mppt vs pwm

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.