Originally Posted by zboss
Take the following solar
Peak Power (+/- 5%) - Pmax 137 W
Rated Voltage - Vmp 24.0 V
- Imp 5.7 A
Open Circuit Voltage - Voc 29.1 V
Short circuit Current
- Isc 6 A
Let assume an MPPT
The panel spec show a rated current of 5.7 AMPS.
If my batteries require a charge of 14.8 volts peak what is the max amperage you can expect to see from the panel?
5.7 amps or 9.25 amps (137 watts / 14.8 volts)?
Although I agree with everyone about SOC of batteries, etc. I think that your key question is why your calculations show you that you will see more then the Imp of the panel.
I see you've labeled Imp as the rated current. I don't know where you got that, but Imp is the current for maximum power, not some sort of maximum current.
Have a look at this typical solar
The maximum power point of the panel is reached when it's running at Vmp and Imp. However, the maximum current is actually at zero voltage (short circuit). For your panel, that's 6 Amps.
A lot of people are telling you that you'll get twice the current at half the voltage (this is a 24V panel). This isn't true. As you drop the voltage, the current will max out at Isc, the short circuit current.
So if you hook the panel up directly to batteries that are at 12 V you'll only get 6x12 = 72W from it.
Now comes the magic of MPPT
(as Ocean Planet so eloquently explained). Your MPPT will keep your panel at the max power point. Your panel will see 24V and will produce 5.7A.
The MPPT will then take that 137W worth of power and downconvert it to the 12V for your battery
(that is all kinds of wrong technically, I'm sure, but it's the easiest way to think about it. The real answer has more to do with impedence, etc). So your batteries will get 137/12 = 11.4A from the panel which is getting 5.7A from the panel. This magic is possible because the power's the same on either side.
This is why MPPT controllers are really required if you're going to use high voltage panels
. Otherwise, you throw away half the power.