Forum Discussion
- In my controller, MPPT is used in the bulk stage only since once you get to the absorb and float stages the battery voltage is high enough and the battery acceptance rate is low enough that MPPT isn't needed and the extra amps wouldn't go into the battery anyways.
The buck converter is always active even in float which is good. At altitude each of my three panels produce a Isc of around 10a but I can pull a load of close to 50a without pulling the battery out of float. - It should also be pointed out that MPPT controllers are also (usually) the ones that offer other features, like battery equalization and long term maintenance. Most PWM controllers, especially the ones being touted as cheap, are simply chargers.
JiminDenver wrote:
The buck converter is always active even in float which is good. At altitude each of my three panels produce a Isc of around 10a but I can pull a load of close to 50a without pulling the battery out of float.
There is no logic to force the panel to higher watts?- On the rig's roof, I want MPPT to catch every single watt coming from the limited space for panels.
When we start getting cheap, large square footage panels, such as ones that roll out with awnings, those can make do with a PWM controller because space isn't as precious. Although I prefer MPPT controllers because they can do equalization, desulfation, and other battery upkeep duties. - With mppt, other than in low light conditions, I cannot imagine battery voltage ever getting even close to panel voltage. So, low lighting conditions aside, during normal operation, a good mppt controller should buck voltage down to the necessary voltage to charge the batteries, until such time as Vbatt reaches the Voltage Setpoint, correct? At this time, voltage would be held at the setpoint, and amps would begin to taper in Abs/cv?
I asked the (initial) question, regarding mppt functioning continually throughout, because some folks operate/cycle their batteries in the 50-80% range, while others keep their bank up in the 85-100% range. With solar, I imagine most operate in the upper soc range. So I'm wondering if this could affect maximum power tracking? In other words, can one think of solar in terms of charging with a charger?
For example, using a charger, a C/5 charge rate would keep the bank in bulk stage up to say 85% soc (I see 83% at a C/4.8 rate, so this a reasonable estimate). A 400Ah bank would require 80 amps for a C/5 rate. Once the Voltage Setpoint is reached, amps would begin to taper. So, if starting at 85% soc, the 80 amp charger would go into abs mode very soon after starting.
Likewise, 1100-1200 watts of solar could supply 80 amps, using mppt. So, if starting from an 85% soc, it seems the bucking feature of an mppt would not function for very long. But can one make such a comparison, or does panel voltage throw a wrench into trying to compare the two? (mppt vs a charger, not mppt vs. pwm) - "With mppt, other than in low light conditions, I cannot imagine battery voltage ever getting even close to panel voltage"
Me either but I continue to see & hear of folks using 17Voc PV arrays with MPPT controllers (which to me makes very little sense} high array temps/heat will/can easily drop that Voc to the battery voltage.
"So I'm wondering if this could affect maximum power tracking? In other words, can one think of solar in terms of charging with a charger?"
When using a quality MPPT controller the battery SOC should have no affect (there are exceptions) on the tracking algorithm. An example of an exception lets say the array is over sized for the controller, high end controllers use the tracking function to reduce output current (by going away from the Vmp) thus saving them selves from destruction.
Sorry I do not understand the "charging with a charger" statement. Depending on the mode the controller is operating in, which should be controlled/mandated by the battery SOC the controller will simply output the Voltage / Current it is setup for or the array can supply. Exceptions abound controller configurations vary.
"Likewise, 1100-1200 watts of solar could supply 80 amps, using mppt. So, if starting from an 85% soc, it seems the bucking feature of an mppt would not function for very long."
Assume an array voltage of 50Vdc - the buck conversion is always active otherwise the battery would see the array voltage. red31 wrote:
JiminDenver wrote:
The buck converter is always active even in float which is good. At altitude each of my three panels produce a Isc of around 10a but I can pull a load of close to 50a without pulling the battery out of float.
There is no logic to force the panel to higher watts?
Correct...
But you missed the point
IIRC his panels are 250w each for a total of 750w
Rating is something near 30v at 9a
50a at 13v+ = 650 watts , which is less than the max output of the panels
Because of the high altitude camping, The panels can out put 30 amps at near 30v , the MPPT converts that extra voltage into the the higher amps, so you can use ALL of the panels output- "Sorry I do not understand the "charging with a charger" statement"
I'm referring to regular everyday charging with a charger, i.e; any charger that plugs into 120vac, be it a portable, or a converter/charger, etc.
An 80a charger converts/bucks 120v/20a service into the appropriate charging volts and amps. The charger is limited by it's V setpoint, and current limit. Likewise, an mppt controller converts/bucks X volts and Y amps into usable volts and amps, and is limited by its voltage setpoint and current limit. But I'm wondering if it's really that simple, or is there more to it?
And I guess the relevant question would be, at what point would a system (array plus mppt) be over-sized, to a point where mppt would lose any advantage over pwm? - The buck convertor is exchanging the extra voltage for more amps, that's why I am getting the 20a extra. I wouldn't need it with a 12v system in PWM because at float voltage the panel is running efficiently and you get all of the power to use on a load. The battery wont accept the power but I can still use it either way.
Now does that mean MPPT is the best way to go, no it's just only thing to consider when designing a system. I use it because I ignorantly bought bought 24v panels to start with, for a year I wished I had bought 12v due to the cost of a MPPT controller.
Since they up'd the Voc spec on my controller I have been wanting to try a pair of 150w 12v panel in series vs my 24v panels. I am losing interest because of seeing the results from others doing the same with my controller as well as more respectable expensive controllers. What I am seeing is the conversion from a Higher Voc in the mid 40's to 12v isn't as efficient as a 24v panels mid 30's voltage. I'm seeing two 150w panels in series only producing a few amps more than my 245w panel. On a better controller I know of a 960w system using 160w panels in pairs for mid 40's producing 48a while I would expect 60a from four 250w 24v panels. Understanding the nuances can make a big difference. - Hi,
I do wish MPPT controllers had a way to dump extra power unneeded for charging into a 12 volt water heater rod that would fit the drain hole on a water heater tank.
When I was not full time I used to deliberately "knock down" the battery bank by running the 120 volt water heater. Since I started full time, I rarely have that luxury.
