If the panels are "too different" (wrt Vmp and Imp), you would need to have two different charge controllers. One for the Vmp=33 volt array and a second for the Vmp=27 volt array. It is the nature of the beast (unfortunately).

If you are willing to accept less than optimum harvest from the solar array, sometimes you can use a PWM controller and mix differing panels to a greater degree--But that could cost you 20% or higher of your solar array's overall output power (due to less than optimum configuration--if it is even possible).

The details here matter... Exact panel specifications, battery bank votlage, controller brand/model/type, etc.

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