(Viewing the PWM controller as a "fast switch" is actually pretty inaccurate in this case, as it really does regulate the output voltage, not just connect or disconnect.)
"
Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a technique used to encode a message into a pulsing signal. It is a type of modulation. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. In addition, PWM is one of the two principal algorithms used in photovoltaic solar battery chargers,[1] the other being MPPT.
The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load." [emphasis added - dwh]
http://en.wikipedia.org/wiki/Pulse-width_modulation
"Fast switching" is the EXACT definition of what PWM does. Fast switching is HOW it regulates the voltage and amperage on the charging loop.
It's not "pretty inaccurate". It is absolutely, certainly, totally and completely 100% precisely accurate.
It is a switch. It regulates by switching on and off at various speeds. It DOES NOT MATTER where in the circuit you put that switch.
For example, if your charge controller (either PWM or MPPT!) is controlling to 14.4v, and you have it mounted to the panel with 25' of wire between it and the battery, then your battery isn't going to see 14.4v, but rather 14.4 minus the voltage drop in whatever wiring you run. Even with fairly oversize wire, as the OP wants to use, there is still SOME voltage drop, and your battery isn't going to charge at the intended voltage.
This is one of the most common errors I see on the net. Been seeing it (and correcting it) for years. I blame the solar forums (and years ago I was offered a slot to be a moderator on the NAWS forum, so I've seen how it happens).
I've said it before, and I'll keep saying it: Voltage drop is A FUNCTION OF LOAD.
Repeat: A FUNCTION OF LOAD.
When the load on the wire is large, then you'll have large voltage drop. But when the load on the wire is small, then you'll have small voltage drop.
As the battery approaches full, the amps flowing - and thus the LOAD on the the wire will reduce - and thus the VOLTAGE DROP will reduce. By the time the battery is above 90% full, there will be almost no amps flowing, and there WILL BE NO SIGNIFICANT VOLTAGE DROP because there is not enough LOAD on the wire to cause any noticeable voltage drop..
You can "calculate" X voltage drop at Y load, but it's misleading because when the battery is almost full, the load might only be 1 or 2 amps. So recalculate the voltage drop using the ACTUAL load. Then do it again for .5a, .25a, etc.
Thus...NO! Voltage drop WILL NOT cause a battery be "Xv low".
(Voltage drop through a wire that is...voltage drop through a diode is a different story.)
It will NOT be, as you say, "14.4 minus the voltage drop", because "there ain't no voltage drop".
Let's say you have .5v of drop due to the distance of wire that you must run, and you put the controller right at the battery. It is likely that your panels are going to put out ~16-17v or more, so even with significant voltage drop in the wire, your charge controller still has to regulate the supplied voltage down to 14.4v, and your battery gets 14.4v and charges quickly and fully. If you move the controller to the panel instead, it still controls to 14.4v, but your battery will only sees 13.9v due to the .5v voltage drop in the wiring. At 13.9v, charging your battery to 100% will take much longer! With solar, you're limited on charge time, so you want charge voltage to be correct so you get to 100% in the shortest time possible! From an engineer's perspective, that ALWAYS means charge controller AT the battery, AND temperature compensated!
That is NOT how it works...NOT AT ALL.
With PWM, there is
a single charging loop. ONE BIG LOOP (a.k.a., circuit). That loop includes the battery, the wiring, and the PV module. The BATTERY will regulate the voltage on that charging loop - until the battery voltage rises to the charge controller's set point, at which time the charge controller will take over and regulate the voltage on the loop by fast switching to hold the loop voltage at the set point.
UNTIL the battery voltage - and thus, the voltage of the ENTIRE LOOP - rises to say, 14.4v, the PWM charge controller will be operating basically "wide open" (much more duty cycle time spent "on" than time spent "off").
Because of this, the PV module will NOT be operating (
EVER) at Vmp. It will be operating at whatever voltage the battery is at.
What is the voltage drop between the charge controller and the battery? Effectively - none. Big wire, small wire, heavy load, small load. VOLTAGE DROP IS EFFECTIVELY NONE.
Because the charge controller and the wire and the PV module and the battery is all part of the "one big loop" and the ENTIRE loop WILL BE at whatever voltage the battery is at.
Thus...MPPT.
MPPT *splits* the charging loop from one big loop, into two loops - one on the solar side, and one on the battery side. It then uses the MPPT circuit to regulate the voltage on the solar side, allowing the PV module to operate at whatever voltage gets the Max Power. Most sample the circuit every 60 seconds and adjust the load as needed to maintain the voltage of that loop at whatever it needs to be to get Vmp under current conditions.
It then takes that higher voltage, and runs it through a buck converter to lower the voltage (which also causes the handy side effect of increasing the amperage), then feeds the output from the buck converter (usually through a PWM circuit) to the battery.
Unlike PWM, with MPPT the PV module actually DOES operate at Vmp. This gets you more total watts from the PV module. Reducing the voltage from Vmp to battery voltage, increases the amps to the battery. It's a win/win. But it costs more for the MPPT controller, AND the lower the battery voltage, the more benefit you get from the amp "boost" of lowering the voltage. If the battery is never below 50%, and the PV is less than 200w, then it's generally not worth it to pay the extra for the MPPT controller.
So with an MPPT controller, which has TWO LOOPS, then yes, it's best to make the battery charging loop as short and thick as possible. But that's NOT the case with PWM, which is just a switch in the middle of ONE BIG LOOP.
(And yes, I am aware that HandyBob screwed up on that one. But he's using MPPT, and he's used to doling out generalized "best practice" advice to a bunch of knuckleheads in Quartzite. So it's an understandable, minor and completely forgivable error on his part that he simply lumps all charge controllers together and makes the same recommendations for both. After all, even through it isn't needed, it won't actually HURT anything to put a PWM charge controller as close as possible to the battery and use bigger wire on a short section of that one big loop. Hell, it might even gain someone a fraction of a percentage point in extra amp*hours, so what the hey...go for it Bob.)
People have a picture in their mind, of "Point A to Point B". That's totally wrong. It's a circuit; a loop. So the correct picture is "Point A all the way around and back to Point A".
So what is the ACTUAL EFFECT of this
theoretical voltage drop
when charging a battery? The increased resistance of too small a wire, will cause it to take longer for the battery to reach 14.4v. It WILL NOT prevent the battery from reaching 14.4v, but it WILL take longer to get there.
And this is why you see this error all over the solar forums. Those guys are trying to squeeze every watt out of a LIMITED TIME WINDOW of "good sun" by VERY CAREFULLY balancing the variables of <PV output|wire size|battery size|time>. So they keep repeating the Holy Mantra of, "voltage drop = bad" and "max allowable voltage drop = 3%".
But that's NOT because the battery won't get to full charge, it's because it won't get to full charge IN THE LIMITED TIME WINDOW.
Another reason this error is so common, is because most people never realize that voltage drop behaves ONE WAY when running loads FROM a battery (voltage drop increases as battery voltage goes down), and behaves THE OPPOSITE WAY when feeding power TO a battery to charge it (voltage drop decreases as battery voltage goes up).
(And don't mind the caps. I just use them for a quick and dirty emphasis - I'm NOT actually yelling at anyone.
)
(You now owe me: One Cold One. :ylsmoke: )
