Questions about solar power

[blodn1]

From Solar Charge Controller Basics

The obvious question then comes up - "why aren't panels just made to put out 12 volts". The reason is that if you do that, the panels will provide power only when cool, under perfect conditions, and full sun. This is not something you can count on in most places. The panels need to provide some extra voltage so that when the sun is low in the sky, or you have heavy haze, cloud cover, or high temperatures*, you still get some output from the panel. A fully charged "12 volt" battery is around 12.7 volts at rest (around 13.6 to 14.4 under charge), so the panel has to put out at least that much under worst case conditions.

*Contrary to intuition, solar panels work best at cooler temperatures. Roughly, a panel rated at 100 watts at room temperature will be an 83 watt panel at 110 degrees.

The charge controller regulates this 16 to 20 volts output of the panel down to what the battery needs at the time. This voltage will vary from about 10.5 to 14.6, depending on the state of charge of the battery, the type of battery, in what mode the controller is in, and temperature.

This is why I claimed that you need higher volt panels - the volts are the amp-pushers. If you don't get 16-20 volts from that 17 volt panel, the battery will never fully charge. As the battery gets close to full, it needs higher voltage to push in the remaining charge. Just like squeezing into a tight pair of pants - need to jump higher with more force to reach the goal.

[peterh]

"As the battery gets close to full, it needs higher voltage to push in the remaining charge."

Er, no. First, solar panel output will always be in the same, fairly limited voltage range. (This is classical physics, Einstein's photoelectric effect.) The only exception to this rule is when the panel isn't getting a lot of sun and the panel isn't producing much power. So, even when your 100w solar panel is only producing 50, 25, or even 10w, it's still producing that power at about 17.5 volts.

Beyond that, batteries have a charging cycle that runs through three or four phases: bulk charging, absorbtion, equalization (the optional phase), and float, and none of these charging phases ever requires more than the 17.5 volt output from a solar panel.

During the initial phase of battery charging, when the battery is run down, it'll take in the full output -- volts and amps -- of a battery charger or single-panel solar panel system without over-heating or damaging the battery. This is the "bulk" charging part of the cycle, when the battery's resistance to charging is almost zero and the charging system's full output is converted almost directly into battery charge.

Once the battery gets to about 60-75% charge, the battery's internal resistance to charge increases and, unless the charging voltage drops to 14.5-14.9 volts, the excess energy will go into creating battery-damaging heat. This is the "absorbtion" phase, when the battery's resistance gradually climbs and the number of amps the battery can accept as charge declines until the battery reaches around 95% charge. It's this part of the charging cycle where an MPPT charger can make a big difference, increasing the actual current passed to the battery by 15% or so. More on that later.

When the battery's resistance flattens out at about 95% of full charge, batteries (AGM batteries in particular) can still accept just a little bit more charge, but the charger really has to work to make this happen quickly. Some charging controllers oblige them by boosting their output voltage just a bit, by a couple volts or so, for a short period of time, packing that last little bit of juice into the system in the shortest period of time. This is the equalization phase, when the charge stored in the battery's cells is "equalized," and spread evenly over the entire surface of the battery's lead plates. This charge phase has to be kept short, or the battery will overheat and become damaged.

The last phase is the "float" phase, where the charger pushes around 13.6 volts into the battery to prevent its gradual self-discharge, but, like the equalization phase, it'll also pack more charge into the battery if it has remaining storage capacity. It'll just happen much more slowly.

If you're a tech type, you can read details on how this works on the BatteryTender website, here: http://batterytender.com/resources/battery-basics.htm

Here's the difference between a less expensive Pulse-Wave Modulation (PWM) solar charge controller and an Maximum Power Point Tracking (MPPT) controller:

When the battery charge hits 60-75% and it's time to enter the absorbtion charging phase and reduce the charging voltage, PWM controllers reduce the voltage by rapidly switching the power coming from the panels on and off, so they're only fully "on" about 85% of the time, so the average voltage passed to the battery is about 14.6 volts. This means you lose that last 15% of the panel's charging capacity. When the battery charge hits 95% and it's time to enter the float phase, PWM controllers reduce the "on" part of the cycle to about 78% of the time, so 22% of panel capacity is lost.

I do not know of any PWM controller that performs an equalization step.

MPPT controllers take the full output of the solar panels and run it through a special inverter/converter circuit that pumps out that same 14.6 volts, this time without turning the panels on and off, so you get the full charging potential of the panels. They do this same thing throughout the absorbtion, equalization, and float charging phases.

So, here's the question: Is an MPPT controller worth it? Here's my take:

IF your expected daily solar panel output comfortably exceeds your daily energy consumption, no. You already have excess charging capacity and don't need an ultra-efficient charger.

IF your expected average solar panel output is much (25%) less than your daily energy consumption, no. Your solar system will only rarely enter the absorbtion part of the charging cycle, and you're just throwing money at buying charge controller features you'll never need.

[blodn1]

Maybe I could have stated it better - that you should BUY higher voltage panels, because you won't get 16-20 out of a 17 volt panel.

[Jon Vermilye]

The GoPower 30 amp PWM controller is a 4 stage unit. It does equalization when ever the battery voltage drops below 12.1 volts and every 28 days.

[Mike Magee]

Peter, thanks. I had somehow gotten the mistaken idea that the MPPT controller would act like a transformer during bulk charge, converting excess voltage into more current (keeping wattage the same). Probably read that on the internet!

[Carol H]

Although interesting the discussion of PWM vs MPPT solar controllers is staring to sound a lot like the big debate on whats the best type of converter to use to get a the elusive fully charged battery we hear so often

[Briantb]

For a small RV system use an inexpensive controller and save your money for the new battery you're eventually going to need.

[peterh]

There have been a lot of innovations in the solar market. Affordable PWM controllers were a nice step forward, and is what I have on my Scamp. Only recently, however, have MPPT controllers come down in price to where they're not more expensive than the solar panels they support.

MPTT controllers are like many tools. They are very good at what they do. The question, as always, is, "Are they the right tool for the job?"

The answer to that can be yes or no, depending on things like money, the size of your solar panels and battery, and how you use the power your batteries store and panels make. What follows here is a long-ish, boring-ish discussion you can skip (just head down to the "=========" line below the boring stuff) explaining what scenarios an MPPT controller makes a lot of sense in.

==========

First off, a good MPPT controller will charge your batteries at least as well as a good PWM unit. It may not do it better, but it will do at least the same job. Whether you get more out of an MPPT controller has a lot to do with the size of your solar panels, the size of your batteries, and the amount of electricity you use from your solar system and batteries.

MPPT controllers make sense when your solar panel output is just adequite to getting your battery fully charged. So, let's say you have a 100w panel and get the equivalent of four hours of full sunlight. After doing the math, that means your solar system can optimally provide about 16 Amp-Hours (AH) of battery power each day. If you have a fully-charged 50 AH battery that you discharge down to 12 volts or so by the next morning (30AH used) you will never use the features of your MPPT charge controller, and your battery will go dead over time.

Double the size of your battery, to 100 amps and use the same amount of power, and you'll get some small benefit from an MPPT controller on the first day or two, but your battery will still go dead, and in slightly less than double the time compared to using a 50 AH battery.

If, on the other hand, you discharge that same 50 AH battery to 12.4 volts by the next morning (20AH used) you will use the MPPT features for the first several days while it trys to keep your battery charged. Your battery will still eventually go dead, but it'll take longer, so you do get some value out of your MPPT controller. Bump your battery size up to 100AH, and it'll still discharge in slightly less than half the time, but will again use the MPPT features a lot during those first several days.

Let's say your daily usage is right at 16 AH, the amount of power your solar panels can produce. Because more energy is lost during the absorbtion and float phases of battery charging, your solar system won't quite keep up for the first several days running on solar, but then you'll get back from a day in the great outdoors and discover your battery voltage is about 12.5 volts (80% charged), same as the day before, and same as the day before that. This is the break-even point where an MPPT charger makes the most sense, when your solar panel power production and daily consumption are just about in-balance, regardless of the size of your battery.

What, you might ask, gives you the most bang for the buck if your solar panels make more electric power each day than you use? Say your 100w panel makes 16AH each day but you use just 12AH. In that case, it doesn't matter whether you have an MPPT controller or a PWM controller. Both will bring your battery up to full charge each day.

==========

In the end, MPPT charge controllers make a lot of sense if your solar panel generating capacity more or less matches your average daily consumption, say plus or minus 25%.

If you have an abundance of solar compared to your daily demand, your solar system is already beefier than you need, so why spend more on an MPPT controller? If your solar panel capacity is less than 75% of your daily demand, you won't get much out of those fancy MPPT features, and perhaps your money would be better spent buying a larger solar panel, a second panel, or buying a larger battery.

Yes, it gets complicated. And, rather than working all the details out I can really see the logic of saying "Screw it. I'm spending the extra $50 and getting an MPPT controller."

Which is, in the end, what I decided to do for our Surfside project trailer.

[multi-task]

This white paper makes more sense. I use the mppt controller mentioned and it works. Not for everyone though.

https://s3.amazonaws.com/Morningstar...Whitepaper.pdf

[Mike Magee]

Originally Posted by multi-task

This white paper makes more sense. I use the mppt controller mentioned and it works. Not for everyone though.

https://s3.amazonaws.com/Morningstar...Whitepaper.pdf

That's a very good document. Well worth reading. I see that MPPT can indeed convert excess voltage into current. But there is no easy way to quantify the charging advantage of MPPT over PWM because it depends on several factors that are in flux throughout the charging day. Thus they summarize with some 'rules of thumb' based on assumptions that will usually be valid.

Carol H, I would say there's no debate here... just discussion and learning.

[Darwin Maring]

It's beginning to look like this site is turning into a Fiberglassrv Science Forum.

[Paul Braun]

Nice. The discussion of the different controller types is causing me to rethink my selection process. I was set on this controller, Blue Sky Sun Charger 30, but now I will review that choice.

The biggest difficulty seems to be measuring different usages and discharge rates. For that reason it seems hard to hit the "sweet spot".

Mike, Peter and Jon. Thank you for the discussion and the informative links you provided. Carol, I have to admit I am one of those junkies who loves getting into all of the intricacies of battery charging and getting the battery fully charged.

[peterh]

Originally Posted by Mike Magee

. . . I had somehow gotten the mistaken idea that the MPPT controller would act like a transformer during bulk charge . . .

That's a yes and no kind of thing. When you have a higher-voltage set of solar panels, say a "24" or "48" volt system (with actual panel output in the 35 and 70 volt range), MPPT controllers will transform and down-convert the output voltage to match the voltage of your batteries. This is great for fixed installations in homes because, counter-intuitively, running higher voltage systems allows you to use less-expensive, lighter-guage wire to connect the panels to the charge controller and other elements. MPPT controllers are practically required in applications like these because you don't want to charge a lead-acid battery at voltages more than around 1.75 times the voltage they hold when fully charged. (Above this number the current overcomes the resistance of the battery, causing a short circuit that is dangerous to the battery and any people nearby.)

You can chain solar panels on a trailer, connecting two panels in series (e.g. two 60-watt panels with the negative output of one connected to the positive output of the other) one after the other to create a "24 volt" panel set, and making the system more house-like. The problem with this approach is that, if one panel winds up in the shade, it'll draw the output of the second panel down, and you won't get as much power. Connect them in parallel (positive to positive, negative to negative), and shade on one panel won't steal power from the other.

On the Morningstar "White Paper."

Something to keep in mind: Morningstar's "white paper" is marketing material. Hype. It's largely accurate, but over-values the role of adaptive voltage regulation during bulk charging.

Specifically, their claim that MPPT controllers offer 15% increase in efficiency is accurate, but only during later adaptive, equalization, float phases of operation and at the beginning of the bulk charging cycle when the battery is heavily discharged. During the majority of the bulk charging cycle it doesn't make one bit of difference.

This is because the ideal, maximumly efficient input voltage for charging a battery is around 1.25 to 1.5 times the discharged battery's voltage. Much below this value and the battery will resist accepting a charge (chemists will recognize this as the minimum "activation energy" required), much above and you lose some of the power you're pumping in as heat.

Solar panels pump out around 17-17.5 volts (depending on the panel). That means, right out of the box, a standard panel already puts out the optimal charging voltage for a discharged battery with a voltage of 11.3-11.7 volts (15-25% of its total capacity) or more. That means you would have to very heavily discharge your battery, discharge it to the point where you're damaging it, before the MPPT circuitry offers any real advantage during the bulk charging over simply dumping panel voltage directly to the battery.

That's a complicated concept for a marketing paper to explain, and it doesn't help them sell their expensive MPPT controllers, so they kinda left that little detail out.

[peterh]

Originally Posted by Paul Braun

The biggest difficulty seems to be measuring different usages and discharge rates. For that reason it seems hard to hit the "sweet spot".

Yea. As I said in earlier post, "It gets complicated. And, rather than working all the details out I can really see the logic of saying "Screw it. I'm spending the extra $50 and getting an MPPT controller . . . Which is, in the end, what I decided to do for our Surfside project trailer."

When it came time to choose a charge controller, I had two choices on my list, the Morningstar SunSaver 10 PWM controller (about $50) or a Tracer1210RN MPPT controller (about $90). Frankly, neither controller was a bad choice, but I opted for the MPPT controller because, on some days, it might make a difference.

[MCDenny]

I appreciate all the discussion too. Maybe the people who really know this stuff could comment on what I envision my scenario to be:

Start camping with a full battery. During the day my (didn't buy it yet) 100 w panel will put out about as much energy as I consume. Load is intermittent 4a draw from the fridge (maybe on 1/3 of the time) and (worst case) a couple of hours of 6a draw to charge the computers. Assuming a 12 hr day that's 28ah consumption. The panel should give me exactly that "on average" in S Fl in the winter. So I'm thinking I'll still have a full battery when the sun goes down. Night time use would be fridge on 1/4 of the time, 12ah and maybe another 2ah for lights. I'll get up in the morning with the battery at a soc of around 92%. Every day my soc will drop another 8%. After a week I'd still be at 44% and be thinking about plugging into shore power somewhere.

Does this sound right? Would PWM or MPPT make much difference?

Thanks.

[MCDenny]

Another question, I'm liking this

http://www.renogy-store.com/100watts.../rng-100db.htm

It's the same output and area as a regular 100 w panel but is only about 1/4" thick and weighs 4 lbs instead of 16.5 for their regular glass panel. I could store it under the bed and it's light weight would make it easy to carry outside and place in the sun.

It would seem ideal to stick to the roof too but I think the hassle of setting it up would not be worse than keeping the rooftop panel clean. Plus with the portable we would get more sun exposure.

Anybody have experience with these kind of panels?

Thanks.

[carlkeigley]

WOW............LOL
And I was hoping there would be a one size
fits all system out there somewhere.....lol.

[Briantb]

[QUOTE=" After a week I'd still be at 44% and be thinking about plugging into shore power somewhere.

Does this sound right? Would PWM or MPPT make much difference?

Thanks.[/QUOTE]

Should be thinking about plugging in before you get to 44%...well just try not to go below 12.1 without a load or you will be replacing your battery sooner than you should

[padlin00]

Unless I missed it, it looks like Renogy panel doesn't come with a controller so you'll have to account for one. I like the idea of the semi flexible panels, any idea if they need an air space behind them like the rigid ones for cooling?

[Carol H]

Originally Posted by Paul Braun

Carol, I have to admit I am one of those junkies who loves getting into all of the intricacies of battery charging and getting the battery fully charged.

Oh don't get me wrong guys I understand! I find the discussion equally as interesting as I am looking to upgrade my solar system in the near further as well.

My comment was just my observation on how the discussion over which solar controller to use and the cost associated with the differences between the two types is not unlike the discussions that more often than not takes place when the topic of power converters comes up. There is the price difference and the fact no two end users are the same in regards to their power needs. At the end of the day one party with fairly average solar power needs may find that the PWM does the job just fine for them & have no issues with replacing a battery every 4 years or so ... while another user may be one who watches their power usage carefully and feels they need that extra little punch of power that the MPPT controller will give and as such its well worth the extra money spent. An argument can be made for both types but at the end of the day it really comes down to what the end user feels they really have to have & how deep of pockets they have to get it.