solar panels advice to charge 24v battery bank

[fuss]

I was thinking of adding some solar panels to charge my battery bank (and lessen the amps used in 24hrs etc) and after some research and a bit of a look around the forum I have made some asumptions and I have some questions.

I am limited for space, so the amount of room the take plays a big role in any purchase decision.

I see that there is some 20v panels (I guess these are the 12v ones) and more than 30v panels (the 24v ones) but both seem to be around the same physical size. This means that it is better to buy the 30v ones that buy the 20v ones and put them in series because they would take up much more room and would cost much more.... is that all true?

A MPPT controller will gve me more amps for longer as it can convert the available watts into a voltage that will charge the batteries when the output is low as in later in the day.... is that true?

Am I on the right track here or have I missed the point?
Thanks in advance
John

 

[Playtime]

The watts output from a solar panel depend on the surface area and the type of crystalline structure (monocrystal (better) or amorphous (cheaper)).

For the same type and size of panel you would get the same watts whether it was structured as 12v or 24v (i.e. you would get half the amps from the 24v panel in the same conditions).

Does that help?

 

[VicS]

I think if you compare like with like you will find that nominal 12 volt panels have an open circuit voltage of around 20 volt while the maximum power output is delivered at around 16 or 17 volts. A similar 24 volt panel will have a OC voltage of around 40 volts while the max paower is produced at around 32 to 34 volts.

The power is related directly to the surface area so for a given power output a 12 volt panel will have the same area as a 24 volt panel of the same power. The current output of the 24 volt panel will be half of that of the 12 volt panel.

It will make little difference if you choose 12 volt panels and put them in series or 24 volt panels in parallel to make up your requirements

(perhaps you'd have to watch out for the loses imposed by two built in blocking diodes if putting 12 volt panels in series though)

I think you have to compare what is on offer from various suppliers to find panels which will make the most use of the spaces you have available.

As I understand it the MPPT controller will make use of the difference between the (approx) 14 volts required for charging and the 16- 17 (or thereabouts) volts at maximum power by converting the otherwise wasted power into extra amps.
(Don't know how they do that ... probably would not understand it if I found an explanation !! )

From a website I chanced upon:

Maximum Power Point Tracking controllers are the latest evolution in the solar pv industry. In some conditions MPPT technology can give a power boost of around 33% over a normal 'shunt' or 'pwm' controller.

MPPT controllers convert the incoming power from the solar panel into the optimum voltage to charge the battery bank. Most solar panels typically produce approx 17.5v at maximum power, however a 12v battery only needs around 14v to charge it.

With a normal 'pwm' solar controller this voltage difference is effectively 'lost' - it just disappears in the charging process. However with an MPPT controller, the excess voltage is converted into extra current, which results in more power going into the battery.​

 

[fuss]

I think if you compare like with like you will find that nominal 12 volt panels have an open circuit voltage of around 20 volt while the maximum power output is delivered at around 16 or 17 volts. A similar 24 volt panel will have a OC voltage of around 40 volts while the max paower is produced at around 32 to 34 volts.

The power is related directly to the surface area so for a given power output a 12 volt panel will have the same area as a 24 volt panel of the same power. The current output of the 24 volt panel will be half of that of the 12 volt panel.

It will make little difference if you choose 12 volt panels and put them in series or 24 volt panels in parallel to make up your requirements

(perhaps you'd have to watch out for the loses imposed by two built in blocking diodes if putting 12 volt panels in series though)

I think you have to compare what is on offer from various suppliers to find panels which will make the most use of the spaces you have available.
[/INDENT]

Thanks,

I also thought that 2 12v panels in series would be the same area roughly as a 24v of the same wattage but look at this example....

The first is a 12v panel suntech 140w
1482mm x 676mm
http://www.energymatters.com.au/suntech-140watt-12volt-monocrystalline-solar-module-p-1405.html
and the 2nd is a 24v panel suntech 170w
1580mm x 808mm
http://www.energymatters.com.au/suntech-170watt-24volt-polycrystalline-solar-panel-p-2500.html
It looks to me like the 24v uses less area than if I put 2 12v panels in series.
I am sorry that I could not find 2 exact wattage ones

So it looks to me that once you get to around 140w panels, it is much better to go with 24v than 2 x 12v in series as it will take much more room.
What do you think?

 

[VicS]

The two you compare are pretty much the same in terms of power output per square metre (the dims given are the overall dims of the panel not just of the area of the cells so not going to agree exactly)

When you put the two 12 volt panels in series you will get the same current but at double the volts so the power you are producing will be doubled.

So you have to compare two 12 volt 140 watt panels in series with a 24 volt 280 watt panel.

If you want to compare 12 volt panels in series with your 170 watt 24 volt then you must compare a couple of 12 volt 85 watt panels.

OK they dont do 85 watt ones but look how 80watt or 90watt panels compare


(BTW if connecting in series the two must be the same power)

 

[fuss]

If you want to compare 12 volt panels in series with your 170 watt 24 volt then you must compare a couple of 12 volt 85 watt panels.

OK they dont do 85 watt ones but look how 80watt or 90watt panels compare

Yes they do make an 85w one.

I checked the specs and you are completely right the 85w 12v one in series is nearly exactly equal to the 170 w 24v one.

Thanks again for the help and clarification.

 

[mitiempo]

fuss
As VicS posted the MPPT controller will convert high voltage to the proper voltage at a higher amp rate. A less expensive controller will just waste the extra voltage as heat. The reverse is also true. With a less expensive controller if the output is 12 volts it cannot charge. With a MPPT controller it will pulse charge a higher voltage from the lower voltage current so you get some charge. Overall they are about 30% more efficient or better.

 

[William_H]

Soolar panels

The difference between the PV voltage ie 20volts and the actual battery voltage ie 14v is lost in resistive heat in the PV cells. Or if you use a regulator in the regulating transistor.
So the MPPT controller takes whatever it gets from the PV cells and converts or transforms this into the voltage required for the battery. It would appear to get more power out of the PV cells at any sunshine level.
However any switch mode type transformation has inefficiencies in itself of 80 to 90% so it is not all gain.

I think for the OP the best deal will be 12v (20v) panels 2 in series in a size he can afford and fit on the boat. If the panel wattage is small compared to the battery size then do not fit a regulator but just connect direct. (assuming the panels have a blocking diode fitted.) (usually).
No the 2 blocking diodes in 2 panels will not make any significant losses. Though really only one is needed.
good luck olewill

 

[mitiempo]

William
While there are losses in any system an MPPT controller will give you less losses than any other option. The rule I've found on whether a controller is needed is if the panel's approximate daily output is greater in amp hours than 1/60 of the size of the battery bank you should have one. Here's a link with some good info.
http://blog.dankim.com/2009/02/02/solar-power-on-boats/

 

[VicS]

The rule I've found on whether a controller is needed is if the panel's daily output in amps is greater than 1/60 of the size of the battery bank you should have one

that should of course read " .... if the panels daily output in ampere hours (Ah) ...."

The author of the link would do well to get his units sorted out as well!

Another rule of thumb is that if the panel output in watts is smaller than 1/10 of the battery capacity in Ah then a regulator is not required BUT it is advised with any panel over 10 watts anyway regardless of the 1/10 rule.

 

[mitiempo]

VicS
I would have thought the use of the words "daily output" would have sufficed but I changed the wording to please you.