Buck Gizmo -Volts vs Amps ? UPDATED

jrnymn7 wrote:
BFL said,

"-$105 buck converter with fancy readouts attached, which has way higher voltage input limit of 60v (so it can run two 12v panels in series, which the Eco-W at 42v cannot)"

What am I missing here? Two 12v panels (say 2 x 18v) would provide up to 36v, less than the E-W's 42v.



It appears to me, matching up solar panel output voltages with buck converter input voltages is not the problem... lots to choose from. The issue seems to be with finding a buck that can handle the amps output from the panel. And like with everything else, big amps cost big bucks (pun intended).

The solar controller (because of/same as its buck converter) has its rated limit for voltage input and amps output. In the case of solar panel input, this is based on the panel's Voc (not the Vmp)

The usual Voc for a "12v" panel is about 22v so two is too much for the 42-45v Eco-W. this is especially true when you read the small print. Your controller Voc limit is based on the actual Voc at the time, which is different from the rated Voc taken at 25C. Voc goes up with colder temperatures. so they advise to figure out the coldest temps you will be camping in, calculate the Voc for that, and use that higher Voc number for picking your controller based on Voc limit. So you want some margin.

The buck converter's intake from a high voltage panel would have low amps (within the wattage VA) the buck output limit is still the watts but now with its reduced output voltage, the amps are higher, so the rating limits to look at are max volts in and max amps out.

When you buy a solar controller (which is a buck converter) you look at the same thing-- its "size" in amps (output limit) and its Voc input limit. It will also state its watt limit for the panel.

The buck converter's watts "size" is based on its output amps and voltage limit, since you could set the output voltage to one volt less than the max input voltage, so eg, that 60v, 20a one is shown as a 1200 watt job.

The Eco-Worthy 20a MPPT is rated for a 250w panel (which has 38Voc and 8.5 amps Isc approx.) 38 x 8.5 = 323 but output 20a at 15.5v (the upper CV, oops called a "set-point" here, is 310. So why only a 250w panel?

The 250w comes from the panels MPP (Vmp x Imp) not from its Voc and Isc which are higher than Vmp and Imp. And you must use the Voc for the input voltage limit not the Vmp.

Yes, the MPPT is to try to keep the panel at its MPP to get the best intake for the controller's buck converter. It does not control the buck converter.