solar panel help...

[crosser]

hi,  can anyone tell me if these solar panels will be ok for my narrowboat.. my boat is a 24v system...  thinking of buying 2...  thanks

 

https://www.cityplumbing.co.uk/p/longi-solar-405wp-full-black-solar-pv-module-lr5-54hib-405m/p/642893?gmcpid=642893&gclid=CjwKCAjwyqWkBhBMEiwAp2yUFsUGCvjigcjy5056mZK-thZaNEc_ZWX_FsM0sIcl_z7glClR6kufThoC4REQAvD_BwE&gclsrc=aw.ds#fo_c=2832&fo_k=ea6315bfdac7735b056f500e98bb00da&fo_s=gplauk

[Tony Brooks]

But what controller will you be using. They seem to be 30 volt panels so you will probably connect them in series and use an MPPT controller. I feel doing them in parallel to give 30 volts is a bit close to the near 29 volts needed to fully charge a LA battery bank. Others may have a different idea.

[IanD]

6 minutes ago, Tony Brooks said:

But what controller will you be using. They seem to be 30 volt panels so you will probably connect them in series and use an MPPT controller. I feel doing them in parallel to give 30 volts is a bit close to the near 29 volts needed to fully charge a LA battery bank. Others may have a different idea.

Definitely not enough margin above battery voltage, especially allowing for lower light levels and hot panels when the voltage will be lower -- need to connect in series with an MPPT controller rated to at least 80V input, preferably 100V to get some margin.

Edited June 14, 2023 by IanD

[matty40s]

Yes, a pair of these will be absolutely fine, espwcially is used in series, but will perform absolutely fine in parralel if  you use an quality MPPT (victron, outback etc). With the bluetooth Victron and the app you can set the parameters for ideal charging regime, but it works fine straight out of the box.

1 hour ago, IanD said:

Definitely not enough margin above battery voltage, especially allowing for lower light levels and hot panels when the voltage will be lower -- .....blah blah

Not correct 

[IanD]

2 hours ago, matty40s said:

Yes, a pair of these will be absolutely fine, espwcially is used in series, but will perform absolutely fine in parralel if  you use an quality MPPT (victron, outback etc). With the bluetooth Victron and the app you can set the parameters for ideal charging regime, but it works fine straight out of the box.

Not correct 

Victron MPPT data sheet says the PV voltage must exceed battery voltage by 5V for the controller to start up.

 

Those 30V (nominal) panels do not meet this requirement in a 24V (nominal) system, especially with LA batteries close to 100% SoC, or in equalisation/absorption, when you consider minimum panel voltage and maximum battery voltage. 2 panels are needed in series to guarantee that the MPPT will start up in all conditions.

Edited June 14, 2023 by IanD

[matty40s]

29 minutes ago, IanD said:

Victron MPPT data sheet says the PV voltage must exceed battery voltage by 5V for the controller to start up.

 

Those 30V (nominal) panels do not meet this requirement in a 24V (nominal) system, especially with LA batteries close to 100% SoC, or in equalisation/absorption, when you consider minimum panel voltage and maximum battery voltage. 2 panels are needed in series to guarantee that the MPPT will start up in all conditions.

That's a 6v difference to start with....

 

These 30v nominal panels will never ever show 30v once the sun hits them, unless everything has changed in the last month following my 12 years succesfully installing (100's) of solar panels to many happy boaters boats.

😆⁹

[IanD]

1 minute ago, matty40s said:

That's a 6v difference to start with....

 

These 30v nominal panels will never ever show 30v once the sun hits them, unless everything has changed in the last month following my 12 years succesfully installing (100's) of solar panels to many happy boaters boats.

😆⁹

OK, have your way, I'm not going to argue the point with you.

[matty40s]

1 minute ago, IanD said:

OK, have your way, I'm not going to argue the point with you.

I really wish you would, as I am incredibly interested in your workings to back your statements up.

[IanD]

25 minutes ago, matty40s said:

I really wish you would, as I am incredibly interested in your workings to back your statements up.

It depends on the exact V-I curves of the solar panel, for example this:

 

https://www.victronenergy.com/blog/2020/02/20/pv-panel-output-voltage-shadow-effect/

 

The example given has a maximum power voltage of about 33V, and at high light levels the open-circuit voltage is about 40V -- but this drops at lower light levels down to maybe 36V or a bit lower. For the panels posted Voc is about 37V so the open-circuit voltage could be around 33V at low light levels.

 

On top of this the voltage drops by about 0.3%/C (0.1V/C), so when hot the O/C voltage could easily drop another couple of volts down to maybe 31V.

 

To guarantee MPPT startup this means the maximum battery voltage would be 26V, which can easily be exceeded in a 24V system especially with LA cells.

 

Most of the time not everything will add up worst-case and the MPPT will start up OK especially in bright sunlight, but there's little margin and under some conditions (e.g. low light levels) this might not happen, especially with LA batteries.

 

So you're right, most of the time this will probably be fine -- but with 2 panels in series it's guaranteed to work under *all* conditions 🙂

Edited June 14, 2023 by IanD

[blackrose]

If the MPPT requires solar panel input voltage to be at least 5v above the battery voltage to start pumping charging current into the battery bank, that's around 18v or so. 

 

Once the MPPT switches ON, it then only requires the input to remain at least 1v above the battery voltage. 

 

Therefore a series panel connection (which will have a higher total voltage than a parallel connection) will naturally reach 18v as the sun comes up sooner than a parallel connection. And vice versa in the evening. Hence a higher overall solar harvest from a series connection.

[IanD]

10 minutes ago, blackrose said:

If the MPPT requires solar panel input voltage to be at least 5v above the battery voltage to start pumping charging current into the battery bank, that's around 18v or so. 

 

Once the MPPT switches ON, it then only requires the input to remain at least 1v above the battery voltage. 

 

Therefore a series panel connection (which will have a higher total voltage than a parallel connection) will naturally reach 18v as the sun comes up sooner than a parallel connection. And vice versa in the evening. Hence a higher overall solar harvest from a series connection.

It's a 24V system, not 12V...

 

Also the panel voltage doesn't drop much with light level, but the current does. So long as there's enough margin of panel voltage over battery voltage as I described, having more panels in series won't start up any sooner or carry on any later.

[ditchcrawler]

12 minutes ago, blackrose said:

If the MPPT requires solar panel input voltage to be at least 5v above the battery voltage to start pumping charging current into the battery bank, that's around 18v or so. 

 

Once the MPPT switches ON, it then only requires the input to remain at least 1v above the battery voltage. 

 

Therefore a series panel connection (which will have a higher total voltage than a parallel connection) will naturally reach 18v as the sun comes up sooner than a parallel connection. And vice versa in the evening. Hence a higher overall solar harvest from a series connection.

I don't think it works quite like that,