mppt vs pwm

Salvo wrote:
You're only at the maximum power point of the V-I curve during bulk. At that time the battery can take a lot of current. Once you get into absorption mode, the battery can't handle high current. The mppt controller now outputs a fixed voltage, say 14.4V. The operating point of the V-I curve is no longer at the maximum power point.

mppt has a slight advantage over pwm only in bulk mode. There's no advantage in absorption or float

What is the panel voltage during Absorb or Float when not in MPPT? It is no longer Vmp for sure with no MPPT but what is it? (I asked earlier but got no takers)

The "Rogue Guy" seemed to say earlier that the controller lets the panel voltage equal battery voltage when it drops out of MPPT. ???

But then how does the buck converter get back in business- but without MPPT- whenever you turn on a big load?

( I hope Salvo has not annoyed the "Rogue Guy" so much he has gone away--Salvo, there is no need for that sort of behaviour. Agree to disagee-- We need to hear from these expert people.)

You're only at the maximum power point of the V-I curve during bulk. At that time the battery can take a lot of current. Once you get into absorption mode, the battery can't handle high current. The mppt controller now outputs a fixed voltage, say 14.4V. The operating point of the V-I curve is no longer at the maximum power point.

mppt has a slight advantage over pwm only in bulk mode. There's no advantage in absorption or float.

BFL13 wrote:

Anyway, yes we have a communication problem somewhere here :)

BFL13 wrote:

...
Anyway, yes we have a communication problem somewhere here :) Maybe it is more clear in the owner's manual?

I've learned a lot from the Rogue manuals and this forum.
You can download a PDF of all their manuals from their web site.

"Exactly. The Rogue is different. Your MPPT kicks in at 13.2V, the Rogue kicks in at whatever the FLoat voltage is set for. In my case, 13.6V when boon docking"

That's where I got the idea if you set Float at 14.8 (if it will set that high), then it will not go to PWM but stay in MPPT.

Usually the only time an MPPT controller is in MPPT is during Bulk, so I am doubting the Rogue would be in MPPT during Float but don't know.

Perhaps it goes up and down during Float between your low set and a higher set point, so all that means is your chosen point during Absorb or Float when it would kick up and go back into Bulk becomes the same voltage for the lowest it goes during Float?

But why would it go to MPPT at all during Float just to maintain the voltage?

Anyway, yes we have a communication problem somewhere here :) Maybe it is more clear in the owner's manual?

Again, using higher voltage panels with a 12V pwm system is like putting salt water fish in a fresh water tank. It's ludicrous!

I just installed pwm solar for my son-in-law last year using two 140W panels for $318 (280W) and a $30, 30A pwm charge controller.

You can't come close to that price. Hell, your controller costs almost as much as the two 140W panels and the pwm controller.

As far as where the 5% comes from, you need to look at my earlier post to you. It's all spelled out there.

lorelec wrote:

Salvo wrote:
Your logic is nonsense. Not worth bring up. No one in their right mind will use a 72 cell panel in a 12V pwm system.

The same rules apply when comparing 400W 36 cell panels using pwm and 400W 72 cell panels using mppt. The difference in gain will be on average only 5%.

Where are you getting 5% from?

Some aluminum foil on that plywood would have made a huge difference, too. And you could have cooked your meals on it!

Summer of 2013, it got up to 104F in Toronto. I was parked on the road, on brand new black top, so I know what you mean by getting it from both ends! :)

jrnymn7 wrote:
"If you cover the roof with panels you will be making your own shade."

Provided the panels are well above the roof and well ventilated. Otherwise it would be much like the heat build up that occurs in an attic. Just running an attic fan, and no a/c, can lower house temperature significantly.

I'm looking into actually harvesting this solar heat for winter under-floor hydronic heating. There are solar panels available with built in heat exchangers/collectors. In summer, the heat would be exhausted to the outdoors using a fan/heater core combination.

I covered the roof with 1/2" plywood raised 4 inches above the roof. It knocked 6 degrees off the 104F. Very few days the sun stays vertical. The stupid ground radiation is nothing to fool with either. It's mean and running for the trees is not an option. Panels on the roof and trees don't get along either. A coconut a green one, the size of a laundry basket with the weight of a bowling ball don't bounce too good offa glass. Even a falling dead frond will take out a panel.

You don't have that problem up there and you don't have 86 degree January days either. A park with good electrical hookups is an oxymoron. They do not allow A/C. But in the shade it is near paradise (in the winter). Boondocking with a few wrinkles and warts.

Salvo wrote:
Your logic is nonsense. Not worth bring up. No one in their right mind will use a 72 cell panel in a 12V pwm system.

The same rules apply when comparing 400W 36 cell panels using pwm and 400W 72 cell panels using mppt. The difference in gain will be on average only 5%.

Higher voltage arrays are virtually all that are being installed anymore -- everyone's doing it. It's getting difficult to even find good 12v panels, at least at a price that's anywhere near the $/watt that you'd pay for higher voltage ones. Gains are MUCH greater than 5% when converting a higher voltage array to a lower voltage bank using an MPPT, versus what you would get with a direct connection or PWM controller. The latter is not even a consideration because the losses would be off the charts.

Take a panel with these specs and charge a 12.6v battery with it:
Pmax: 185 Watts @ STC
Vmpp: 36.0 Volts
Impp: 5.15 Amps
Voc: 44.8 Volts
Isc: 5.50 Amps

With PWM (if you don't smoke your controller in the process) you'll have about 69 watts (12.6 * 5.5). With MPPT you'll have 185 watts (36 * 5.15). This is ignoring the effect temperature has, of course. The listed tempco of Pmax is -0.905watt/degC. So assume a 40degC temp rise over STC and you'll lose about 36 watts, which still leaves you with 149 watts. That's 215% of what you'd be getting with PWM. Where are you getting 5% from?

In that 'mainecruising' mppt vs pwm test on LFP batteries, are the panels wired in series or parallel?

BFL13 wrote:

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If you disable Absorb and can set your Float voltage to 14.8 as well, then it seems the Rogue would get to 14.8 in Bulk, then stay in MPPT doing Float at 14.8 till dark?
...

Maybe just a language thing, but I don't think it "stays in MPPT doing FLOAT". Similarly it does not use MPPT to maintain the fixed 14.8V ABSORB voltage. Rather it does whatever it can to maintain a fixed voltage, which is quite different from MPPT mode as I understand it, where max Amps are what is wanted.

But by disabling both ABSORB *and* FLOAT, and using identical setpoints for the two, as I described before, you should be able to keep the controller in MPPT as long as the sun did shine.

Haven't tried that as I doubt the batteries would appreciate a full day sitting around 14.8V. We don't usually have high loads during the day to drag the battery V down; no inverter to speak of.