PWM or MPPT controller for solar

Submitted: Wednesday, Sep 19, 2012 at 19:44
ThreadID: 98121 Views:10760 Replies:6 FollowUps:12
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Hi all,

An earlier post got me some very good info regarding connecting multiple charging sources at the same time. Now next question, an MPPT regulator or a PWM. It appears that a good quality MPPT is going to be a fair bit more expensive than an equivilant quality PWM. Now some say that the amount of gain in typical Australian conditions (mainly northern latitudes in winter) does not warrant the extra dollars. I am happy to shell out some extra bucks for quality components but not if the gain is negligible.
Has anyone had experience with both types and feel that the extra outlay is worth it. All I am looking at charging is a 105ah AGM that mainly keeps a 40lt Engel going. It will be connected to the vehicles main cranking battery with a VSR while travelling so extra charging will only be needed when stopped for a few days.

Thanks once again for any real world experience.

Cheers
Pop
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Reply By: Bigfish - Wednesday, Sep 19, 2012 at 20:14

Wednesday, Sep 19, 2012 at 20:14
Mate...if your happy to shell out a few extra dollars for quality then you really should be putting a second battery on your vehicle. I would never chance running a fridge off my main battery. At the most important time you need to start your car....old mate Murphy,s Law walks in and stuffs ya!!!
AnswerID: 495250

Follow Up By: pop2jocem - Wednesday, Sep 19, 2012 at 20:48

Wednesday, Sep 19, 2012 at 20:48
Bigfish,

Nah mate the 2 main cranking N70'S take care of starting and get isolated by the VSR.
Had experience with ya mate Murphy...lol gets around doesn't he...(;-))

Cheers
Pop
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Reply By: Member - Toyocrusa (NSW) - Wednesday, Sep 19, 2012 at 20:15

Wednesday, Sep 19, 2012 at 20:15
Hi Pop. The technical people whose opinions I have read seem to disagree to some extent on which is better. I have a 22ft van with 2x 90 watt panels charging a 100ah Gel battery through an MPPT regulator. In good conditions it gets up to around 8 amps going into the battery. I also have a 120 watt folding portable panel that can put up to 7 amps through a PWM regulator. that is about as much as I know on the technical side but the PWM was under half the cost of the MPPT. I know it is better to have a screen to be able to set voltages and see what is happening. These are the two I have.Cheers,Bob.

My PWM regulator

My MPPT Regulator
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Follow Up By: pop2jocem - Wednesday, Sep 19, 2012 at 20:53

Wednesday, Sep 19, 2012 at 20:53
Hi Tc,

Now they are well priced, haven't had a look at e bay yet. The Plasmatronics PL 20 PWM was around the $385 mark and the Morningstar MPPT 20 amp job was about double that hence my enquiry. I want to try and get it right first time....I hope.

Cheers
Pop
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Follow Up By: pop2jocem - Wednesday, Sep 19, 2012 at 21:06

Wednesday, Sep 19, 2012 at 21:06
And yeah you're right about the "experts" not agreeing about which is best value for money. Some say put your dollars into an extra panel which is also worthy of consideration


Cheers
Pop
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Reply By: Nomadic Navara - Wednesday, Sep 19, 2012 at 23:58

Wednesday, Sep 19, 2012 at 23:58
Quality MPPTs are more for larger arrays where you can really make a saving. For a couple of panels then a quality simple regulator and bigger panels are mostly cheaper (unless of course yo don't have the real estate for the extra panel size.)

PeterD
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Reply By: Member - Boobook - Thursday, Sep 20, 2012 at 05:32

Thursday, Sep 20, 2012 at 05:32
The premium for MPPT has almost gone away. Search for Tracer MPPT which is a true, good quality charger from about $110. ( the same unit as the $380 Projecta one)

Many years ago I switched and I can 100% assure you that in RV applications for 80W - 400W approx, you will get between 15 and 30% more power from the same solar panels.

The reason is simple, panels work at about 20V, batteries charge at about 12 to 14V. The difference in the resulting power is thrown away in PWM chargers, where as MPPT's work like an inverter in reverse and preserve the power.

The best thing of all is that their advantage is best when the panels are cool and the batteries are low - or more accurately that is when PWM's are least efficient and waste the most energy.

The thing about that is that you will get about 30% more amps into your batteries in the mornings when it is still cool and you are charging your batteries from overnight use. Ie they will charge 30% quicker.

Don't let anyone tell you they are only for large sites, that has changed with the prices reducing.

One word of warning is that there are a lot of fake MPPT chargers like the Jupta ones sold around and on ebay. Avoid them.
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Follow Up By: Member - Boobook - Thursday, Sep 20, 2012 at 05:33

Thursday, Sep 20, 2012 at 05:33
I meant to say search for the Tracer on ebay.
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Follow Up By: Nomadic Navara - Thursday, Sep 20, 2012 at 10:07

Thursday, Sep 20, 2012 at 10:07
Quote - "The reason is simple, panels work at about 20V, batteries charge at about 12 to 14V. The difference in the resulting power is thrown away in PWM chargers, where as MPPT's work like an inverter in reverse and preserve the power."

That 20 V is a falacy. Someone has been feeding you duff gen. The peak voltage of 12 V panels is 20 or so volts, generally around 21 V. In that condition no current is being drawn from so they are generating SFA power.

Solar panels are a dynamic device. When you start working them their terminal voltage drops somewhat. When they are producing their maximum power the terminal voltage drops to around 17 V, this is a far cry from your 20 V.

When you charge a battery with a series regulator (PWM is just one form of them) the terminal voltage drops to around 15 V or a little under. When you redo your maths I suggest you also take into consideration that at 15 V the panel is also supplying more current than it does at the MPP voltage. I think if you recalculate your gains on those figures then you will see that an MPPT regulator will only supply 15% more power than a series regulator and then only under the most favourable conditions

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Follow Up By: pop2jocem - Thursday, Sep 20, 2012 at 11:12

Thursday, Sep 20, 2012 at 11:12
Thanks for the info so far guys, I have been trying to edgumacate my rapidly fading grey matter a bit more. Although I inhabit an aging body I am pretty green with regard to solar energy.
I have measured the open circuit or what ever it is called with the 85w (rated??) panel I have at the moment. I am reading 21v with a multi meter. Now as has been pointed out that voltage will drop as a load is applied, this I can understand VxA=W???
I have also read that an extra panel, if I need one can be installed in either series or parallel. Obviously the regulator would have to be capable of handling the now doubled input voltage but some people are saying the higher input voltage with a series connection gives less power loss than a parallel connection.
Now does any of that make sense and does it matter whether the regulator is MPPT or PWM?
I am not trying to get a cheap lesson in electronics (far toooo late for that..lol)

Thanks
Pop
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Follow Up By: Member - Boobook - Thursday, Sep 20, 2012 at 13:53

Thursday, Sep 20, 2012 at 13:53
Sorry Peter, you may be a retired radio tech, but as an Electronics Engineer with direct experience with this I can tell you that you are incorrect.

True a PWM will drag the panel down to the charge voltage plus a little, but you have obviously never used or measured the panel voltage when connected to a MPPT controller. If working properly, the panel voltage will be very close to it's MPP level when at full load, and that is about 19 - 20 V generally.

Have you ever tired it? Obviously not.

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Follow Up By: Nomadic Navara - Thursday, Sep 20, 2012 at 14:47

Thursday, Sep 20, 2012 at 14:47
Pop,

Whether you can series or parallel panels depends upon which style of controller and also the specifications of the controller. If you are using a simple series controller you should parallel your panels. The charge current is not boosted by the controller like it is in an MPPT controller. You will only get an increase in current if you parallel your panels.

With MPPT controllers there are a couple of things to watch. Firstly you have to consider its input rating. If you are using something like a Ctek D250S that only has a maximum input voltage rating of around 21 V, you will zap it if you series the panels. One bloke in CF reported he had blown his, he thought "MPPT, series panels mean less power loss" but forgot to read the specifications of his unit.

The second thing to watch is shading. We all encounter camping in places where our panels get a little shading throughout the day. The current from a series array is dependant upon the output of the weakest panel. If one panel gets a little or complete shading the whole of the arrays power output is reduced by that amount. If you have a parallel array and only one panel is shaded the other will still contribute its full power. You will only suffer the loss of power from half of the array.

In addition if you have panels of different power output the current that is generated in a series array will be largely controlled by the panel with the lesser power rating. You can parallel panels of different capacities but there is little point of connecting them in series.

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Follow Up By: Member - Boobook - Thursday, Sep 20, 2012 at 15:24

Thursday, Sep 20, 2012 at 15:24
Pop, the kind of regulator I referred to will cope with inputs up to about 150V ( check the type you buy if you go this way, some are 60V, 100V etc)

All of them would easily handle 2 panels in series or parallel and will deliver a charge at nominally 12v to your batteries with the same power.
The advantage of wiring the panels in series ( ie about 40V input) is that the current is low and you have less losses over the cable from the panel to the charger which can be considerable. However as Peter points out shading is your enemy. You will lose almost all power, even if only one panel is shaded. Bypass diodes will minimise this effect somewhat.


The advantage of putting them in parallel ( ie about 20V in) is that they will operate independently wrt shade so you will still get half power even if one panel is totally shaded. The cost to this is the increased losses due to higher current or the price of thicker wire.
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Follow Up By: Nomadic Navara - Thursday, Sep 20, 2012 at 22:51

Thursday, Sep 20, 2012 at 22:51
Boobook, quote "If working properly, the panel voltage will be very close to it's MPP level when at full load, and that is about 19 - 20 V generally. "

The MPP voltage for a 12 V panel is around 17 V, not 19 - 20 V. If you use a digital meter you can get all sorts of funny readings when working with pulsing circuits. Try using something like an AVO model 7 to do your readings.
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Follow Up By: Member - Boobook - Friday, Sep 21, 2012 at 05:38

Friday, Sep 21, 2012 at 05:38
Peter

The MPP of a panel will depend on the manufacturer and construction. 19V is VERY Common and observable. There are literally thousands of panels with 19V plus MPP. I have personally observed it regularly on mine.

Have you ever seen a MPPT controller in action rather than relying on internet based theory?

Better stick to Radios with that Avo :-) Yoiu are trying to argue against facts, not opinions. It is pointless arguing with you on this. Try some real world measurements.
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Follow Up By: Member - Boobook - Friday, Sep 21, 2012 at 06:08

Friday, Sep 21, 2012 at 06:08
Apologies, for completeness I should mention the other, and probably most significant factor in the maximum power point voltage of a panel, the temperature, Panels have a higher temp when cooler, hence my earlier comment about the effectiveness in the mornings.
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Reply By: Member - Bruce C (NSW) - Thursday, Sep 20, 2012 at 12:58

Thursday, Sep 20, 2012 at 12:58
Hi Pop,
I have used and still use several PWM and MPPT types of solar regulators.
I have found little difference between them under most normal or ideal conditions, normal sunlight as we experience most days.

However the MPPT will produce more power in low light, that is overcast conditions or shady conditions,

Currently I have a PWM, a Steca copy, in the van and an MPPT fitted on the house 12 volt solar setup.

I prefer the PWM as it has an “At a glance” fuel gauge type display which means it is easy to see where the system is at from several meters away from the regulator.
All my units have LCD screens on them for ease of use and so that I can see what is going on if I wish as flashing LEDs do not fill me with confidence,

This is the one I prefer

As you can see from the link they are not overpriced and they can handle 30 amps of input and 30 amps of output. They also look better than my MPPT units.

As far as the cheap Juta units go I do not be bother with them.

Hope this helps,
Cheers, Bruce.
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restless and lost on a track that I know. HL.

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Reply By: pop2jocem - Thursday, Sep 20, 2012 at 16:03

Thursday, Sep 20, 2012 at 16:03
Gentlemen,

Firstly allow me to thank you all for taking the time and effort to reply to my inquiry. All good info and also has prompted me to have a bit more of a look into factors that I hadn't considered. It sounds like reasonable quality controllers of both persuasions can be had for a lot less money than I had realised. If I am correct it sounds like the performance of both types is not that far apart but the MPPT variety would come into it's own in less optimal conditions. As far as to losses in the wiring, going to a good size is not going to cost much more because of the relatively short run involved. Also I will mount the controller as close to the battery as possible.
At the moment I will try the 85w panel I have and an MPPT controller and see how I go. If I am still not getting enough to keep the battery up I have the room to mount an additional panel.

Once again thanks for your valued input.

Cheers
Pop
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