Another MPPT question

[BlueStringPudding]

I'm going to be upgrading my solar panels soon to 480w. The new panels are designed to charge a 12v system

 

I have a 20 amp or 30 amp MPPT controller capable of taking up to very high voltages (50 volts plus at the moment) and I don't want to have to buy another ideally.

 

Although the panels are unlikely to ever chuck out their full current and there's a very long cable run to the controller which means some voltage drop probable; I want to be sure the set up is safe for the controller I have.

 

There will be four x 120 watt panels. Should I wire them in series or parallel? And how does that affect the amount of charge that will be going into the batteries from the controller?

 

Thanks leccy gurus

[nicknorman]

We'd need to know the exact spec of the controller and panels in terms of the controller's max input voltage, and the panels' max no load voltage. However my first guess would be a series - parallel configuration ie two parallel strings of two series panels.

 

Are the 4 panels all the same make/ model?

Edited January 28, 2015 by nicknorman

[BlueStringPudding]

Yes, same make and model panel. If the controller can cope with 50v at the moment it's not going to have a problem with 24v instead of 12v, surely?

[nicknorman]

Yes, same make and model panel. If the controller can cope with 50v at the moment it's not going to have a problem with 24v instead of 12v, surely?

 

Panels designed to charge a 12v battery are likely give their max current at around 14v, and under no load conditions this will rise substantially, maybe 20v or more. When a controller decides that the batteries are full and stops taking current from the panels, the panel voltage will rise to the open circuit voltage. This obviously depends on the type of panel, but also on the temperature (higher voltage in colder weather). So with 2 in series you would be OK up to 25v max per panel. Whether it is within this limit we can't say until you give us the spec of the panels. Yes, it will probably be OK but it is only a guess until we have the info.

[BlueStringPudding]

The four solar panels have this rating each:

 

Maximum Power: 120 watts
Maximum Power Voltage: 16.9 volts
Maximum Power Current: 7.10 amps
Open Circuit Voltage: 21.5 Voc
Short-circuit Current: 7.45 amps

 

My present MPPT controller has this spec:

System Voltage: 12v or 24v

Min.Battery Voltage 6v

Max.SolarVoltage 60V

Max.SolarCurrent 20A
Max.LoadCurrent 20A

 

I have a back up controller which I thought blew up, but after I discovered a wire had been stepped on and broken, it may well be fine. That one I think has this spec:

 

System Voltage 12v/24v

Rated current 30A

Rated load current 30a

Max Voltage 50v

 

So which controller should I use (the 20a is easiest because it's already wired in) and if I wire them in series or parallel - how does that affect the amount of charge that will be going into the batteries from the controller?

[nicknorman]

Using the series/parallel configuration I mentioned you would have max open circuit voltage of 43v (before allowing for any temperature coefficient) and max current of 14.2A. So the 20A controller will be fine, and it has the advantage of a higher max input voltage in the event of full batteries, bright sunshine and cold temperatures.

 

If you connect the panels all in series, the voltage will be far too much

 

If you connect the panels all in parallel, the max current becomes 28.4A thus ruling out the 20A controller. Also, you will need heavier duty wiring and/or have more power loss in the wiring due to the higher current and lower voltage.

 

Thus the series / parallel config is a no-brainer!

[BlueStringPudding]

Fab thank you, Nicknorman Very clearly put, which is what I needed.

 

Now what does a series/parallel configuration look like? I can't picture it. Can you (or anyone) link to a diagram of such a set up please?

 

Just so I know, I have another question. The cable run is long (50ft each way maybe) and is either 4mm or 6mm thickness cable - I can't remember which. The delightful Mister Matty made the cable up for me so might remember. So could this theoretically carry the parallel set up with 28.4A? Although I supposed voltage drop may still be an issue.

Edited January 28, 2015 by BlueStringPudding

[nicknorman]

Fab thank you, Nicknorman

 

Now what does a series/parallel configuration look like? I can't picture it. Can you (or anyone) link to a diagram of such a set up please?

Diagram 3 on page 9 of this document illustrates it. Of course the doc is about batteries, so you have to mentally substitute panels for the batteries!

 

http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf

[BlueStringPudding]

Now I get it - brilliant. Thank you.

[spadefoot]

Excellent topic. Well asked questions & straightforward answers. Cheers.

[Boredrider]

Do the panels have diodes fitted? I have four 100w wired in series-parallel and I was advised to include a couple of diodes in the circuit because the panels did not have them.

[BlueStringPudding]

I don't know.