mppt vs pwm

red31 wrote:
Duty cycle changes, less current = more OFF time, need more current the duty cycle increases to more ON time.

So width modulation itself could be supplying some (or all) of the extra current, over and above float current?

jrnymn7 wrote:
BFL said,

"(I never quite understood how that panel voltage change happens)"

... that's where I get stumped, too.

I think BFL needs to add an oscilloscope to his battery of tests and redo, report.

BFL said,

"(I never quite understood how that panel voltage change happens)"

... that's where I get stumped, too.

jrnymn7 wrote:
Jim,

Were you using an eco-worthy, or was that happening with a different model mppt?

Eco-worthy or Eco-worthys. One per panel and using two or more meant that most of the time both of my banks sat in float all day long unless I pulled a big load with the inverter.

Duty cycle changes, less current = more OFF time, need more current the duty cycle increases to more ON time.

jrnymn7 wrote:
BFL,

If panel Isc is say 7a, at 14.8Vabs, then in series it would remain at 7a, but in parallel it would double to 14a. So in my example, when the switch happens from bulk to abs, there would still be a sudden and significant drop in current... 14a down to 7a, instead of 16.4a down to 8.2a. So the issue would still exist, correct?

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I do keep forgetting about that 13.x volt kick in setting.

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But, apparently, Jim sees current above and beyond Isc... while still in pwm mode. How is this possible? What, other than the bucker, could do this?

It is the buck converter, that's the whole point. Isc means nothing when you are using the controller with a buck converter. The amps you get to the battery is always output power/Vbatt. All the MPPT does in Bulk is fine tune the input power to improve the output power somewhat.

In Bulk, series or parallel, your input power is Vmp x Imp. Isc is meaningless except to say that the Imp is an Isc on the Isc curve, but at a higher voltage so it is fewer amps. Ignore "Isc" with MPPT. (Also ignore Imp and Vmp with straight PWM and no buck converter--now you use "Isc")

Ok so in MPPT Bulk at 14.8 you are at some input power depending on panel temperature. Then MPPT stops so what happens with input power? The sun is still shining on the panel, no change there.

That is what I was asking earlier with questioning the panel voltage once no longer at Vmp. Answer was panel voltage goes up higher than Vmp so input and output power is less and so output amps is lower at that Vbatt. But if there is a load greater than battery charging, panel voltage goes down (back up the knee on the IV curve) allowing output amps to go up. (I never quite understood how that panel voltage change happens)

It is still a 24v panel. Once your MPPT controller goes to PWM mode, that does not make the panel a 12v panel with only half the power. The power is still there.

At 14.8 you are at a high SOC at solar charging rates. Amps will taper steeply in the remaining few percentages of SOC to Full. If amps seem to drop off as soon as you go out of MPPT, it would be from that fast taper and also from the reduction in output power from not being at Vmp anymore but at a less favourable V farther down the knee of the IV curve for that panel.

Your point was about the charging rate being high enough that the bank reached 14.8 sooner at a lower SOC. So then battery acceptance is higher and that would be the demand, so panel voltage would not go as much above Vmp, so amps would stay high (but still less than in Bulk at Vmp) and taper less steeply.

Another question might be is there anything better or worse about the IV curve of a 24v panel of 280w than the IV curve of a 140w panel where the second panel in series makes it a "24v" array, but it is not a single 24v panel which has its own IV curve.

Jim,

Were you using an eco-worthy, or was that happening with a different model mppt?

I first noticed it when I started using the 230w panel alone, could turn on all the lights, fans and furnace in the trailer without pulling the battery out of float. In the beginning I thought it was the altitude giving my panel superman's cape, but it was the buck converter still doing its job of converting the excess voltage to amps. With the three systems I can run the AC and it pulls more than the 30a the Isc should provide.

BFL,

If panel Isc is say 7a, at 14.8Vabs, then in series it would remain at 7a, but in parallel it would double to 14a. So in my example, when the switch happens from bulk to abs, there would still be a sudden and significant drop in current... 14a down to 7a, instead of 16.4a down to 8.2a. So the issue would still exist, correct?

------------

I do keep forgetting about that 13.x volt kick in setting.

------------

But, apparently, Jim sees current above and beyond Isc... while still in pwm mode. How is this possible? What, other than the bucker, could do this?

If the battery wanted more it is there. the fact the battery is not taking it all means it would not help to create more headroom for making amps, unless you want to run more loads. The buck converter is still there, so just because you are in PWM does not limit you to panel Isc like with a PWM controller (no buck converter) Also, in Float the panel voltage is higher than Vmp.

The Isc at 14.8 is not still 8.2 You have to look at the IV curves for that model/brand panel to see what the Isc would be at various panel voltages. Should be 7.x amps and ready to drop off the cliff at 15v.

BTW 8.2 Isc rating is the same as for the 130w poly. 140/130 x 8.2 = 8.8 amps. The voltage must be a little higher with that panel so you get a lower Isc.

The MPPT controller goes back into Bulk when battery voltage drops to whatever, say 13.2v If the load demand in Float goes above what the battery is taking, you get more amps up to what the solar can do at the time. That will not lower battery voltage. You get the same thing with PWM and 12v for some "free solar" to do things. Good time to recharge the laptop for instance.

Once the load demand goes over what the solar can do, battery voltage will decline. "It depends" how long it takes for that to get down to 13.2 and kick the MPPT back on. So I don't think it has anything to do with the buck converter in/outs.

Salvo, that is a good diagram! The micro-controller does two things: MPPT and battery charge control. I was calling that micro-controller the "MPPT part" ahead of the buck converter and was asking if it did two jobs. Things are becoming more clear.