mppt vs pwm

Salvo wrote:
No, not correct. Both the pwm and mppt controllers use pulse width modulation to control output voltage during absorb and float.

The 3048 manual discusses changing pulse widths during the maximum power point discussion. Do you understand what changing pulse widths means? It's pwm!!! The mppt controller uses pwm to achieve the maximum power point and it also uses pwm to achieve constant voltage output during absorb and float.
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Again, forget output voltages. Look at panel voltages.

During the float or absorb stages the mppt controller maintains voltages on the panels that are different from the battery, whereas the pwm controller does not. They are obviously not doing the same thing.

The mppt controller is certainly using some form of pulse width modulation, but that doesn't make it's behaviour identical to a PWM controller during float or absorb states.

O.K., I think it's becoming more clear to me. The mppt is a matter of "sweeping", or "tracking", or "searching", but the method used to actually "regulate" is the pwm itself? And it's an "on/off" thing, that manipulates voltage by regulating spurts of current?

But then there is also the bucker. Does it make up for what pwm cannot do on it's own, because of too large a spread between voltages?

12thgenusa wrote:

brulaz wrote:
Apparently a pwm controller applies the battery voltage to the panels during float or absorb.

No. The panel's voltage is dragged down to battery voltage. . . always. At higher SOC the battery voltage is climbing, current is dropping and panel voltage will continue to climb. This is the major reason for a controller. Batteries can't tolerate voltage beyond a certain level and certainly not approaching Voc.

Right.

No, not correct. Both the pwm and mppt controllers use pulse width modulation to control output voltage during absorb and float.

The 3048 manual discusses changing pulse widths during the maximum power point discussion. Do you understand what changing pulse widths means? It's pwm!!! The mppt controller uses pwm to achieve the maximum power point and it also uses pwm to achieve constant voltage output during absorb and float.

I didn't mention that the mppt controller goes back into mppt mode while in float or absorb when the setting sun doesn't supply adequate power.

brulaz wrote:


Apparently a pwm controller applies the battery voltage to the panels during float or absorb. But the MPPT controller does not.

jrnymn7 wrote:
So. is it fair to say, when the controller is in mppt mode, it tracks for where it can output max amps at a minimum voltage above Vbatt, but when it drops out of mppt mode, it tracks for where it can maintain a constant voltage; Nppt (Necessary power point tracking), with power to spare?

No. In MPPT it tracks for maximum power. When not in MPPT it doesn't track at all.

brulaz wrote:
Apparently a pwm controller applies the battery voltage to the panels during float or absorb.

No. The panel's voltage is dragged down to battery voltage. . . always. At higher SOC the battery voltage is climbing, current is dropping and panel voltage will continue to climb. This is the major reason for a controller. Batteries can't tolerate voltage beyond a certain level and certainly not approaching Voc.

So. is it fair to say, when the controller is in mppt mode, it tracks for where it can output max amps at a minimum voltage above Vbatt, but when it drops out of mppt mode, it tracks for where it can maintain a constant voltage; Nppt (Necessary power point tracking), with power to spare?

At zero duty cycle the Vpanel = Voc, as the duty cycle increases from zero to 100% the panel should spend more time at Vbatt and less time at Voc, a slow v meter would read the ave so going from 100% Voc to 100% Vbatt the voltage would decrease as current increase from Ioc of zero to Ipanel > 0.

Salvo: You seem to be missing the distinction between output/battery voltage and panel voltage.

Apparently a pwm controller applies the battery voltage to the panels during float or absorb. But the MPPT controller does not. It maintains a constant battery voltage (or tries to) by adjusting the panel voltage. Here's quote from earlier in this thread where the panel voltage decreases as needed to the MPP voltage:

... slowly added loads while in float. As demand increased, PV voltage decreased. At about 17 amps (still only 2 going to the batteries), the controller gave up and switched back to MPPT to supply the demand.

The Rogue manual has a graph showing panel voltages during the day, including during Absorb and Float. It shows the same behaviour described above. The panel voltage drops with panel power output rising to maintain the battery at the Float voltage.

It's easier to understand when you have the graphs in front of you.
http://www.roguepowertech.com/documents.htm

Salvo wrote:
That's the description to search for the maximum power point. In order to do a complete search, the controller needs to do a sweep. There may be several peaks and valleys in the I-V curve. The first peak that the controller finds may not be the larges peak (meaning it's not at the maximum power point). Hence the sweep.

However, this has no relevance when the controller is in absorption mode. The controller doesn't care about the maximum power point. It outputs a constant voltage.

brulaz wrote:

jrnymn7 wrote:

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So, is the "mppt-capable" controller still doing something the pwm cannot do? IOW, Can the pwm raise or lower panel voltage to accommodate for variances in lighting, or just for variances in Vbatt?

dunno about pwm controllers.

But JiminDenver's quote about MPPT controllers pretty much agrees with Rogue's manual. Rogue talks about occasionally doing a "sweep" of the panels (guess that means changing the voltage and seeing what happens), especially when changing modes. And it the frequency of "sweeps" can be adjusted.

But it also "dynamically" checks panel output as well without a sweep. Much of this is probably proprietary, but my guess is that in MPPT mode is constantly trying adjacent voltages to see if the current output increases.

Salvo wrote:
That's the description to search for the maximum power point. In order to do a complete search, the controller needs to do a sweep. There may be several peaks and valleys in the I-V curve. The first peak that the controller finds may not be the larges peak (meaning it's not at the maximum power point). Hence the sweep.

However, this has no relevance when the controller is in absorption mode. The controller doesn't care about the maximum power point. It outputs a constant voltage.

brulaz wrote:

jrnymn7 wrote:

...
So, is the "mppt-capable" controller still doing something the pwm cannot do? IOW, Can the pwm raise or lower panel voltage to accommodate for variances in lighting, or just for variances in Vbatt?

dunno about pwm controllers.

But JiminDenver's quote about MPPT controllers pretty much agrees with Rogue's manual. Rogue talks about occasionally doing a "sweep" of the panels (guess that means changing the voltage and seeing what happens), especially when changing modes. And it the frequency of "sweeps" can be adjusted.

But it also "dynamically" checks panel output as well without a sweep. Much of this is probably proprietary, but my guess is that in MPPT mode is constantly trying adjacent voltages to see if the current output increases.

Would there not be any adjusting done then, such as lowering amps, to maintain that constant voltage, in the case of low light?