MPPT v PWM

[ROBDEN]

What difference would you expect to see when charging 4x110amp batteries with 2x100 watt solar panels

using an MPPT controller against a PWM controller?

 

Thanks Rob....

[nicknorman]

It will depend on the V (max current) of the panels. If close to the voltage of batteries under charge, say 13.5 to 14.5v plus whatever the PWM drops then the answer is very little. The further away the V (max c), the more benefit. MPPT excels when you have panels in series giving a high voltage. This is not only efficient from a power transfer point of view, but also means you need much less copper in the wires from panel to controller.

Edited February 18, 2015 by nicknorman

[rogeriko]

Hi ROBDEN hope everything is working OK now.

 

Solar panels can only give the Amps rating on the label. If the label says 5 Amps then thats all you can get. ie 5 Amps times 12v = 60 watts even if the panel is 250 watts you will never get more than 60 watts into a 12v battery. A PWM controller will not help in this instance only disconnect the panel when the battery is fully charged. A MPPT controller will take the Voltage into consideration as well as the Amps and can in the right circumstances give you more power into the batteries. ie 5A times 30v = 150w the MPPT controller wil probably give you 100w into your 12v battery or 8.5A. Tell us what the label says on the back of your panels. MPPT controllers are expensive cheaper to just buy more solar panels.

Edited February 19, 2015 by rogeriko

[by'eck]

Hi ROBDEN hope everything is working OK now.

 

Solar panels can only give the Amps rating on the label. If the label says 5 Amps then thats all you can get. ie 5 Amps times 12v = 60 watts even if the panel is 250 watts you will never get more than 60 watts into a 12v battery. A PWM controller will not help in this instance only disconnect the panel when the battery is fully charged. A MPPT controller will take the Voltage into consideration as well as the Amps and can in the right circumstances give you more power into the batteries. ie 5A times 30v = 150w the MPPT controller wil probably give you 100w into your 12v battery or 8.5A. Tell us what the label says on the back of your panels. MPPT controllers are expensive cheaper to just buy more solar panels.

 

Please explain how I occasionally got 16 amps (regularly over 10 amps) charge current into my 12 volt battery bank from two 130 watt panels (in parallel) and an mppt controller, given your fixed current theory.

[smileypete]

 

Please explain how I occasionally got 16 amps (regularly over 10 amps) charge current into my 12 volt battery bank from two 130 watt panels (in parallel) and an mppt controller, given your fixed current theory.

 

Seemed OK enough to me, maybe not crystal clear though. Breaking the paragraph down would help:

 

Hi ROBDEN hope everything is working OK now.

 

Solar panels can only give the Amps rating on the label. If the label says 5 Amps then thats all you can get. ie 5 Amps times 12v = 60 watts even if the panel is 250 watts you will never get more than 60 watts into a 12v battery.

 

A PWM controller will not help in this instance only disconnect the panel when the battery is fully charged.

 

A MPPT controller will take the Voltage into consideration as well as the Amps and can in the right circumstances give you more power into the batteries. ie 5A times 30v = 150w the MPPT controller wil probably give you 100w into your 12v battery or 8.5A.

 

Tell us what the label says on the back of your panels. MPPT controllers are expensive cheaper to just buy more solar panels.

 

An additional clearer explanation may be helpful.

 

To answer the OPs question, an MPPT can give up to 30% more power at times, but maybe say 15-20% on average.

 

It also allows the use of the large domestic panels which work out cheaper per watt. A decent programmable MPPT can also be a good off-the-shelf solution to batt equalisation.

 

cheers, Pete.

~smpt~

Edited February 20, 2015 by smileypete

[nicknorman]

Please explain how I occasionally got 16 amps (regularly over 10 amps) charge current into my 12 volt battery bank from two 130 watt panels (in parallel) and an mppt controller, given your fixed current theory.

Magic?

 

I think most of Roger's post related to PWM and was thus correct if a little unclear. However I disagree with his comment about "just get more panels" since, if you actually use your boat for cruising, a lot of clutter form panels can be a PITA even disregarding the aesthetics.

[Staarek]

I used to have Morningstar PWM Controller in the past, changed for Outback MPPT and tbh I don't think there is a significant difference. On the other hand, it's quite hard to compare it as we CC and we moor in places that differ in terms of sun/shade etc. I only started taking readings last Feb so soon I should be able to have more data to compare it.

 

For now:

1-9 Feb 2014: 58Ah on average/day

1-9 Feb 2015: 56.8Ah on average/day

 

I changed the controllers in May 2014

[nicknorman]

I used to have Morningstar PWM Controller in the past, changed for Outback MPPT and tbh I don't think there is a significant difference. On the other hand, it's quite hard to compare it as we CC and we moor in places that differ in terms of sun/shade etc. I only started taking readings last Feb so soon I should be able to have more data to compare it.

 

For now:

1-9 Feb 2014: 58Ah on average/day

1-9 Feb 2015: 56.8Ah on average/day

 

I changed the controllers in May 2014

But the crucial question is, what is the panel voltage for max current?

[Staarek]

But the crucial question is, what is the panel voltage for max current?

 

Around 30V as long as I'm aware

[nicknorman]

Around 30V as long as I'm aware

If that's the panel voltage for max current, as opposed to the open circuit voltage, I would certainly expect a significant gain from MPPT, but of course there are so many other variables (weather, mooring location etc) that I think you will need a lot of data to daw any conclusions.

 

Ed: and of course presuming a 12v electrical system.

Edited February 20, 2015 by nicknorman

[Staarek]

If that's the panel voltage for max current, as opposed to the open circuit voltage, I would certainly expect a significant gain from MPPT, but of course there are so many other variables (weather, mooring location etc) that I think you will need a lot of data to daw any conclusions.

 

Ed: and of course presuming a 12v electrical system.

 

Yes it's 12V system. As you said it will be quite difficult to establish what the gain really is but I will be keeping an eye on current and old records for the next couple of months.

 

The panels specs:

 

• Vmp 29.62V
• Isc 8.56A
• Voc 36.75V

[nicknorman]

Yes it's 12V system. As you said it will be quite difficult to establish what the gain really is but I will be keeping an eye on current and old records for the next couple of months.

 

The panels specs:

 

 

• Vmp 29.62V
• Isc 8.56A
• Voc 36.75V

Well in theory you should get approaching twice the AH with the MPPT. Out of interest what is the total of all the panels' power?

 

Ed: of course that presumes that the batteries can actually take the current. Looking at you Feb daily AH I would think your batteries would be getting pretty full (and hence not taking much current) mid way through a summer's day? Once the batteries' demand has fallen to what a PWM could provide, the MPPT becomes pointless.

Edited February 20, 2015 by nicknorman

[costalot]

My simple way of looking at the difference between a PWM and MPPT controller is that the MPPT converts some of the excess voltage to amperes

 

Example 100w panel rated at max 27 volts (W = V x A) so the maximum amps from the panel is 100/27 = 3.7 Amps The batteries can't accept 27 volts so the PWM controller reduces the voltage to an acceptable level (say 14.8V) by rapidly cutting in and out during the period the voltage is exceeding 14.8. The MPPT controller is more sophisticated and converts the additional volts to amps. 100/14.8 = 6.7 Amps. In practice the MPPT isn't quite that clever in converting to excess voltage to amps and usually you get approximately an additional 30%. Furthermore, not all MPPT controllers are the same.

[Staarek]

Well in theory you should get approaching twice the AH with the MPPT. Out of interest what is the total of all the panels' power?

 

I've got 6 of these, but mounted flat on the roof. So give or take 1400W

[by'eck]

Magic?

 

I think most of Roger's post related to PWM and was thus correct if a little unclear. However I disagree with his comment about "just get more panels" since, if you actually use your boat for cruising, a lot of clutter form panels can be a PITA even disregarding the aesthetics.

 

Not really magic surely - making the theory fit the practice we have:

 

2 x Kyocera KC-130 pv panels rated 7.39 amps each at 17.6 volts (Vmpp) = 130 watts

Sunny skies and a cooling breeze to keep panel surface temp down = Caribbean

Blue Sky SB2512i mppt controller holding panels at Vmpp and say 14.5 volts on batteries in absorb mode with typical 95% efficiency = 2 x 130/14.5 x 0.95 = 17 amps

 

Hopefully these facts benefit the OP in highlighting the advantage of mppt over plain pwm controllers. Surely a cheaper route than adding pv panels to compensate (however cheap) when you take into account mounting, cabling and extra power handling, not to mention aesthetics

[nicknorman]

I've got 6 of these, but mounted flat on the roof. So give or take 1400W

Ok so that's a lot of solar, I doubt the MPPT will make much difference to the daily AH in summer, only on the cloudy days, unless you are a heavy power user.

[Staarek]

Well in theory you should get approaching twice the AH with the MPPT. Out of interest what is the total of all the panels' power?

 

Ed: of course that presumes that the batteries can actually take the current. Looking at you Feb daily AH I would think your batteries would be getting pretty full (and hence not taking much current) mid way through a summer's day? Once the batteries' demand has fallen to what a PWM could provide, the MPPT becomes pointless.

 

Good point. During summer I was using low power immersion heater to warm up water and I was getting between 200 - 400 ah/day. When I wasn't heating up the water the batteries were full after 80 - 100ah.

Edited February 20, 2015 by Staarek

[AjW]

A thought....

Batteries are flattest in the morning after a night of tv, lights and pumps. This is when you need max efficiency from your solar setup. mppt works well at this time of day, but tilted panels will add a huge % over flat panels.

Later in the day, or after cruising your panels are effectively trickle charging after a bulk charge by alternator or panels. Mppt isn't going to make much difference over Pwm at this point.

In the evening tilted panels will help catch the last of the rays.

Mppt vs Pwm is dependent on your charge/discharge patterns. You might get more out of a Pwm system just by angling the panels correctly rather than using an mppt.

[Ssscrudddy]

The way I see it PWM cuts off the higher voltage & so that is electricity you arent getting if you need it. MPPT changes the volatge so that you get all of the electricity if you need it.
PWM is older tech than MPPT.
PWM is cheaper than MPPT. On Bimble the equivalent MPPT is £95 & the PWM is £32

Edited February 20, 2015 by Ssscrudddy

[rogeriko]

 

Please explain how I occasionally got 16 amps (regularly over 10 amps) charge current into my 12 volt battery bank from two 130 watt panels (in parallel) and an mppt controller, given your fixed current theory.

Sorry for replying late I have been working all day. You say you have 2 x Kyocera KC-130 pv panels rated 7.39 amps each at 17.6 volts (Vmpp) = 130 watts those are 7.39A panels. you have 2 in parrallel that will give you 14.78A without any controller. I was talking about the more common higher voltage panels, the 30v MPPT panels. Your panels are well matched to your batteries so you get a good charge rate, not everyones panels are matched like yours.

[George94]

My simple way of looking at the difference between a PWM and MPPT controller is that the MPPT converts some of the excess voltage to amperes

 

Example 100w panel rated at max 27 volts (W = V x A) so the maximum amps from the panel is 100/27 = 3.7 Amps The batteries can't accept 27 volts so the PWM controller reduces the voltage to an acceptable level (say 14.8V) by rapidly cutting in and out during the period the voltage is exceeding 14.8. The MPPT controller is more sophisticated and converts the additional volts to amps. 100/14.8 = 6.7 Amps. In practice the MPPT isn't quite that clever in converting to excess voltage to amps and usually you get approximately an additional 30%. Furthermore, not all MPPT controllers are the same.

 

I am not sure that is correct. The much used 30% figure is an average, based on the fact that batteries will stop accepting a high amperage as they get nearer to being fully charged, so the MPPT is then no better than a PWM.

 

When batteries ARE accepting as many amps as you can throw at them, the MPPT might well throw in four times as many amps as the PWM, or more, depending of course on how they are wired up and the specs of the panels.

 

At least that is my understanding.

[Loafer]

Don't forget that most MPPT controllers are also 3-stage battery chargers, and charge your batteries correctly. PWM controllers don't usually do that.

[Ian Mac]

PWM - Pulse Width Modulation, basically a clever on/off switch, if the provided voltage from the panel gets too high the PWM switches the supply off from charging, and the power is wasted.

Cheap

 

MPPT - Maximum power point tracking - uses a computer of some form, can be very complex measurement of the voltage or just three points on a curve to calculate how to get the excessive voltage and its associated current, into the battery by storing the energy (in an inductor usually) and then feeding it into the battery later, happens many times a second. Only any use if you have lots of excess voltage from the panels, and your batteries need lots of current ie are flat!

Expensive

 

There are other variation available such as controllers which charges the batteries first and then use the excess power to heat your water for example You can have either type do this but MPPTs are better at it, but are more complex so therefore cost more!

[nicknorman]

MPPT - Maximum power point tracking - uses a computer of some form, can be very complex measurement of the voltage or just three points on a curve to calculate how to get the excessive voltage and its associated current, into the battery by storing the energy (in an inductor usually) and then feeding it into the battery later, happens many times a second.

I'm not sure it's "very complex"? Don't they just vary the degree of sucking panel current (thus changing the panel voltage) whilst measuring the power? If the power is getting more, the variation is in the correct sense and if getting less, the variation is in the wrong sense, and so it settles at the max power voltage.

[by'eck]

I found this diagram the easiest to understand in explaining how the buck-boost converter at the heart of an mppt controller works, in this case reducing the output voltage.

 

 

The switch is controlled by pulse width modulation as in a plain PWM controller. In the On state energy flows from the pv panel source on the left to the inductor only, due to blocking diode. The ouput is fed from charge in capacitor.

 

In the Off state energy from the inductor flows to charge the capacitor and to the output load on the right. Note current reversal though.

 

The PWM signal (balance between Off and On), controls the current to the inductor.