Buck Gizmo -Volts vs Amps ? UPDATED
Update 5 Nov. (It didn't work out as hoped) I got my little gizmo, see close-up photos here http://www.ebay.com/itm/DC-DC-15A-Converter-Buck-Adjustable-4-32V-12V-to-1-2-32V-3-3V-5V-24V-Step-Down...
Forum Discussion
I said,
"??? What I don't understand is why Isc does not apply when an mppt is employed?"
O.K., I think I understand better now.
The Isc exists. It's there, whether the controller "cares" about it, or not. And, in fact, the MPPT does not concern itself with Isc while in operation, because it is designed to calculate for the most efficient use of ALL the available power (W's) at that moment; which is a mathematical relationship between USABLE ("max") I and V, and the load being called for.
The solar panel is a Source for Current (Amp Hours). But the MPPT is a Voltage Regulator that ONLY operates in CV mode; NEVER in cc mode. The pwm, otoh, is also a voltage regulator, but it operates in BOTH cc mode and cv mode; controlling voltage indirectly thru current regulation, and controlling current indirectly thru voltage regulation.
So, essentially, MPPT's do not ever do BOOST mode, because they never do CC mode. Boost mode = CC mode. So to say the MPPT does such and such while in boost, is really a misnomer. MPPT's simply do not operate in boost mode. They may do such and such, at a time when a PWM would oherwise be doing boost/cc, but that's still two different controllers doing two different things during the same "stage" of battery soc.
There, indeed, appears to be no advantage to using an MPPT, over a PWM (or a basic little buck converter gizmo)... once the batteries have reached a state of charge, such that, they can no longer accept ALL the available current from the panel/controller. At this point, both types of controller are now operating in ABS/CV mode, and amps are tapering.
But there is definitely an advantage to the MPPT while the batteries are capable of accepting more than the available current. For even though the PWM makes use of the higher current rating of the two, Isc vs. Imp, its operating in cc mode, and according to Kirchoff's Law, results in less over all power going to the batteries.
BFL said: "increasing the load does make the voltage drop"
I don't see that happening. When the batteries have a high soc, when either controller would in abs/cv mode, any additional load will simply be supplied by the surplus current available. It's no different than putting a 110v 5a load on a 110v circuit capable of 15a, and then adding another 110v 3a load to the same circuit. Voltage is constant, at 110v, and the current is variable; but limited at 15a... just like a dc circuit in CV mode. It should be no surprise then, when V remains the same, as I increases, while the batteries are being floated IN CV MODE, not cc mode (like a smart charger).
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In order for the OP to make the most of every watt produced by the panel, on a day when sunshine varies, he would have to continually monitor the power available, just like the MPPT does. This could get rather tedious and time consuming. And seeing as the bulk of the Ah's replaced are replaced during the early stages of charging, when the batteries have such a high acceptance rate, and see that at certain times, charge times are limited by hours and quality of day light, there is a definite advantage to making the most of every W available. And this is where the MPPT proves its usefulness.
And yes, if solar is being used to supplement early morning generator charging, then there is no advantage to using an mppt, but if solar is being used for the early stages of charging, as well, then it can be very useful.
"??? What I don't understand is why Isc does not apply when an mppt is employed?"
O.K., I think I understand better now.
The Isc exists. It's there, whether the controller "cares" about it, or not. And, in fact, the MPPT does not concern itself with Isc while in operation, because it is designed to calculate for the most efficient use of ALL the available power (W's) at that moment; which is a mathematical relationship between USABLE ("max") I and V, and the load being called for.
The solar panel is a Source for Current (Amp Hours). But the MPPT is a Voltage Regulator that ONLY operates in CV mode; NEVER in cc mode. The pwm, otoh, is also a voltage regulator, but it operates in BOTH cc mode and cv mode; controlling voltage indirectly thru current regulation, and controlling current indirectly thru voltage regulation.
So, essentially, MPPT's do not ever do BOOST mode, because they never do CC mode. Boost mode = CC mode. So to say the MPPT does such and such while in boost, is really a misnomer. MPPT's simply do not operate in boost mode. They may do such and such, at a time when a PWM would oherwise be doing boost/cc, but that's still two different controllers doing two different things during the same "stage" of battery soc.
There, indeed, appears to be no advantage to using an MPPT, over a PWM (or a basic little buck converter gizmo)... once the batteries have reached a state of charge, such that, they can no longer accept ALL the available current from the panel/controller. At this point, both types of controller are now operating in ABS/CV mode, and amps are tapering.
But there is definitely an advantage to the MPPT while the batteries are capable of accepting more than the available current. For even though the PWM makes use of the higher current rating of the two, Isc vs. Imp, its operating in cc mode, and according to Kirchoff's Law, results in less over all power going to the batteries.
BFL said: "increasing the load does make the voltage drop"
I don't see that happening. When the batteries have a high soc, when either controller would in abs/cv mode, any additional load will simply be supplied by the surplus current available. It's no different than putting a 110v 5a load on a 110v circuit capable of 15a, and then adding another 110v 3a load to the same circuit. Voltage is constant, at 110v, and the current is variable; but limited at 15a... just like a dc circuit in CV mode. It should be no surprise then, when V remains the same, as I increases, while the batteries are being floated IN CV MODE, not cc mode (like a smart charger).
------------
In order for the OP to make the most of every watt produced by the panel, on a day when sunshine varies, he would have to continually monitor the power available, just like the MPPT does. This could get rather tedious and time consuming. And seeing as the bulk of the Ah's replaced are replaced during the early stages of charging, when the batteries have such a high acceptance rate, and see that at certain times, charge times are limited by hours and quality of day light, there is a definite advantage to making the most of every W available. And this is where the MPPT proves its usefulness.
And yes, if solar is being used to supplement early morning generator charging, then there is no advantage to using an mppt, but if solar is being used for the early stages of charging, as well, then it can be very useful.
