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time2roll

Southern California

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Posted: 01/26/21 10:02pm Link  |  Quote  |  Print  |  Notify Moderator

+1 for BFL13. All good info. Don't stress about changing now. The set up will work very well for you.

* This post was edited 01/26/21 10:25pm by time2roll *


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pianotuna

Regina, SK, Canada

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Posted: 01/26/21 10:19pm Link  |  Quote  |  Print  |  Notify Moderator

1L243,

MPPT runs about a 4% loss (i.e. 96% efficient). You may garner only about 10% more by moving to MPPT. Charging may start a little earlier in the morning, and last a bit longer during the day.

If you had room for a 6th panel, then you could do 2 strings of 3 panels, and that would save quite a bit on the cost of the MPPT controller.

MPPT works best when the batteries are low--as they approach fully charged the MPPT changes over to PWM mode.

MPPT may work some what better in low light conditions. But panels are a constant voltage type device.

My vote would be to stay the course with PWM and see if it meets your needs. If it doesn't, then it is a simple matter to rewire to series and get an MPPT controller.

I would do #10 wire from each panel to the combiner box (under the fridge vent), and then #6 or #8 to the input of the charge controller.

* This post was edited 01/26/21 10:51pm by pianotuna *


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My ride is a 28 foot Class C, 256 watts solar, soon to have SiO2 batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

time2roll

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Posted: 01/26/21 10:28pm Link  |  Quote  |  Print  |  Notify Moderator

The only gains in MPPT come during bulk charge anyway and even then mostly just when the battery is very low.
There is no gain in absorption or float as the controller reverts to PWM during these phases.

1L243

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Posted: 01/26/21 10:32pm Link  |  Quote  |  Print  |  Notify Moderator

So, I should keep the PWM and not worry about the distance between the panels and the Charge Controller?

If you see any benefit I can change from 10ga to 8ga wire from the panels to the Charge Controller. (35 feet)


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1L243

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Posted: 01/26/21 10:39pm Link  |  Quote  |  Print  |  Notify Moderator

In researching wire I don't I know the difference between Photovoltaic Wire and AWG when it comes to solar?

pianotuna

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Posted: 01/26/21 10:42pm Link  |  Quote  |  Print  |  Notify Moderator

1L243,

Not much you can do about the distance from panels to controller. The only real variable would be larger wire size. And larger wire size doesn't much matter with PWM, because the system "runs" at what ever the battery voltage is.

If, on the other hand, MPPT is in your possible future, then #8 would be a good choice.

As I said before, that would be from where they combine to where the controller is.

Again, in your shoes I would stay with PWM for now.

pianotuna

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Posted: 01/26/21 10:46pm Link  |  Quote  |  Print  |  Notify Moderator

1L243 wrote:

In researching wire I don't I know the difference between Photovoltaic Wire and AWG when it comes to solar?


You want wire where the insulation can withstand UV light, and is weather proof.

"UL says “PV wire has superior sunlight resistance and low-temperature flexibility in addition to a thicker insulation or jacket and a proven level of flame resistance.”"

* This post was edited 01/26/21 10:59pm by pianotuna *

StirCrazy

Kamloops, BC, Canada

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Posted: 01/27/21 08:20am Link  |  Quote  |  Print  |  Notify Moderator

I tdhink people need to learn more about MPPT chargers before they recomend against them..... yes they benifit more during bulk charging, no they do not switch to PWM mode, they are a three stage MPPT charger so basicly a DC_DC charger that converts DC to AC then back to DC at the most efficent combination ov voltage vs current. in the summer on a nice day they may only give you an aditional 10% but in the winter when it is cold that number can go up to 40% or when its raining or overcast it will be inbetween. also you could basicly run pannels in series lowering the current while raising the voltage which allows you to get away with a small cable and less loss than running in parallel. if you can aford it it is always worth getting a MPPT controler, if you cant then dont worry about it, but you can always upgrade later, would just mean reconfiguring your pannels to take advantage of a series setup.

Steve


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BFL13

Victoria, BC

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Posted: 01/27/21 08:55am Link  |  Quote  |  Print  |  Notify Moderator

MPPT controllers have a buck converter in them, which is used even when doing 12-12 or 24-24 etc.

They only do MPP tracking in the Bulk charging stage, and do not do tracking for Vp in the Absorb/Float stages when the action is by "demand" on the buck converter. If the demand goes up for more of a load in Float then it will supply more amps (if the panels will supply them)

Panel voltage will seem to go up and down with the rapid switching on/off timing durations making for an average kind of reading, not be held to Vp, and that on/off is called PWM.


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FWC

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Posted: 01/27/21 08:59am Link  |  Quote  |  Print  |  Notify Moderator

Well said Steve.

There are several misconceptions in this thread. As stated well designed MPPT does not switch to PWM in absorption and float. It switches to CV (constant voltage) mode. Consider the edge case for a second - a 100V string of panels feeding a 12V battery, for half of the absorption phase the charge current is > 50% of the bulk current, if it was in PWM mode the current would be limited to about 15% of the bulk charge current. Secondly in float charge the current is very low, so you typically don't get any extra power out of an MPPT, but it is still not acting as a PWM charger. Consider if you turn a load on - the charger will ramp up the current to carry the load while keeping the voltage at the float set point (until it maxes out the available power).

The other thing to consider is that solar panels are NOT constant voltage sources, they are closer to constant current sources. This is hard to conceptualize, but will help understand how MPPT harvests 'lost power'.

For the OP, this doesn't make a huge difference as his panels are in parallel and it is a small system, but it is important to provide accurate information.

StirCrazy wrote:

I tdhink people need to learn more about MPPT chargers before they recomend against them..... yes they benifit more during bulk charging, no they do not switch to PWM mode, they are a three stage MPPT charger so basicly a DC_DC charger that converts DC to AC then back to DC at the most efficent combination ov voltage vs current. in the summer on a nice day they may only give you an aditional 10% but in the winter when it is cold that number can go up to 40% or when its raining or overcast it will be inbetween. also you could basicly run pannels in series lowering the current while raising the voltage which allows you to get away with a small cable and less loss than running in parallel. if you can aford it it is always worth getting a MPPT controler, if you cant then dont worry about it, but you can always upgrade later, would just mean reconfiguring your pannels to take advantage of a series setup.

Steve


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