PWM vs MPPT

[axeman33]

First a little background.

My wife and I just bought our first RV; a nice 26' used (2011) Gulf Stream Ameri-lite. We havent actually got it from the dealership we bought it from yet simply because we dont have a place to store it but that's a different story.

One of the things we are looking at purchasing for it is a portable Solar Panel Kit in case we are in a situation we need to recharge the battery. I've been doing my research on the different types of panels and reading about the charge controllers but the one thing I dont seem to find the info on is which one would be best for our needs. I know the difference between them but I am curious if it's worth the extra money for the MPPT controller or does the PWM work just as well and I wouldn't notice a difference?

Anyone out there used both and can notice a difference in how they work for simply recharging a 12V battery? Is it worth the extra money?

 

[Mark_K5LXP]

MPPT offers a bit more juice during some phases of the charging process, namely when there might be some panel shading. MPPT often uses a higher panel voltage which translates to a bit more efficiency in transferring power. I think the statistic I read is that MPPT gains you about 15% more power overall vs PWM, all other things being equal. Maybe you want that, maybe you don't care.

"Recharging a battery" implies a few different scenarios. Is this daily use or sporadic? Is there an expectation of 100% restoration of the charge or just a "boost" to get by? Answering those questions will get you closer to the kind of system that will solve the situation you have.

A couple examples would be if you didn't care about efficiency and this was only a periodic battery boost, you wouldn't need any controller at all. If this were a "hands off" system that you wanted as much daily Ah as you could get, then a consciously selected panel configuration and matched controller would be best.

Mark B.
Albuquerque, NM

 

[axeman33]

Thanks for the reply.

I dont really know how much we'll need to use it because we really haven't done much RVing yet. As mentioned, we're kinda green.

My assumption would be there could be weekends when we go to a spot that doesnt have the power hookups and we would have to run off a battery for our power needs, so we would be recharging the battery during the day as required, so in that regards, it would be a daily usage. I dont expect us to go to many sites without power hookups, but I am just trying to pre-plan for what might come if needed, if that makes sense.

 

[Mark_K5LXP]

OK, so now we peel back a few more layers of that onion.

How many Ah you need to recover is the next question. In a perfect world you'd have enough panel to bring your batteries back 100% in the six hours or so of useable sunlight you get. That would be one scenario. Another would be to pick a convenient size of panel, then whatever you get, you get. Some folks go way oversize on panels so that they're full by noon, or can recover a sizeable charge even on a cloudy day.

Knowing how many Ah you use in a typical 24 hour period is part of the solution. Being 'green' you probably have no idea so kinda hard to judge at this point. So from here my suggestion is to start with a "nominal" size setup, say something around 150W to maybe 250W and run it. You'll know probably during the first trip how close that is to meeting your needs. You can always buy more later if you need to, the downside is that much more to haul around, set up and tear down.

Practically speaking if all you're doing is trying to fill a gap from time to time I wouldn't mess with solar, I'd just charge from the tow vehicle. When I was both tent camping and with a popup I did this exclusively. Solar does work but usually not when or a quickly as you'd like (night, rainstorms) and you have a tow vehicle there anyway so why not use it. There's always an option of a small getset too.

Mark B.
Albuquerque, NM

 

[Ex-Calif]

If you don't go beyond 2 house batteries PWM will be fine. When you get to 4 house batteries MPPT and series wiring of panels becomes useful.

"Simply recharging the batteries" - You don't need to worry about flattening the vehicle battery and if you plan to be hooked up all the time I wouldn't go any further than 200W and PWM controller. You probably could start with 100W and see if you ever use it.

100W should provide 8 amps for like 8 hours and harvest about 55-60 amps a day. 55-60 amps will easily run LED lights, phone/iPad and laptop chargers overnight easily. 2 X 12V batteries are probably 80 amps usable capacity. So 100W should stretch you 2 or 3 days before the solar panel can't keep up with battery drain.

 

[Frank B]

Thanks for the reply.

I dont really know how much we'll need to use it because we really haven't done much RVing yet. As mentioned, we're kinda green.

My assumption would be there could be weekends when we go to a spot that doesnt have the power hookups and we would have to run off a battery for our power needs, so we would be recharging the battery during the day as required, so in that regards, it would be a daily usage. I dont expect us to go to many sites without power hookups, but I am just trying to pre-plan for what might come if needed, if that makes sense.

Ah, another Canuck! Welcome!

Our Arctic Fox 30' TT (with two 12v batteries) came with a single 140 watt solar panel mounted on the roof and a PWM controller installed. It was OK for extending stays when away from power as long as it got good sun exposure during the day. But it really didn't do a whole lot.

If you are new to trailering, you can take an outing or two, or just stay in your rig 'in the yard' if you can, and test it out over a weekend disconnected. You will find out soon enough how much you can do with what you have. With two good batteries, you may find that you have enough for your present needs.

Once you realize how much you can do 'as is' you will have a better idea of what you might need in addition. I would also suggest that you ask here before laying out money on anything solar. Solar is still kind of "The Wild Wild West" when it comes to purchasing something that will truly address your needs. Too many people buy, then realize what they got is not what they need.

What Ex-Calif said is sound advice.

On a side point, if you only want to extend stays, and only occasionally if you get 'stuck' with no power, you can always 'boost' your trailer batteries from your tow vehicle. A truck alternator will do a fair bit of charging if you idle the engine for an hour or two. Not efficient, and not recommended for anything other than a stop-gap solution, but the only additional cost is fuel and a set of cables.

Frank.

 

[Gizmo]

For a portable panel set-up, generally 100-200 watts, you will be fine with a PWM controller.

 

[axeman33]

Ah, another Canuck! Welcome!

Our Arctic Fox 30' TT (with two 12v batteries) came with a single 140 watt solar panel mounted on the roof and a PWM controller installed. It was OK for extending stays when away from power as long as it got good sun exposure during the day. But it really didn't do a whole lot.

If you are new to trailering, you can take an outing or two, or just stay in your rig 'in the yard' if you can, and test it out over a weekend disconnected. You will find out soon enough how much you can do with what you have. With two good batteries, you may find that you have enough for your present needs.

Once you realize how much you can do 'as is' you will have a better idea of what you might need in addition. I would also suggest that you ask here before laying out money on anything solar. Solar is still kind of "The Wild Wild West" when it comes to purchasing something that will truly address your needs. Too many people buy, then realize what they got is not what they need.

What Ex-Calif said is sound advice.

On a side point, if you only want to extend stays, and only occasionally if you get 'stuck' with no power, you can always 'boost' your trailer batteries from your tow vehicle. A truck alternator will do a fair bit of charging if you idle the engine for an hour or two. Not efficient, and not recommended for anything other than a stop-gap solution, but the only additional cost is fuel and a set of cables.

Frank.

Thanks, and yes a Maritimer!

I am pretty sure my truck is setup to actually charge the battery while driving with the standard hookups so I dont think I need any more cables but I could be wrong.

 

[Mark_K5LXP]

Not to be a total Debbie Downer but in the interest of full disclosure, the folks that proclaim the success of solar are often in the context of boondocking in Arizona or somewhere there's bright sunny skies 300 days a year. Not that any Ah are bad Ah but up in the great white north there just isn't that much sun to begin with. Panels are rated in watts under conditions basically equivalent to being on the equator at solar noon, at higher latitudes the output is reduced. Point being is that for a given panel rating you'll be getting less than that, in some cases much less. Just setting the expectation for when a panel gets set out somewhere and the measured current doesn't seem to all be there. That's not factoring clouds on top of it, which I'm guessing around the Maritimes are not an unusual occurrence. I know when I lived in Newfoundland it was claimed to be the foggiest place on earth, guessing there's not many solar installations there (wind farms would do great though...). Probably not a coincidence that now I live somewhere there's 300+ sunny days a year. Given the variables of clouds and camping in places with shade I don't count on solar even where I live, odds are I'll end up running a generator anyway. If I get enough solar during a given day I don't need to then great but I don't plan on it.

Mark B.
Albuquerque, NM

 

[John From Detroit]

Both MMPT and PWM have advantages but with MMPT you can put panels in series for a higher voltage (lower current) between the panel and the controller. then the controller which is a DC/DC (among other thigns) will swap voltage for current so you get the maximum performance from the panel.

Now the other thign. BOTH MMPT and PWM can cause interference to nearby radio/televisons unless properly shielded and installed .

 

[solarman]

Both will work for for low wattage setups. That said, MPPT is more efficient for higher wattages above 200 Watts and will permit use of any voltage panel type be it 36,60,72 or 144 cell panels at voltages up to 600 Volts.. PWM controllers are a simple switch device and are primarily targeted at "12V" panels, these are typically 36 cell and develop a max voltage of around 18 to 20 Volts. The number of "house" batteries is irrelevant. I would suggest you stay with PWM IF you have no need for further expansion. IF you intend to expand the system at a later date then MPPT is the better choice.

 

[Ex-Calif]

To expand a bit on what Mark said about collection.

In general it is accepted to use 90% panel efficiency so a 100W panel becomes a 90W panel.

90 / 12 = 7.5 amps

Then there is collection efficiency generally factored at 80% due to shade and clouds. 90 becomes 72.

72 / 12 = 6 amps

Then there is collection time. In larger lattitudes the sun may never get overhead and the sun may be up for less than 12 hours. I generally discard the first and last 2 hours from sunup to sunset so 8 hours of collection. Orienting the panels towards the sun vs. flat is necessary in larger lattitudes.

So 8 hours of collection at 6 amps gives a 48 amp yield per day. You might get that in summer in Nova Scotia in the spring fall and winter probably not near that.

 

[Frank B]

Thanks, and yes a Maritimer!

I am pretty sure my truck is setup to actually charge the battery while driving with the standard hookups so I dont think I need any more cables but I could be wrong.

I was thinking of the situation where you would be in one spot for a few days with the truck unhitched and batteries in the coach losing power.

Jumper cables will provide much more current for a 'boost' than the wimpy 'charge' wire in the harness.

I did this when winter camping before I bought a generator. Now, however, I have so much solar that I never use the generator anymore, and never plug in either for the 3 months we go south (when we could GO south).

 

[solarman]

To expand a bit on what Mark said about collection.

In general it is accepted to use 90% panel efficiency so a 100W panel becomes a 90W panel.

90 / 12 = 7.5 amps

Then there is collection efficiency generally factored at 80% due to shade and clouds. 90 becomes 72.

72 / 12 = 6 amps

Then there is collection time. In larger lattitudes the sun may never get overhead and the sun may be up for less than 12 hours. I generally discard the first and last 2 hours from sunup to sunset so 8 hours of collection. Orienting the panels towards the sun vs. flat is necessary in larger lattitudes.

So 8 hours of collection at 6 amps gives a 48 amp yield per day. You might get that in summer in Nova Scotia in the spring fall and winter probably not near that.

your figures are off somewhat, I'll explain why .

1. A 100 watt solar panel is 100% efficient at STC ( standard test conditions, 1000 Watts per square Meter ) and produces 100 watts.

2. Collection efficiency is a factor of the sun's irradiance, the energy that finally illuminates the panel is typically between 60 to 80 percent ( 600 to 800 W/M^2 ) depending on where you are. Energy is lost several ways, viz.
a) by way of attenuation through the atmosphere.
b) energy reflecting off the atmosphere.
c) solar panel heating incurring reduced conversion.( STC is 25 Celsius. Normal operating conditions typically 45 Degrees Celsius, panel output reduces with increasing temperature )

3. Sun hours, just because you think you have 8 hours of sun does not mean the energy is constant through the day. NREL produces detailed and globally accepted tables of actual useful hours of sun insolation. A brief study of these will show you why you will not obtain what you say.

 

[Ex-Calif]

If you look at the numbers conservatively they work out for most standard solar panel installs. They are rules of thumb I have been using (with a lot of other guys) for 20 years.

They represent a scenario where you will likely collect the amount predicted. Many people assume full efficiency and never get it. Everything matters including latitude shad cover etc..

 

[Mark_K5LXP]

I see both sides of it. The NREL tables factor not only insolation but local weather so overall whatever's there is going to be pretty close. But not every installation is going to require a by the numbers design review. Sometimes a back of the envelope WAG will tell you how close you might get. Given the variables of location, panel angle and shading you get with a house on wheels most critical design parameters are out the window. So at the end of the day you put in what you can and you get what you get, then tweak as you go.

Mark B.
Albuquerque, NM

 

[solarman]

If you look at the numbers conservatively they work out for most standard solar panel installs. They are rules of thumb I have been using (with a lot of other guys) for 20 years.

They represent a scenario where you will likely collect the amount predicted. Many people assume full efficiency and never get it. Everything matters including latitude shad cover etc..

yes, i did look at the numbers and they are way off..

however, I get were you are coming from, this highlights the difference in DIY and professional approaches. for diy, you rule of thumb it and add/subtract/replace until you get a solution you're happy with. for me, I have paying customers that expect a functionally correct design first time.
I'm just trying to introduce a little education here so you guys can better understand the design process and some of my more technical replies. either way, if you are happy with the end result then that's fine.. goal achieved..

 

[Ex-Calif]

yes, i did look at the numbers and they are way off..

however, I get were you are coming from, this highlights the difference in DIY and professional approaches. for diy, you rule of thumb it and add/subtract/replace until you get a solution you're happy with. for me, I have paying customers that expect a functionally correct design first time.
I'm just trying to introduce a little education here so you guys can better understand the design process and some of my more technical replies. either way, if you are happy with the end result then that's fine.. goal achieved..

Complete beg to differ. Not many people need design 101 theory for 200W of solar. When you get up above 400-800W sure then things get serious in terms of system design.

I have installed this exact system a dozen times and always with happy results.

The problem with professionals is that they confuse people with a tremendous amount of detail that is not necessary to answer a basic question which is, "What can I expect from 200W of solar?" or "What do I need to overnight without having batteries go flat?"

It's perfectly fine to be detailed in peukerts scale and the minutia of solar panel design but for most users the answers are overly complex.

It's not like there are a zillion choices here. 100W, 200W, 400W - pick a number. 1 battery, 2 batteries, 4 or 8 - pick a number. MPPT or PWM...

200W of solar, 200 a/h of battery and a PWM controller will work fine albeit with the caveat about high latitudes.

 

[Mooree]

Either PWM or MPPT charge controllers can recharge solar batteries safely. However, there are several substantial differences. Both controllers charge batteries in three stages: bulk, absorb, and float. Unfortunately, PWM controllers bulk-charge batteries at a constant voltage value. On the other hand, an MPPT controller automatically adjusts the solar panels’ output voltage to recharge the deep cycle batteries. As a result, the MPPT controller ensures more efficient solar energy utilization.

 

[Wasillaguy]

MPPT maximizes charge to the batteries with the same technology as switching power supplies. In simple terms, the flow to the batteries is cut off and reconnected rapidly. This induces an "in-rush" effect, where for a short period of time, the current flow is higher than a steady state connection.
This makes MPPT preferable for LA batteries, including gel and glass mat, in my opinion.