MPPT solar charge controller with 12v panels...

[SJ-Chris]

I wanted to share an MPPT observation/experience (some of you likely are already aware, but many others maybe not...)

I just completed my 3rd solar install on my 3rd RV. (I enjoy these projects, and have been becoming a bit of a solar nerd...and mostly I hate dead batteries!) These are fairly simply installs (the first 2 were 500w with a 2000w inverter on one and a 1000w inverter on the other). They used commercial 250w used panels ($35 each) and an MPPT controller. Doing great for the last year (https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30217540/srt/pa/pging/1/page/1.cfm)

For this latest project, my 23' Class C RV has just two new 6v GC batteries (210AH) which is all I really want/need in there. A while back I acquired via craigslist three 100w used 12v panels (tested prior to purchase) for $25 each. I had a cheap PWM charge controller I was thinking of using. But I decided instead to buy a MPPT controller to use. I'm glad that I did. Even though the solar panels are "12v", they actually produce at 18-21v as you are probably aware. So the MPPT charge controller actually converts that extra voltage into extra charging current.

This system is 300w flat mounted on the roof. After I completed my install, I did a test. I discharged my batteries to about 50% and then I turned on my solar (toggle fuse) yesterday morning. I can see on the charge controller display, for example, that the panels were at 18.5v and the current coming in from the 300w of panels was 9.5amps at 11am. The display then shows the voltage going out to the batteries (ie. 14v) and the amps going to the batteries at 12amps. (Note: Those numbers might be off by just a bit, I'm trying to illustrate a general point.) That translates to roughly 25% MORE charging to the batteries than if I was using a PWM controller.

So I guess that means my 300w MPPT system is actually equivalent to a 375w PWM system had I gone that route.

Just something to think about if you are installing "12v" panels.

(Side note: I actually bought a 4th 100w 12v panel at that time, and I used the cheap PWM controller and 25' of wire and a fuse as a portable system. Currently used to keep my boat batteries charged during storage but in theory could come with me on an extended boondocking trip if I ever thought I needed an extra 100w of portable solar. Handy to have 🙂 )

Happy Camping!
Chris

Two 2015 Thor Majestic 28a Class C RVs
One 2013 Coachman Leprechaun 210QB Class C RV

[Vintage465]

That's great info. One of these days I'd like to upgrade to an MPPT. Then at the same time put in a shunt and a battery monitor. I enjoy solar projects too.

[CA_Traveler]

From this link Click the cell voltage jumps to 0.5V at 1 W per meter square and a cell area is much smaller than a meter squared.

To me in practical terms cell voltage is either 0V or 0.5V.

[CA_Traveler]

From what I've read panel voltage increases from 0V at night to panel voltage at extremely low light. So low that it's way below the lowewest panel IR curve. Therefor I'm of the opion that for practical purposes panel output is either 0V or normal voltage in early morning or late evening. ie Practically you either have usable power or not regardless of parallel vs serial.

Anyone have any documents etc on this subject?

[CA_Traveler]

Subtract MPPT efficiency losses of about 5-10% for MPPT wiring and controller.

For parallel 12V panels with PWM use Isc with no panel to controller wiring loss as the panels are about 18V output. HOWEVER panel amps rise with panel temperature hence at noonish and say 140F panel temperature PWM has a advantage. Vmp decreases with panel temperature. The panel temperature compensation for voltage loss is greater than that for amps gain.

Point being that with all of the variables it's involved to compare various configurations with the same equipment.

[StirCrazy]

SJ-Chris wrote:

I'm not sure you can draw any series vs parallel conclusions from your two systems that are using different panels and perhaps different wiring (or distances) and perhaps different solar charge controllers. Too many variables?

On a separate (related?) note....I solve most of these "inefficient panels", "Series vs Parallel", "MPPT vs PWM", "wire sizing", "tilt vs no-tilt", etc issues with ONE SIMPLE FIX: Just put up 50% or 100% more panels than you actually need and you will be fine! (...and since I like to use very inexpensive used panels (tested) the panels are next to nothing when factoring the expense of everything else and my time to do the install). On my 500w installs, I really only wanted/needed 200-250w but I found 250w used panels for $35 each so I put up two (500w) for each of those solar installs. I think this is a easy solution for most weekend warriors who boondock less than 15 days per year. For those serious (or full-time) boondockers witih serious power needs, yes they want to be more accurate.

Happy Camping!
Chris

exactly, my whole point was that if you can you should go with a 24v MPPT setup as it is much more efficient than the 12V PWM set up. and that series will start charging ealier and charge laiter , when we were talking about actual numbers it should have said the same set each way would get you a bit more in the morning and the evening. but the difference between my two set ups is different where you have 325 watt 24v panel putting out a substancial amount more than 480 watts of 12V. so it might have got a bit confusing and I apoligize for that.

cheep used panels are not available up here. I think there is a guy that keeps advertizing 250 watt panels for 180.00 but I can buy a brand new 325W to 360W for 200.00 to 230.00, I do envy your solar pricing down there.

Steve

[SJ-Chris]

StirCrazy wrote:


no thats not it, it averages about 2amps higher output through out the day also. so if we have a nice sunny day I get the extra say 3AH for the extra time morning and night, then I get an extra 8 to 18AH for the rest of the day depending on the time of year.

Steve

I'm certainly no expert, but that statement doesn't sound right to me. Consider these two examples: (Assume it is not early morning or late afternoon, but rather close to full sun which is most of the day. And we are only talking about sunny days in this discussion and not worrying about shade.)

Example 1: Two 12v panels in parallel producing at 20v and 4 amps each. The MPPT charge controller would see 20v and 8 amps. Assuming the charge controller wants to charge at that moment at 14v, it would supply (20v / 14v) x 8 amps = 11.4286 amps at 14v going to the batteries.

Example 2: Two 12v panels in series producing at 20v each (40v in series) and 4 amps each (which would be 4 amps total). The MPPT charge controller would see 40v and 4 amps. Assuming the charge controller wants to charge at that moment at 14v, it would supply (40v / 14v) x 4 amps = 11.4286 amps at 14v going to the batteries. The SAME amount of charge to the batteries.

I suppose voltage drop on the wire (worse with parallel panels) could have some affect. I plugged in an example into a voltage drop calculator (assuming a 30' run and 10AWG wire) and the series example above was about 2% better. 2% of 11.4286 amps would be about 0.229 amp more for the series example. After a 10 hour day of charging that would be 2.29amps extra total into your batteries if routed in series due to voltage drop savings. So if we add that to the 2-4amps extra due to earlier morning charge and later evening charge and it seems like series would provide about 4-6AH extra to your batteries compared to panels in parallel. This is using the above 200w example. I guess a 400w system would produce double the bonus or close to it. I guess it's something, but to me at this point not worth the hour or two up on the roof to reroute the panels. Maybe if I get bored sometime just for kicks I'll do it and see if I can tell a difference.

StirCrazy wrote:


now I don't know if I switched my curent 12V panels on my 5th wheel to series if it would get that same efficiency , I think part of it has to do with the split cell panel on the camper and it also has a higher effecniency rating, and the ones on the 5th wheel are you basic 12V go power panels, so probably over priced garbage.. Ill see up to 23amp going to my battery in the peak hours of the day with the 24V panel, but the 5th wheel with 30somthing percent more panel wattage at best does 21amps...... but mostly 20.

Steve

I'm not sure you can draw any series vs parallel conclusions from your two systems that are using different panels and perhaps different wiring (or distances) and perhaps different solar charge controllers. Too many variables?

On a separate (related?) note....I solve most of these "inefficient panels", "Series vs Parallel", "MPPT vs PWM", "wire sizing", "tilt vs no-tilt", etc issues with ONE SIMPLE FIX: Just put up 50% or 100% more panels than you actually need and you will be fine! (...and since I like to use very inexpensive used panels (tested) the panels are next to nothing when factoring the expense of everything else and my time to do the install). On my 500w installs, I really only wanted/needed 200-250w but I found 250w used panels for $35 each so I put up two (500w) for each of those solar installs. I think this is a easy solution for most weekend warriors who boondock less than 15 days per year. For those serious (or full-time) boondockers witih serious power needs, yes they want to be more accurate.

Happy Camping!
Chris

[StirCrazy]

CA Traveler wrote:

StirCrazy wrote:
I have seen several tests and such on the benifits of serial over paralell. there actualy will be a difference in morning votage between the two even if using 24V panels. my controler can take 140V in so that would let me stack three 24V panels and still be save. 4 and I am asking for trouble. but lets say I have three and early morning they are putting out 6V each, that adds up to 18V which is over the threshhole to start charging. if I had three paralell then it would only be 6V and wouldn't get a charge untill the sun came up more. not sure if thats what you were getting at but thats how I read it so I apoligize if I misunderstood what you were trying to say.

Steve

My apologizes as I've seen that also. I should have stated that the irridation is very low including amps in the early morning and hence the power output is small but it does contribute.

I've never seen 6V on a 30V panel. It has to happen of course since it's 0V at night. I've seen 90V (3 panels) at very low amps in the early morning light and no sun.

I've seen charging at 20V and 30V for 2 and 3 of the 9 panel sections w/o sun due to a tree and the bypass diodes activated for the other sections. I might have a graph of that event.

you won't see the 6V unless you are directly measuing the panel output with a seperat meter. most of the controlers have a threshold voltage where they do nothing till that min voltage is met. so 6V everything sits dead, but 18V it starts charging as the threshold is met. I played with my panel before i installed it to see what the max open circut voltage was and then I just wanted to see ifmy led lights in the garage would make it work haha

[StirCrazy]

SJ-Chris wrote:

StirCrazy wrote:

SJ-Chris wrote:
That is a good question. Strangely, I didn't even think about connecting them in series. I connected them in parallel. I can appreciate how in series they would collectively get to say 14v+ earlier in the day and stay at 14v+ later in the evening. That would generate a little extra charging. Does anyone have any real life data on how much/little gain this ends up being in the real world? Would be interesting to know. Since all the panels are connected on the roof it wouldn't be terribly hard to re-wire them in series.

I guess the gain comes from the time between when the individual panels are producing ~5v to the time they are producing ~14v (and the reverse as the sun goes down). I'm curious....is this ~10 minutes in the morning and ~10 minutes in the evening? 20 minutes on each end?? Anyone know?

I suppose when I go boondocking next if I'm up before the sun I can keep an eye on my solar charge controller and measure the time...

PS: Solar is addictive.

Happy camping!
Chris

don't have any real numbers only observation. my 5th wheel has 480 watts of 12v panels on a PWM controler and my camper has a 325 watt 24V split cell panel on a MPPT controler. the overall output total per day is more from the 325 watt set up. for morning my camper will start charging at 7 am with about 0.5 amps where the 5th wheel wont put that out till about 8 to 8:30 up here. so at least 1 to 2 AH more just from reaching that higher voltage sooner

So maybe 1-2 extra AH in the morning and 1-2 extra AH in the evening.
2-4AH extra per day.

On a sunny day, your 325w system likely has the ability (batteries permitting) to generate ~100AH of charge to the batteries. So 2-4 extra AH would be about 2-4% more overall charge series vs parallel. Certainly it's not a bad thing, but for me it's not worth getting up on the roof and rewiring to series for just 2-4%. If I was doing it from scratch, yes I would probably do them in series next time.

Happy Camping!
Chris

no thats not it, it averages about 2amps higher output through out the day also. so if we have a nice sunny day I get the extra say 3AH for the extra time morning and night, then I get an extra 8 to 18AH for the rest of the day depending on the time of year. now I don't know if I switched my curent 12V panels on my 5th wheel to series if it would get that same efficiency , I think part of it has to do with the split cell panel on the camper and it also has a higher effecniency rating, and the ones on the 5th wheel are you basic 12V go power panels, so probably over priced garbage.. Ill see up to 23amp going to my battery in the peak hours of the day with the 24V panel, but the 5th wheel with 30somthing percent more panel wattage at best does 21amps...... but mostly 20.

Steve

[CA_Traveler]

StirCrazy wrote:
I have seen several tests and such on the benifits of serial over paralell. there actualy will be a difference in morning votage between the two even if using 24V panels. my controler can take 140V in so that would let me stack three 24V panels and still be save. 4 and I am asking for trouble. but lets say I have three and early morning they are putting out 6V each, that adds up to 18V which is over the threshhole to start charging. if I had three paralell then it would only be 6V and wouldn't get a charge untill the sun came up more. not sure if thats what you were getting at but thats how I read it so I apoligize if I misunderstood what you were trying to say.

Steve

My apologizes as I've seen that also. I should have stated that the irridation is very low including amps in the early morning and hence the power output is small but it does contribute.

I've never seen 6V on a 30V panel. It has to happen of course since it's 0V at night. I've seen 90V (3 panels) at very low amps in the early morning light and no sun.

I've seen charging at 20V and 30V for 2 and 3 of the 9 panel sections w/o sun due to a tree and the bypass diodes activated for the other sections. I might have a graph of that event.

[SJ-Chris]

StirCrazy wrote:

SJ-Chris wrote:
That is a good question. Strangely, I didn't even think about connecting them in series. I connected them in parallel. I can appreciate how in series they would collectively get to say 14v+ earlier in the day and stay at 14v+ later in the evening. That would generate a little extra charging. Does anyone have any real life data on how much/little gain this ends up being in the real world? Would be interesting to know. Since all the panels are connected on the roof it wouldn't be terribly hard to re-wire them in series.

I guess the gain comes from the time between when the individual panels are producing ~5v to the time they are producing ~14v (and the reverse as the sun goes down). I'm curious....is this ~10 minutes in the morning and ~10 minutes in the evening? 20 minutes on each end?? Anyone know?

I suppose when I go boondocking next if I'm up before the sun I can keep an eye on my solar charge controller and measure the time...

PS: Solar is addictive.

Happy camping!
Chris

don't have any real numbers only observation. my 5th wheel has 480 watts of 12v panels on a PWM controler and my camper has a 325 watt 24V split cell panel on a MPPT controler. the overall output total per day is more from the 325 watt set up. for morning my camper will start charging at 7 am with about 0.5 amps where the 5th wheel wont put that out till about 8 to 8:30 up here. so at least 1 to 2 AH more just from reaching that higher voltage sooner

So maybe 1-2 extra AH in the morning and 1-2 extra AH in the evening.
2-4AH extra per day.

On a sunny day, your 325w system likely has the ability (batteries permitting) to generate ~100AH of charge to the batteries. So 2-4 extra AH would be about 2-4% more overall charge series vs parallel. Certainly it's not a bad thing, but for me it's not worth getting up on the roof and rewiring to series for just 2-4%. If I was doing it from scratch, yes I would probably do them in series next time.

Happy Camping!
Chris

[StirCrazy]

CA Traveler wrote:
Serial vs Parallel Thoughts

Using 2x24V panels for 12V charging yields the exact same power input to the controller so I would not expect any difference based on the panels, ie the voltage is not less in the early morning. So twice the voltage or twice the amps. Wiring is different and a given MPPT controller could have different efficiencies depenting on the input voltage or amps.


For 2x12V panels for 12V charging could use a PWM controller for parallel and a MPPT controller for serial which involves wiring and controller differences. It also involves temperature differences and could favor either one. With morning temperatues above or below 77F (STC) as the panel output can favor one or the other. If the same MPPT controller is used for serial vs parallel then likely the same except for wiring and efficiency.

Just my 2 cents. I don’t recall seeing in testing on this subject.

I have seen several tests and such on the benifits of serial over paralell. there actualy will be a difference in morning votage between the two even if using 24V panels. my controler can take 140V in so that would let me stack three 24V panels and still be save. 4 and I am asking for trouble. but lets say I have three and early morning they are putting out 6V each, that adds up to 18V which is over the threshhole to start charging. if I had three paralell then it would only be 6V and wouldn't get a charge untill the sun came up more. not sure if thats what you were getting at but thats how I read it so I apoligize if I misunderstood what you were trying to say.

Steve

[StirCrazy]

SJ-Chris wrote:
That is a good question. Strangely, I didn't even think about connecting them in series. I connected them in parallel. I can appreciate how in series they would collectively get to say 14v+ earlier in the day and stay at 14v+ later in the evening. That would generate a little extra charging. Does anyone have any real life data on how much/little gain this ends up being in the real world? Would be interesting to know. Since all the panels are connected on the roof it wouldn't be terribly hard to re-wire them in series.

I guess the gain comes from the time between when the individual panels are producing ~5v to the time they are producing ~14v (and the reverse as the sun goes down). I'm curious....is this ~10 minutes in the morning and ~10 minutes in the evening? 20 minutes on each end?? Anyone know?

I suppose when I go boondocking next if I'm up before the sun I can keep an eye on my solar charge controller and measure the time...

PS: Solar is addictive.

Happy camping!
Chris

don't have any real numbers only observation. my 5th wheel has 480 watts of 12v panels on a PWM controler and my camper has a 325 watt 24V split cell panel on a MPPT controler. the overall output total per day is more from the 325 watt set up. for morning my camper will start charging at 7 am with about 0.5 amps where the 5th wheel wont put that out till about 8 to 8:30 up here. so at least 1 to 2 AH more just from reaching that higher voltage sooner

[CA_Traveler]

Serial vs Parallel Thoughts

Using 2x24V panels for 12V charging yields the exact same power input to the controller so I would not expect any difference based on the panels, ie the voltage is not less in the early morning. So twice the voltage or twice the amps. Wiring is different and a given MPPT controller could have different efficiencies depenting on the input voltage or amps.


For 2x12V panels for 12V charging could use a PWM controller for parallel and a MPPT controller for serial which involves wiring and controller differences. It also involves temperature differences and could favor either one. With morning temperatues above or below 77F (STC) as the panel output can favor one or the other. If the same MPPT controller is used for serial vs parallel then likely the same except for wiring and efficiency.

Just my 2 cents. I don’t recall seeing in testing on this subject.

[CA_Traveler]

Another factor favorable to serial is greater shadow tolerance than parallel. Virtually all panels have bypass diodes that isolate the panel into sections, 3 for my panels. This means that a shadow on a section causes the diode to conduct bypassing that section. In other words the voltage of my panel would be reduced by 1/3 but with full amps. The same effect for parallel would effectively eliminate that panel due to it's voltage decrease.