MPPT vs PWM for Smaller Systems

[SteveJ]

I recognize the value of going MPPT with a series panel set up. Particularly with non optimum, read hazy, cloudy days.

Not so sure on a single or parallel set of collectors. It's been discussed that there is waste with the PWM controllers because the input may show at 16(for example) but only have 14.6 going to the battery.

What I have observed with my system is that there seems to be no difference in the input outpot of the controller when in bulk charging but the difference comes into play in absorption, when the controller has throttled back the amps.

So in effect, the extra power could not be used to charge the battery as the controller is tapering the amps down, correct? or no?

I am thinking about converting the panels into a series set up with an MPPT controller to better harvest the solar on not perfect days. Not so much now, but if and when I go to a compressor fridge. But was mostly just curious.

My system has 150 watts collected from three 50 watt panels, A Renogy PWM controller with temperature compensation and separate direct to the battery volt readings. Battery is a group 29 Wally flooded (non maintenance) deep cycle, group 29DC. I do have the poor mans version of a battery monitor that measures incoming and outgoing watt hours on a cumulative basis along with watt and amp in and output and voltage, using a shunt.

[booster]

Originally Posted by SteveJ

I recognize the value of going MPPT with a series panel set up. Particularly with non optimum, read hazy, cloudy days.

Not so sure on a single or parallel set of collectors. It's been discussed that there is waste with the PWM controllers because the input may show at 16(for example) but only have 14.6 going to the battery.

What I have observed with my system is that there seems to be no difference in the input output of the controller when in bulk charging but the difference comes into play in absorption, when the controller has throttled back the amps.

So in effect, the extra power could not be used to charge the battery as the controller is tapering the amps down, correct? or no?

I am thinking about converting the panels into a series set up with an MPPT controller to better harvest the solar on not perfect days. Not so much now, but if and when I go to a compressor fridge. But was mostly just curious.

My system has 150 watts collected from three 50 watt panels, A Renogy PWM controller with temperature compensation and separate direct to the battery volt readings. Battery is a group 29 Wally flooded (non maintenance) deep cycle, group 29DC. I do have the poor mans version of a battery monitor that measures incoming and outgoing watt hours on a cumulative basis along with watt and amp in and output and voltage, using a shunt.

Good question and observations.


Re the wasted energy with a PWM. Any given system may or may not have it based on quite a few things. Basically, as I understand it, at any voltage from the panels higher than the absorption setting on the controller the PWM charger will just reduce the volts, essentially using just the amps that the panel can generate. Since the panel is really generating watts of power, volts times amps, you have less watts out to the batteries than out of the panels. As you say, if the batteries have tapered their acceptance to lower than the panel output you don't lose anything. The only time you would be losing power would be if you were at absorption voltage at the batteries but higher than that at the panels with the same amps on each. In that case and MPPT controller would covert some of the watts to higher amps at absorption voltage and you would have higher amps out of the controller than into it.


A lot of systems never run in that relatively narrow band of operation for long. Most commonly, at least for us, is that if we have more solar than the batteries are willing to take, that is the time to charge all the devices and such, and turn on the separator to give the starting battery a little charge if needed. We have an MPPT controller so we can take advantage of that window.


Many systems don't even ever get to the tapering point of the batteries on solar low amount of solar compared to batteries. The indicator would be if the panel voltage and controller output voltage are the same, which means the panels are getting pulled down by more load than they can handle at full voltage. Through all of that an MPPT is of no benefit as there is no excess voltage to get higher watts.


Going series with an MPPT does have some advantages beyond the above stuff, as the higher voltage of series drops the amps, in your case to 1/3 of what you have now. This will lower the voltage and energy drop in the wiring from the panels and increase output. Be aware that nominal 17v panels will have an open circuit (full battery same) voltage of over 20 volts in most cases, so you need a controller with a max voltage of near 65 volts to run three in series. We ran into that problem, so I have had to switch to parallel when I added the third panel as our controller wouldn't be able to handle 60+ volts.

[Winston]

Originally Posted by SteveJ

I am thinking about converting the panels into a series set up . . .

In addition to the high voltage issue of a 'series' connection mentioned by Booster, verify that you won't have a 'shading' problem. We placed our panels (3x270watt Kyocera) in parallel because ours were highly prone to shading. Covering just 5% of a panel all but shut it down.

Some, we're told, have diodes that avoid this problem. If yours don't, a small amount of shading over one panel will effectively shut the entire series string of three, down - - as opposed to shutting-down just the affected shaded panel in a parallel arrangement.

[SteveJ]

A lot of systems never run in that relatively narrow band of operation for long. Most commonly, at least for us, is that if we have more solar than the batteries are willing to take, that is the time to charge all the devices and such, and turn on the separator to give the starting battery a little charge if needed. We have an MPPT controller so we can take advantage of that window.

From what I've seen, the PWM allows the same from my system. Example: Turning on the Fantastic fan immediately drops the panel voltage and increases the amps flowing through the controller, but does not change the current or voltage flowing to the battery, at least after things stabilize which takes maybe a second. This assumes that I am not drawing more amps than the panels are supplying. When we fire up the stick vacuum from the 110v inverter, about 175 watts, the voltages do drop at controller input, output, and at the battery.

In view of this, perhaps the advantages of an MPPT controller are not as strong as would first appear, at least in 200 watt or less systems. I had forgotten about the reduced current with the higher voltages incurred with series panels. Definitely a benefit, especially as the systems get larger than, say, 2 or 300 watts.

Anyhow, my system seems to be meeting my current needs and camping style quite nicely. If and when I go the compressor fridge route it will be time to update/upgrade and enlarge my system. Not on the immediate horizon as I seem to be one of the lucky few that is getting good performance from the absorption fridge.

Thanks for the input. Have fun, and don't let "them" get you down.

[booster]

Originally Posted by SteveJ

From what I've seen, the PWM allows the same from my system. Example: Turning on the Fantastic fan immediately drops the panel voltage and increases the amps flowing through the controller, but does not change the current or voltage flowing to the battery, at least after things stabilize which takes maybe a second. This assumes that I am not drawing more amps than the panels are supplying. When we fire up the stick vacuum from the 110v inverter, about 175 watts, the voltages do drop at controller input, output, and at the battery.

In view of this, perhaps the advantages of an MPPT controller are not as strong as would first appear, at least in 200 watt or less systems. I had forgotten about the reduced current with the higher voltages incurred with series panels. Definitely a benefit, especially as the systems get larger than, say, 2 or 300 watts.

Anyhow, my system seems to be meeting my current needs and camping style quite nicely. If and when I go the compressor fridge route it will be time to update/upgrade and enlarge my system. Not on the immediate horizon as I seem to be one of the lucky few that is getting good performance from the absorption fridge.

Thanks for the input. Have fun, and don't let "them" get you down.

That sounds like a good plan to me, and lets more effectively be ready if you do change frig and maybe battery capacity in the future. You must be a quite low power user with a single gp29 battery, although the solar certainly helps that situation and 150 watts is fine for that much battery.

[SteveJ]

Originally Posted by Winston

In addition to the high voltage issue of a 'series' connection mentioned by Booster, verify that you won't have a 'shading' problem. We placed our panels (3x270watt Kyocera) in parallel because ours were highly prone to shading. Covering just 5% of a panel all but shut it down.

Some, we're told, have diodes that avoid this problem. If yours don't, a small amount of shading over one panel will effectively shut the entire series string of three, down - - as opposed to shutting-down just the affected shaded panel in a parallel arrangement.

Thanks for the reminder, but yeah, shading gets to be more of an issue with panels in series.