mppt vs pwm

pianotuna wrote:
Hi Salvo,

I'd love to see an analysis of Mppt vs Pwm at a vmp of 33 with a panel temperature of 75C. To make it a level playing field use 16.5 v for the pwm.

i.e. MPPT configuration series/parallel @ 33 vmp
and PWM configuration parallel @ 16.5 vmp.

Salvo wrote:
Fig 14 pretty much sums up the differences. The RV crowd are for the most part fair weather campers. That means cell temperature is in the range 40 to 80C. In that range, for the most part, mppt gains are between 0 and 10% better than pwm. On average it could be about 5%. Nothing to crow about.

PT , please clarify. Vmp does not apply to PWM and the IV curve for a 12v panel has its amps (Isc) fall off a cliff at 15v (which is battery voltage)

At 25C my panel was at 51C, so if panel is 75C I would not like to be RVing in what the ambient for that would be! ( Mena would probably even think it was a bit warm out)

Setting up a "fair fight" is complicated to say the least. The only thing they have in common that can be compared is amps to the battery.

You can't do PWM on a "24v" panel since the PWM controller has no buck converter. You can do MPPT vs PWM with 12v panels, but you would have to decide if it is "fair" to leave the panels in parallel and just swap controllers, or if it is "fair" to switch the panels to series for when the MPPT controller is used.

You have the wiring from panel to controller. Do you leave it the same? What if it is enough gauge for the lower amp MPPT but not enough for the higher amp PWM? The controller-battery wire would stay the same, both being at 12v.

My own comparison is just based on same panel wattage 230w MPPT vs 230w PWM for what amps I get to the battery in the same conditions.

But I am not using the same panels and I am using the same wire from panel to controller, good for the PWM and more than enough for the MPPT.

The part in the reference that has PWM equal or almost equal to MPPT within that panel temperature range never has PWM doing more amps to the battery than you would get with MPPT. Yet at 25C ambient, and 51C panel with 230w I get more amps to the battery with the PWM 14.5 vs 13.5 amps.

I know this is due to panel temperature because at 44C panel, I get more amps with the MPPT than with the PWM at 15.5 vs 14.5 amps.

So how come the fancy lab tests show only where PWM can be almost equal but never better? IMO it is because they are using "power" instead of amps to the battery as the comparison. The battery only cares about amps. 25C ambient is not terribly hot out either.

At a certain panel temperature in the high 40s I would get 14.5 amps either way from PWM or MPPT. Power at the battery would be the same using amps to the battery and battery voltage. Power from the panel would be different.

Power there cannot be compared to decide who is the winner. It is what the battery sees that matters and that is the amps. What power that comes to doesn't matter, it is just a thing you can calculate. The battery is charged by the amps passing through it to do the chemistry in there that raises the battery voltage. Power doesn't do that--amps do that. So the thing to measure in the fair fight is amps to the battery.

Since MPPT creates amps in an entirely different way from PWM you can't compare them earlier in the process, you have to wait until you see what comes out in each case.

Setting conditions for the fair fight is the hard part. Is it even possible? PWM amps stay the same while MPPT amps are all over the map depending on conditions. So you can get the answer you want by picking the conditions!

So that is ok too. You can pick your set-up based on the conditions you expect to be RVing in.