Whats the difference between mppt and pwm

[nb celestine]

A friend of mine has been advised to get a 30 amp mppt controller for his solar panels. I looked on ebay for him and there is stacks to choose from.

 

Whats the difference between the mppt and the cheaper pwm in regards to controlling the charge?

 

The pwm is half the price of the mppt.

[Nickhlx]

A quick Google revealed this which is a pretty rounded summary of the differences...

 

http://www.ecodirect.com/Charge-Controller-Comparison-s/251.htm

 

Nick

[Chris Pink]

A friend of mine has been advised to get a 30 amp mppt controller for his solar panels. I looked on ebay for him and there is stacks to choose from.

 

Whats the difference between the mppt and the cheaper pwm in regards to controlling the charge?

 

The pwm is half the price of the mppt.

 

one 'p', a 't' and a 'w'

 

 

If you use high voltage panels - the ones commonly used on house arrays then you need an MPPT to match the voltage to your batteries.

 

With the so-called "12V panels", ie with VOC around 22V, it is moot whether the extra cost could be better spent getting another panel. A good MPPT controller, and it has to be a good one, will more accurately match the panels to the batteries and claim up to 30% extra power in but the effect will be less with these panels.

 

The Tracer MPPT controllers (with meter) are, in my opinion, a good buy.

 

forum Member Moomin Papa also recommends a make (can't remember off the top of my head)

 

If I start talking about kittens and tomatoes you might look at me funny.

[Chris Pink]

A quick Google revealed this which is a pretty rounded summary of the differences...

 

http://www.ecodirect.com/Charge-Controller-Comparison-s/251.htm

 

Nick

 

Not sure I agree with the thrust of what they are saying which seems to be

 

...that quality PWM controllers provide an equalisation charge and MPPT ones don't the Morningstar Tristar MPPT certainly does.

 

...and PWM doesn't provide high voltage "in bursts" it allows the batteries to control the voltage up to the point of bulk cutoff, 14.4V usually after that the multistage ones provides an averaging current by switching the panel on and off rapidly. I have never put a scope on the output but it certainly doesn't make a voltmeter twitch. The cheap PWM controllers simply cut off at a certain voltage and with some of the really cheap ones that's quite low.

[bizzard]

Good riddle this. Simple though. There's four letters in one and three in the other. One has two P's in it and one in t'other, one hat no T in it and one has no W in it.

[Nickhlx]

Not sure I agree with the thrust of what they are saying which seems to be

 

...that quality PWM controllers provide an equalisation charge and MPPT ones don't the Morningstar Tristar MPPT certainly does.

 

...and PWM doesn't provide high voltage "in bursts" it allows the batteries to control the voltage up to the point of bulk cutoff, 14.4V usually after that the multistage ones provides an averaging current by switching the panel on and off rapidly. I have never put a scope on the output but it certainly doesn't make a voltmeter twitch. The cheap PWM controllers simply cut off at a certain voltage and with some of the really cheap ones that's quite low.

 

 

 

I am sure it will come down to "you get what you pay for", with the bigger manufacturers producing products which are more feature rich and higher quality...

 

I think the main answer to the question is "why MPPT" and that is probably " better matching of the panels to the battery bank" and " better efficiency in conversion of the light captured ", compared to PWM...

 

Nick

[MtB]

Unfortunately, the the psychology of price at play here.

 

When buying something they don't understand, the human been has a habit of using price to assess the quality of whatever he's buying. Wine springs to mind, 'this wine MUST be good it's £18 a bottle" effect.

 

I am concerned a similar effect can occur in solar. Certainly people seem to think if it says MPPT on the box it MUST be better, and I suspect certain manufacturers are well aware of this. I wouldn't be surprised if there are PWM controllers around that are well worth buying. I expect the more expensive PWMs are better. Oh, hang on....

 

 

MtB

[by'eck]

A friend of mine has been advised to get a 30 amp mppt controller for his solar panels. I looked on ebay for him and there is stacks to choose from.

 

Whats the difference between the mppt and the cheaper pwm in regards to controlling the charge?

 

The pwm is half the price of the mppt.

 

mppt = max power point tracking (usually employed in a controller to hold solar panels (or other device) at their most productive voltage and provide the controlled lower voltage to batteries or other load)

 

pwm = pulse width modulation (a means of controlling the power supplied to a device by high speed digital switching of the current on and off for differing periods giving a variable average)

 

You can't compare the two. It could well be that an mppt controller employs pwm as do alternator regulators.

 

In your solar controller example though a stand-alone pwm device would only control the voltage charging the batteries leaving the solar panels working at less than max efficiency for much of the time.

Edited March 24, 2013 by by'eck

[Chris Pink]

 

In your solar controller example though a stand-alone pwm device would only control the voltage charging the batteries leaving the solar panels working at less than max efficiency for much of the time.

 

That's not true.. A PWM controller will only kick in when the batteries are charged, most of the time it leaves the panels and the batteries to their own devices. You can see when this happens on a Tristar controller, the meter tells you so and on the Prostar you can hear it.

 

The MPPT controls the voltage so that the voltage in and the voltage out are at different levels.

[dmr]

I know nothing about solar controllers but in the tradition of this forum will not see this as a reason not to post!!

 

I would assume that if a panel can produce 10amps at 22 volts (220Watt) then a PWM controller will reduce this to 10 amps at 14v (140W) to match the batteries, whilst a MPPT will convert it to 15.7amps at 14v (220Watt). Is this correct??????

 

...........Dave

[Chris Pink]

I know nothing about solar controllers but in the tradition of this forum will not see this as a reason not to post!!

 

I would assume that if a panel can produce 10amps at 22 volts (220Watt) then a PWM controller will reduce this to 10 amps at 14v (140W) to match the batteries, whilst a MPPT will convert it to 15.7amps at 14v (220Watt). Is this correct??????

 

...........Dave

 

no, it is the battery that would reduce the voltage to 14V (or whatever) in a PWM controller's case and when it gets to 14.4V (if that's what it's set at) it switches the panel on and off so the terminal voltage doesn't exceed this and the current starts to drop.

 

The theory behind MPPT is that there is a (varying with current) voltage at which panels produce the most power, the MPPT controller provides a variable load so that the panel is working at this voltage all the time.

 

With all charging systems it is essentially the battery, or rather its internal resistance, that controls the current (power) in. The principle (in all charging systems) is that the voltage should stay below the gassing voltage because any higher and they will take more current but that turns electrolyte into hydrogen gas rather than charging the battery. Solar is no different.

[by'eck]

That's not true.. A PWM controller will only kick in when the batteries are charged, most of the time it leaves the panels and the batteries to their own devices. You can see when this happens on a Tristar controller, the meter tells you so and on the Prostar you can hear it.

 

The MPPT controls the voltage so that the voltage in and the voltage out are at different levels.

 

Not sure what's not true in my statement but if you want to be pedantic

 

I was simply making the point that a controller employing pwm only does not hold the panels at a particular voltage.

[Chalky]

Very simple difference.

 

MPPT is a power converter that can change the input voltage to a different one to match what the battery requires. The conversion ratio is variable and is adapted to get the best/optimum power transfer from the solar panel to the battery. The conversion optimises for peak power transfer. - Think of it like an electronic CVT gearbox.

 

PWM - pulse width modulation relies on turning the power from the solar panels on and off at speed so that the voltage that the battery sees is correct. The voltage supplied to the battery may be higher than wanted however if its only turned on part of the time the average voltage that the battery sees is correct (the peak may be considerably higher). The rest of the power is not transferred an is lost. A bolinder uses a PWM speed controller.

 

MPPT controllers require switch mode power supplies to do the conversion and electronic controllers to determine the best conversion ratio and adjust the conversion. PWM is just a switch and a fairly simple timer / comparator circuit.

Edited March 24, 2013 by Chalky

[Timleech]

Beware that some of the cheaper controllers which claim to be MPPT are not, or at least have no benefit over a simple controller.

 

Tim

[smileypete]

A friend of mine has been advised to get a 30 amp mppt controller for his solar panels. I looked on ebay for him and there is stacks to choose from.

 

Whats the difference between the mppt and the cheaper pwm in regards to controlling the charge?

 

The pwm is half the price of the mppt.

As mentioned Tracer/EP Solar have a good reputation, strangely Amazon is another source of UK supplied ones:

 

http://www.amazon.co.uk/SainSonic-TRACER-3215RN-Charge-Controller/dp/B008RJEXAE/ref=sr_1_1?ie=UTF8&qid=1364141176&sr=8-1

http://www.amazon.co.uk/SainSonic-TRACER-3215RN-Charge-Controller/dp/B008RJEXAE/ref=sr_1_1?ie=UTF8&qid=1364141328&sr=1-1

 

These people may work out cheapest if you can collect:

 

http://solar.bimbleinn.com/solar/mppt

 

With any controller the charge voltage and absorption charge time are worth knowing, no good buying a cheap controller if it doesn't put enough charge into the batts.

 

cheers, Pete.

~smpt~

Edited March 24, 2013 by smileypete

[Nickhlx]

The output efficiency of a panel depends on it being loaded optimally.

 

As I understand it, if you connect a 22 Volt panel to a battery bank that is in a low SoC, and when a conventional charger will be in its BULK stage, it will drag the voltage down to say 13.5 volts, but for optimum output power from the panel it needs to be at (say) 17.5 volts. The MPPT controller allows the panels to work at their optimum 17.5 volts where they develop/produce most power...

 

Nick

[dmr]

no, it is the battery that would reduce the voltage to 14V (or whatever) in a PWM controller's case and when it gets to 14.4V (if that's what it's set at) it switches the panel on and off so the terminal voltage doesn't exceed this and the current starts to drop.

 

The theory behind MPPT is that there is a (varying with current) voltage at which panels produce the most power, the MPPT controller provides a variable load so that the panel is working at this voltage all the time.

 

With all charging systems it is essentially the battery, or rather its internal resistance, that controls the current (power) in. The principle (in all charging systems) is that the voltage should stay below the gassing voltage because any higher and they will take more current but that turns electrolyte into hydrogen gas rather than charging the battery. Solar is no different.

 

Not sure this makes sense to me?

Are you suggesting the PWM controller does nothing to control voltage till a "fully charged/14.4" voltage is detected when it just shuts down?

I realise that the battery will drag the solar voltage down, just like it drags the alternator down in bulk mode, but if there is no control why is it PWM??? PWM to me suggests a variable voltage control!!!

 

Also why does everybody go on so much about gassing and fear it so much??? I generally charge at 14.9 or 15 volts, there is a bit of gassing which I believe to be a really good thing, but I still only top the batteries up two or three times each year! Is this because people have those silly sealed batteries???

 

...........Dave

[Chris Pink]

Not sure this makes sense to me?

Are you suggesting the PWM controller does nothing to control voltage till a "fully charged/14.4" voltage is detected when it just shuts down?

I realise that the battery will drag the solar voltage down, just like it drags the alternator down in bulk mode, but if there is no control why is it PWM??? PWM to me suggests a variable voltage control!!!

 

 

 

It does the PWM bit during the absorption phase when the voltage would otherwise rise above the regulator voltage, at that point it switches the panel on and off to regulate the voltage. It doesn't appear to be a timed cycle so I assume it stays in this mode until the current drops below a certain level

 

There is no particular reason why this is 14.4V other than that is the average they are set to. The Tristar is selectable in 0.2V steps 14.0 to 15.0V

 

As I say the cheaper regulators don't even do this and simply switch off at a point considered to be charged, usually 13.8V

 

A good PWM controller will be in effect a 3 stage battery charger.

 

The Tristar manual states "charging includes a bulk charging stage, PWM absorption, float and equalisation"

 

 

 

 

I rate Morningstar kit and if you want to read up on it you could do worse than download the Tristar manuals - there is an MPPT and a PWM version.

 

 

PWM: http://www.morningstarcorp.com/en/support/library/TS.IOM.Operators_Manual.04.EN.pdf

 

MPPT: http://www.morningstarcorp.com/en/support/library/TSMPPT.IOM.EN.04.pdf

Edited March 24, 2013 by Chris Pink

[nb celestine]

It does the PWM bit during the absorption phase when the voltage would otherwise rise above the regulator voltage, at that point it switches the panel on and off to regulate the voltage. It doesn't appear to be a timed cycle so I assume it stays in this mode until the current drops below a certain level

 

There is no particular reason why this is 14.4V other than that is the average they are set to. The Tristar is selectable in 0.2V steps 14.0 to 15.0V

 

As I say the cheaper regulators don't even do this and simply switch off at a point considered to be charged, usually 13.8V

 

A good PWM controller will be in effect a 3 stage battery charger.

 

The Tristar manual states "charging includes a bulk charging stage, PWM absorption, float and equalisation"

 

 

 

 

I rate Morningstar kit and if you want to read up on it you could do worse than download the Tristar manuals - there is an MPPT and a PWM version.

 

 

PWM: http://www.morningstarcorp.com/en/support/library/TS.IOM.Operators_Manual.04.EN.pdf

 

MPPT: http://www.morningstarcorp.com/en/support/library/TSMPPT.IOM.EN.04.pdf

 

Thanks for all the replies.

 

I now realise why I am a bricklayer and not an electrician.

 

I will pass on the advice. Regards, Paul.

[smileypete]

Also why does everybody go on so much about gassing and fear it so much??? I generally charge at 14.9 or 15 volts, there is a bit of gassing which I believe to be a really good thing, but I still only top the batteries up two or three times each year! Is this because people have those silly sealed batteries???

It's a trade off between sulphation and wear and tear. So a particular charge cycle may be ideal for a once-a-day charge cycle from deep discharge,

 

But a solar controller or shoreline charger may go through quite a number of charge cycles a day in response to loads being switched on, where the discharge is actually quite shallow. And to perform exactly the same charge cycle as above may result in a lot of unnecessary wear and tear, even to the point of failure for cheap leisure batts.

 

Given that sulphation isn't a big problem for much of the year with a solar setup, or year round with a shoreline charger, I'd use a more conservative charge method for these times particularly with unsealed batts.

 

cheers, Pete.

~smpt~

[dmr]

It's a trade off between sulphation and wear and tear. So a particular charge cycle may be ideal for a once-a-day charge cycle from deep discharge,

 

But a solar controller or shoreline charger may go through quite a number of charge cycles a day in response to loads being switched on, where the discharge is actually quite shallow. And to perform exactly the same charge cycle as above may result in a lot of unnecessary wear and tear, even to the point of failure for cheap leisure batts.

 

Given that sulphation isn't a big problem for much of the year with a solar setup, or year round with a shoreline charger, I'd use a more conservative charge method for these times particularly with unsealed batts.

 

cheers, Pete.

~smpt~

 

I am charging at about 15v in the winter but dropping down to less than 14.8 in the summer when we are moving almost everyday.

I would love to know a bit more about how much extra wear (plate corrosion?) I suffer as a result of this harder charging but I imagine very few people have this information. Its sad that Gibbo has gone even though he was a bit argumentative (though some were seriously good entertainment).

As an off grid liveaboard, fighting off sulphation is a big struggle in the winter, and I reckon I spent well over £100 of diesel this winter doing the 100% charge. Thats getting on for a new Trojan! I am not convinced that solar would work for me as we only need it in the winter.

I think I would be happier to shorten the life of the batteries a bit rather than have them die of suplphation, a concious sacrifice is somehow more acceptable than neglect.

I suppose this is really the subject for a new topic another day.

 

...........Dave

[MoominPapa]

forum Member Moomin Papa also recommends a make (can't remember off the top of my head)

 

Ebay linky.

 

MP.

[smileypete]

I am charging at about 15v in the winter but dropping down to less than 14.8 in the summer when we are moving almost everyday.

I would love to know a bit more about how much extra wear (plate corrosion?) I suffer as a result of this harder charging but I imagine very few people have this information. Its sad that Gibbo has gone even though he was a bit argumentative (though some were seriously good entertainment).

Would expect that's fine for a once a day charge for Trojan semi traction batts, the recommend charge voltage is best temperature compensated too. If the batts are around 4°C the charge voltage could be raised to 15.4V, would have thought some gassing is good in those circumstance too.

 

As an off grid liveaboard, fighting off sulphation is a big struggle in the winter, and I reckon I spent well over £100 of diesel this winter doing the 100% charge. Thats getting on for a new Trojan! I am not convinced that solar would work for me as we only need it in the winter.

I think I would be happier to shorten the life of the batteries a bit rather than have them die of suplphation, a concious sacrifice is somehow more acceptable than neglect.

I suppose this is really the subject for a new topic another day.

Wonder how much current or Ah is fed into the batts during equalisation? If it's not a huge amount I'd look into equalising half the bank or even just a pair of batts at a time, trying a DC-DC constant current converter. Think we exchanged a PMs about it a few months back.

 

cheers, Pete.

~smpt~

[dmr]

Would expect that's fine for a once a day charge for Trojan semi traction batts, the recommend charge voltage is best temperature compensated too. If the batts are around 4°C the charge voltage could be raised to 15.4V, would have thought some gassing is good in those circumstance too.

 

 

Wonder how much current or Ah is fed into the batts during equalisation? If it's not a huge amount I'd look into equalising half the bank or even just a pair of batts at a time, trying a DC-DC constant current converter. Think we exchanged a PMs about it a few months back.

 

cheers, Pete.

~smpt~

 

Yeah I've just ordered one of those Chinese DC-DC converters off eBay so will try to build something before next winter. Need to negotiate with Gillie first to give up a bit of storage space so I can have more batteries!! Also want to build a better alternator controller...too many projects and too little time!

 

.............Dave

[peterboat]

Hi there is one other advantage of mppt and that is you can connect the solar panels in series up the voltage and have smaller cables which are cheaper and less visable. I have to say that I have household panels which are 36v so have no choice I have to have mppt. It all works well even in winter

 

Peter