Ok, there is a recent thread out there MPPT vs. PWM and some post were a little confusing to me. Instead of hijacking his thread I started a new one.My set up is two 208 watt Sony panels with 28.71 volts (I.E. 24 volt panel) which is 14.5 amps. (V * A = Watts).What I don't understand is the difference in amp production between the MPPT and PWM. In the other thread there was some math that did not add up or followed Ohm's law formula. Someone mentioned with the higher voltage it was better to go with a MPPT controller rather than a PWM. But why? I know some will say the MPPT handles the extra voltage and turns it to amperage. HUH?Regardless of what voltage is going into the controller it will come out at 14.4 - 14.8 volts. Right? Thus ohm's law says my system amperage, on a perfect day, would produce 28 amps out from the controller, no?I understand using such a higher voltage panel for 12 volt charging system is an overkill, but these panels are left over from a previous residential install I own. As long as the controller can handle the voltage a PWM should work fine. For example I am using the Bogart 2030 controller with the Trimetric monitor. On a side note another rant.I read Handy Bob's information and other research about wire size. Ok, I get that, but if 10 ga works why in the hell are people using 8 or even 6 ga to run their solar panels when the loss is 2-5% with 10 ga? After doing the voltage drop calculations with my system with a run of 30 feet (being very liberal here) I only have a 3% loss of voltage. Also, why are some using 1/0 ga for inverter and battery wiring? HOLY ****! We are talking about a cable that can handle 150 amps. Tell me what 12 volt load could anyone be possibly using in an RV that produces that much amperage? Wouldn't 2 ga be just fine?

A PWM controller adjusts the voltage by essentially cutting off (and discarding it) the voltage above what you need. Your maximum output current will be whatever your panel's current can put out.An MPPT controller has essentially a DC to DC transformer that takes the voltage and current from the panel and converts it to the voltage and current output you need. If the input is 28 volts at 10 amps, the output would be 14 volts at 20 amps.This is a very simplified description, but hope it helps.You can allow varying amounts of loss between panel and controller without too much concern; but between controller and battery use the largest you can get. Doug

The simple answer is that with PWM the panel wattage is not what drives the system. You have to look at the panel output current (Isc) which for your panels would be about 7.24 amps each. You can look at your panel's IV curve and check where your battery voltage intersects the curve. So at the best of conditions you have 14.5 amps available to you. Add in wiring losses and you can see that things go down hill rather quickly.Watt output IS the driver for an MPPT system as wattage in equals wattage out minus losses. Voltage loss from inadequate wiring and panel voltage degradation due to panel cell temp are the biggest killers for the MPPT system. It's true that for a PWM system, wiring losses from the panels to the controller are not as critical as for a MPPT system. However voltage drop for an inverter is critical at the high amp draw as voltage loss is proportional to amperage. This will cause the inverter to operate poorly and introduce premature low voltage shutdown.Your panels are kind of in no man's land for RV use. With PWM there is considerable waste, but the Voc is too low to derive much benefit from a MPPT controller, but you would probably get on the plus side of 20 amps from your system. Of course if the controller can handle the voltage you could run the panels in series and on a good day could see 28 amps.BTW, running the microwave will get you close to 150 amps. On high draw circuits it's not about ampacity but all about voltage drop.

(one panel)In bright light the panel will produce 7amps (208 watts /29v, adjust the amps scale on the graph below). PWM will operate at battery volts and 7 amps (input) and ouput the same. Mppt has the ability to accept all the watts at the peak power point and transform this to battery voltage (208 watts/14 = 14+amps.So while charging at 14v, PWM yields 7 amps * 14v while MPPT yields 208 watts less the ineffiencies of the 'transforming'. ie PWM would limit the panel's power to about 1/2.Your voltage drop calc should use 7 amps for the input to the mppt controller.and 14 amp from the controller to the batteries. PWM would be 7A both sides of the controller.because the controller turns on and off to limit power when the battery approaches full charge, the controller needs to be rated to accept the Voc of the panel(s) and how connected, series exceeds the limit of that controller!Parrellel two panels, PWM yields 14A. Series PWM yields 7ASolar panel open-circuit voltage..................55V maximum Solar panel nominal voltage.......................12V - 24V, matched to the battery voltage: Vpp about 16-18+Volts for 12v systems, Vpp about 32-36+ for 24V systemsin short, for 12v system, wrong controller.

Calkidd wrote:Also, why are some using 1/0 ga for inverter and battery wiring? HOLY ****! We are talking about a cable that can handle 150 amps. Tell me what 12 volt load could anyone be possibly using in an RV that produces that much amperage?Toaster, coffee maker, MW.I have a 48v system so I can use much smaller cable.

Calkidd wrote:My set up is two 208 watt Sony panels with 28.71 volts (I.E. 24 volt panel) which is 14.5 amps. (V * A = Watts).Regardless of what voltage is going into the controller it will come out at 14.4 - 14.8 volts. Right? Thus ohm's law says my system amperage, on a perfect day, would produce 28 amps out from the controller, no?The NO is correct. Look up the Isc specification. Probably around 7 amps. This is what you get with PWM controller. Voltage is cut down to battery voltage and amps remain the same (Isc).MPPT actually converts wattage by reducing voltage and increasing amps like a step down transformer. Conversion uses about 5% of the power.

Voltage vs. PWM controller & wire size | Good Sam Community

2oldman
Explorer II
Apr 22, 2016
Calkidd wrote:
Also, why are some using 1/0 ga for inverter and battery wiring? HOLY ****! We are talking about a cable that can handle 150 amps. Tell me what 12 volt load could anyone be possibly using in an RV that produces that much amperage?
Toaster, coffee maker, MW.

I have a 48v system so I can use much smaller cable.
red31
Explorer
Apr 22, 2016
(one panel)
In bright light the panel will produce 7amps (208 watts /29v, adjust the amps scale on the graph below). PWM will operate at battery volts and 7 amps (input) and ouput the same. Mppt has the ability to accept all the watts at the peak power point and transform this to battery voltage (208 watts/14 = 14+amps.

So while charging at 14v, PWM yields 7 amps * 14v while MPPT yields 208 watts less the ineffiencies of the 'transforming'. ie PWM would limit the panel's power to about 1/2.

Your voltage drop calc should use 7 amps for the input to the mppt controller.
and 14 amp from the controller to the batteries. PWM would be 7A both sides of the controller.

because the controller turns on and off to limit power when the battery approaches full charge, the controller needs to be rated to accept the Voc of the panel(s) and how connected, series exceeds the limit of that controller!

Parrellel two panels, PWM yields 14A. Series PWM yields 7A

Solar panel open-circuit voltage..................55V maximum
Solar panel nominal voltage.......................12V -
24V, matched to the battery voltage:
Vpp about 16-18+Volts for
12v systems, Vpp about 32-36+ for 24V systems

in short, for 12v system, wrong controller.
12thgenusa
Explorer
Apr 22, 2016
The simple answer is that with PWM the panel wattage is not what drives the system. You have to look at the panel output current (Isc) which for your panels would be about 7.24 amps each. You can look at your panel's IV curve and check where your battery voltage intersects the curve. So at the best of conditions you have 14.5 amps available to you. Add in wiring losses and you can see that things go down hill rather quickly.

Watt output IS the driver for an MPPT system as wattage in equals wattage out minus losses. Voltage loss from inadequate wiring and panel voltage degradation due to panel cell temp are the biggest killers for the MPPT system.

It's true that for a PWM system, wiring losses from the panels to the controller are not as critical as for a MPPT system. However voltage drop for an inverter is critical at the high amp draw as voltage loss is proportional to amperage. This will cause the inverter to operate poorly and introduce premature low voltage shutdown.

Your panels are kind of in no man's land for RV use. With PWM there is considerable waste, but the Voc is too low to derive much benefit from a MPPT controller, but you would probably get on the plus side of 20 amps from your system. Of course if the controller can handle the voltage you could run the panels in series and on a good day could see 28 amps.

BTW, running the microwave will get you close to 150 amps. On high draw circuits it's not about ampacity but all about voltage drop.
doughere
Explorer
Apr 22, 2016
A PWM controller adjusts the voltage by essentially cutting off (and discarding it) the voltage above what you need. Your maximum output current will be whatever your panel's current can put out.

An MPPT controller has essentially a DC to DC transformer that takes the voltage and current from the panel and converts it to the voltage and current output you need. If the input is 28 volts at 10 amps, the output would be 14 volts at 20 amps.

This is a very simplified description, but hope it helps.

You can allow varying amounts of loss between panel and controller without too much concern; but between controller and battery use the largest you can get.

Doug

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