Solar panel current calculations

[Wunja]

This question is prompted by another thread: http://www.ybw.com/forums/showthread.php?p=3312726#post3312726

The technical specification on the panel has:

Pmax: 40w

Open circuit voltage(Voc): 20.7V

Optimum power voltage(VMP):18V

Short circuit current(Isc): 2.44A

Max operating current(IMP): 2.22A​

So at 18 volts it generates 2.22 amps 18*2.22 = 39.96 watts. Fine.

My question is, what amp/watt output should be used when calculating how much energy can be used to charge a 12 volt battery.

12*2.22 = 27 watts: The panel has a lower rating than expected

or

40/12 = 3.33 amps: The panel generates more current than quoted.​

I appreciate that solar never generates what it says on the label. But this does have a major effect in consumption/replacement calculations.

 

[bedouin]

Unless you have a very posh controller - the Amps figure is what matters - so peak charging current will be no more than 2.2A

 

[VicS]

You really need a curve of output current vs terminal volts to say.

Charging a battery will pull the terminal volts down below the VMP value. In doing this the current will be very slightly higher than the IMP value.
The Isc value is the highest you can get but that will be a zero terminal volts so not a lot of good.

 

[Chalker]

Realistically assume the 2.22A for no regulator of a simple regulator.

You can do better using a MPPT regulator, they do not seem to be expensive in the low power ranges (ebay for MPPT).

http://www.blueskyenergyinc.com/uploads/pdf/BSE_What_is_MPPT.pdf

John

 

[William_H]

Solar current

Funny I explained some of this yesterday
http://www.ybw.com/forums/showthread.php?p=3311802#post3311802
2,2 amps is all you will get so power depends on voltage of battery olewill

 

[noelex]

As others have said the most current you will see when operating will be 2.22A. This can be produced in theory at 18v, but not in practice. Solar cell specs are quoted for a cell temperature ( not air ) of 25C in practice the temperature of the cells will be about 40C in full sun. This temperature reduces the voltage significantly and the current slightly. So when operating in very bright conditions (although you can get brighter than the specs are quoted for with exceptional days) with no shadows we might expect 16.5V @ 2.1 A. A conventional controller cannot make use of the 16.5V, so we must use battery voltage This leaves typically say
2.15 X 13.9v (The extra 0.05 because its not MPPV) = 29.9w

With an MPPT controller you can use the 16.5v, but they have some inefficiency converting the voltage , so we have to deduct say 5%
(2.1 X 16.5) – 5%= 32.3W

MPPT also suffer some episodes of mistracking and there complicated circuits consume more power under low light, so the overall gains will be smaller than these numbers suggest

The bottom line is you will not often see more than 30w from a 40w panel