Derating 24v Solar Panels?

jrnymn7 wrote:

BFL13 wrote:
The panel's Isc goes up a bit with more panel heat but voltage goes down more so power goes down too.

With PWM you are getting current, so more is better. As long as the panel heating leaves you enough voltage overhead you are good.

With MPPT it doesn't matter what Isc does, since you are not using that. You are using panel output power (reduced by heating), bucking that, then using controller output divided by battery voltage to get whatever amps that comes to for charging the battery.

But how about when the mppt controller is in pwm/abs? Is it still bucking, or simply passing thru something very close to Isc? (up to what the bank can accept)

The buck converter in the MPPT controller is still in effect when the MPPT tracking is not on (only on in "Bulk")

The input amps to the controller with an MPPT controller (in MPPT or PWM mode) is whatever it comes to when you divide panel watts by "panel voltage" The "panel voltage" is whatever the controller chooses at the moment. In MPPT mode it will choose Vmp. In PWM mode it will choose a voltage (which will be higher than the Vmp for MPPT) that provides the amps to match the "demand" on the output of the controller.

The "demand" can be just battery charging or that plus loads, so you can still get lots more amps even though you are in Float (no MPPT) if you increase the demand above what the battery wants by adding load.

(You can run loads more than what the battery wants with any ordinary PWM controller too up to the panel's current output at the time. You can see this when the controller's "solar amps" readout is higher than the Trimetric's amps (the Tri shows only the amps the battery is taking)

The actual voltage the MPPT controller can pick in PWM mode is something less than the Voc you would measure if the panel were disconnected. That Voc is affected by panel heating. EG, I would see about 34v Voc with a panel rated 37 Voc on a typical day. This is a limit on "overhead"