mppt vs pwm

So, Isc will decline even before amps taper? So I should not expect to ever see 16 amps from my two 140w panels, except for maybe the first few minutes of bulk?

The Isc on 12v PWM starts to decline in the 13s and is a bit lower into the mid 14s then falls off a cliff around 15v. (see a panel's IV curve)

"It depends" whether you lose a bit of amps from that or from the battery being nearly full so it won't accept anymore and tapers the amps from that. Or both reasons for amps tapering can be happening. It depends on the charging rate, being the amps the panel can do wrt the "size" of the battery bank.

It just occurred to me that in the video, 14.2 volts were used for charging. But if a more appropriate voltage was used, like Trojan's recommended 14.8v, that would lower the mppt's amp output even further.

So, correct me if I'm wrong, but wouldn't the numbers look more like this:

PWM:

4a (Isc), regardless of the higher charging V.

MPPT:

17.4v X 3.6a (Imp) = 62.64w / 14.8v = 4.23a

... for a gain of 5.8% (vs. 24%)

Sure would be nice to have more options in the 40 and 60 amp size.

Mark

Good to hear from you - it's been a while
What new stuff for 2015 has Rogue got to offer us?
Always in the market for new toys

Salvo wrote:
Right, Don needs to review that thread.

I think Rogue dropped his gains after the discussion from 20% down to 8%; but it's been a while.

No, it's entirely possible to get 20% gain with MPPT. Much more than that, actually, if you're using an array with a Vmp far above the battery voltage (which only MPPT controllers will allow you to do with any measure of efficiency). If you've been stuck in the boondocks for a long time, you might not realize that the popular thing to do these days is to use panels meant for grid-tie installations -- which do indeed have a higher Vmp than traditional panels meant for off-grid installs.

For CONSISTENT gain, with an array that has a Vmp closer to the battery voltage (17v Vmp, for example, with a 12v battery), around 10% is typical when you factor in the effects of temperature on Vmp. This is 1990's thinking, though. Time to move on.

Bingo!

Add in the decline of output power with temperature, mppt gains over pwm should be only about 8 to 5%, not 20%

This is nothing new! The 'experts' should have known this from the get-go. The Rogue guy used the same erroneous calculation to show how much better mppt was. We set him straight.

Salvo,

If you're referring to the 4 amps used in the comparison, he does say it's roughly equivalent to a 60w panel. However, if I understand the point you're making; and if I understand how mppt works; the Imp used by mppt would be lower than the Isc used by pwm, and therefore, the power supply should have been adjusted accordingly, thus resulting in lower output wattage from the power supply, and therefore lower amperage out of the mppt.

We've been over and over this issue many time. It doesn't hurt to have you think about it and try to figure out why this test doesn't represent the real world.

I'll give you a hint, mppt and pwm operate on different points of the V-I curve. Why use Imp for pwm?

red31 wrote:
Small shunt controllers do not do abs charging, only bulk.

Yes, and that is how my modded pm4b behaves. There is, of course, a point at which amps begin to taper, but it has nothing to do with the setpoint being reached. Voltage simply continues to rise, as amps taper, until near full charge.

It was the line on page 13 that said the battery only gets what it needs, instead of having a higher voltage forced into it, that caught my curiosity.