Calling on Solar Techies
I have been thinking of going solar for some time and decided to pull the plug. My wants are a 400 to 500 watt system. I have four 12 volt batteries with 340ah. I am not planning on changing b...
Forum Discussion
I am sure Pianotuna and BFL get it, as do I. MPPT primarily gets you more power when the battery voltage is less than Vmp, as Pianotuna succinctly explains. The extra charging early and late is a secondary and minor effect.
In your example below with Vmp 17.9V and 11.5A assuming your battery is in bulk charge (14V) with a PWM charge controller you can harvest 11.5A x 14v = 161W. With an MPPT controller you can harvest 11.5A x 17.9V = 205W, so an extra 25% or so more power.
With MPPT you absolutely can control the current. You can't produce more current than is available from the solar panel, or more than the battery will accept, but within those limits you can set what current you want. This is useful because it allows you to over-panel your system.
In your example below with Vmp 17.9V and 11.5A assuming your battery is in bulk charge (14V) with a PWM charge controller you can harvest 11.5A x 14v = 161W. With an MPPT controller you can harvest 11.5A x 17.9V = 205W, so an extra 25% or so more power.
With MPPT you absolutely can control the current. You can't produce more current than is available from the solar panel, or more than the battery will accept, but within those limits you can set what current you want. This is useful because it allows you to over-panel your system.
Gdetrailer wrote:
Wrong, you cannot "control" the current even with MPPT.
You are "assuming" that the battery is like a fixed resistor, it is not. You cannot FORCE more current into the battery than what it can "absorb".
The ONLY real advantages to using MPPT is it allows you to use mismatched panel voltages from the battery voltage, allows panel voltages twice or higher than the battery voltage, the higher panel voltage allows for use of smaller ga wire but yet yield more panel wattage, allows charging to commence EARLIER in the day and to continue LATER toward evening which allows you to harvest longer and netting a bit more yield.
Example, two 100W "12V" panels (17.9V VMP) in parallel gets you about 11.5A (about 5.75A each panel)
same panels in series now gets 35.8V VMP at 5.75A..
Both add up to 200W..
17.9 x 11.5 = 205.85W
38.8 x 5.75 = 205.85W
The difference with MPPT is once the series panel gets at least 2V higher than the battery voltage it can start charging.. While it may start charging, it may not have enough current from the panel to be able to harvest full 200W until later when the sun is more direct on the panel.
MPPT has ZERO to do with how or what "modes" it is in for battery charging but more with getting enough voltage from the panels to start charging FASTER and LONGER over the entire day..
Your arguments are not valid because you are ASSUMING that the MPPT cares about the load attached and is regulating the load current attached, it is not.
In reality, MPPT is regulating the VOLTAGE just like PWM does to the load which regulates the CURRENT but the CURRENT is HIGHLY DEPENDENT ON THE BATTERY STATE OF DISCHARGE (IE voltage of the battery) and or load..
Voltage AND current work hand in hand, you cannot separate the relationship without affecting the other.. The battery adds in an unknown dynamic to the whole mess.
Watts is watts, you can't get more watts with MPPT only a bit longer charging cycle.
Piano gets it, BFL gets it, I get it, you don't get it..
