PWM versus MPPT

[sstuller]

If your average daily AH use is 20% or less of your battery capacity then your controller spends most of its' time in the absorption and float modes. If that's the case then a PWM controller is just as effective as an MPPT and a lot cheaper. Fair enough? Thanks.

[Paul Elliott]

I would say you are correct -- if the PWM and MPPT chargers are equally good at managing these stages of battery charging, and, your solar panels provide enough power to fully charge the battery. The MPPT charger will probably get you through the bulk phase more quickly, and that will give you a better chance at completing the remaining charging. If your panels are big enough that this isn't a factor, then there's no advantage to MPPT.

[noelex 77]

Originally Posted by sstuller

If your average daily AH use is 20% or less of your battery capacity then your controller spends most of its' time in the absorption and float modes. If that's the case then a PWM controller is just as effective as an MPPT and a lot cheaper. Fair enough? Thanks.

Yes, there is little or no advantage to MPPT controllers in the scenario you describe, just bear in mind that PWM controllers will only work if the solar panel voltage is appropriate for the battery voltage. So 36 cell panels for a 12v battery system.

Larger solar panels are often cheaper, or sometimes even only available in higher voltages and these will need an MPPT controller.

[Bleemus]

Until you have a cloudy day.

[socaldmax]

Originally Posted by sstuller

If your average daily AH use is 20% or less of your battery capacity then your controller spends most of its' time in the absorption and float modes. If that's the case then a PWM controller is just as effective as an MPPT and a lot cheaper. Fair enough? Thanks.

WTF???

Unless you fill in the blanks, this is all a bunch of wishing in one hand, politicians in the other.

How big is the battery bank? How small is the solar panel? 2000AH bank and a 50w solar panel? If You're using 200A/day, you're going to spend all day in bulk mode, wishing you had bought REAL solar panels that require an MPPT controller since their Vmp is a lot higher than 15v.

In that scenario, someone spent a ton of money on the battery bank and scrimped on pennies on the solar controller. As cheap as MPPT controllers are now, I don't recommend PWMs for anyone. So what if you're going to save $50 or 60 buying a PWM vs an MPPT, your primary concern when installing solar panels on a boat is maximum efficiency for your limited space. Even if you can't get Sunpower panels, there are other brands that are more efficient in the 300w size than in the 50 or 100w size. As has already been pointed out, these need an MPPT controller due to their higher Vmp, so why even bother with PWM controllers that might be adequate under limited conditions?

Honestly, if someone called me up and said, "I want you to install 6 100w panels on my boat with a 45A PWM controller" I would say, "You are much better off with 2 300w panels each with it's own 20A MPPT controller." If he insisted, I'd say, "Have fun with that!"

A month later, the increased solar harvest of the 2 300w panel setup would be abundantly clear.

[sailorchic34]

From someone using a PWM controller for three 100 watt panels feeding 4 gc6's, I will say I'm very happy with PWM. My batteries are at adsorption by 11 ish, so having a MPPT controller would give just a tiny bit more for the first few hours. As I'm at float by 1:30 for my small boat with simple systems and a 120V fridge, it's more then adequate. So for about $250 to $300 you could have a 300 watt system with PWM controller included that works just fine and dandy, thank you very much.

It works for me, and is inexpensive. If you had a big boat with lots of electronics, etc, maybe MPPT would be the way to go. For the lower cost option PWM keeps my batteries happy and MPPT would be a waste of funds.

[socaldmax]

Originally Posted by sailorchic34

From someone using a PWM controller for three 100 watt panels feeding 4 gc6's, I will say I'm very happy with PWM. My batteries are at adsorption by 11 ish, so having a MPPT controller would give just a tiny bit more for the first few hours. As I'm at float by 1:30 for my small boat with simple systems and a 120V fridge, it's more then adequate. So for about $250 to $300 you could have a 300 watt system with PWM controller included that works just fine and dandy, thank you very much.

It works for me, and is inexpensive. If you had a big boat with lots of electronics, etc, maybe MPPT would be the way to go. For the lower cost option PWM keeps my batteries happy and MPPT would be a waste of funds.

My 320w monocrystalline panel and 20A MPPT controller costs $216 total. Why spend more for PWM?

My solar setup is charging 6 GC batteries and was going into float mode by about 10 or 1030am. I realized I was essentially wasting all of that free solar power from that point on, so I bought an ice maker that I turn on at breakfast and turn off at dinner time. It makes about 20 lbs of free ice and uses a lot of that energy that would not have been harvested with the system in float mode, with the batteries still being fully charged by late afternoon.

[Adelie]

Originally Posted by socaldmax

WTF???

Unless you fill in the blanks, this is all a bunch of wishing in one hand, politicians in the other.

How big is the battery bank? How small is the solar panel? 2000AH bank and a 50w solar panel? If You're using 200A/day, you're going to spend all day in bulk mode, wishing you had bought REAL solar panels that require an MPPT controller since their Vmp is a lot higher than 15v.

In that scenario, someone spent a ton of money on the battery bank and scrimped on pennies on the solar controller. As cheap as MPPT controllers are now, I don't recommend PWMs for anyone. So what if you're going to save $50 or 60 buying a PWM vs an MPPT, your primary concern when installing solar panels on a boat is maximum efficiency for your limited space. Even if you can't get Sunpower panels, there are other brands that are more efficient in the 300w size than in the 50 or 100w size. As has already been pointed out, these need an MPPT controller due to their higher Vmp, so why even bother with PWM controllers that might be adequate under limited conditions?

Honestly, if someone called me up and said, "I want you to install 6 100w panels on my boat with a 45A PWM controller" I would say, "You are much better off with 2 300w panels each with it's own 20A MPPT controller." If he insisted, I'd say, "Have fun with that!"

A month later, the increased solar harvest of the 2 300w panel setup would be abundantly clear.

As the OP stated, if you are only using 20% of the bank capacity, there is no difference in performance. During absorption charging MPPT controllers work the same as PWM.

Also if you have less than about 200w of panels the low end efficiency advantage of PWM equals or outweighs the voltage to wattage conversion advantage of MPPT.

[Maine Sail]

At what point in the SOC curve the bank attains absorption voltage is entirely dependent upon the available current. There are many installed arrays that won't hit absorption until 95% + SOC.... In this scenario the MPPT can still be providing boost until that point....

Contrary to popular misconceptions absorption does not automatically occur at "80% SOC".. With a low current source, many marine solar arrays, it can happen as high as 98-99% SOC and with a large current source, huge alternator, absorption can be attained as low as 50% SOC...

Here are two examples of charge rate and the absorption to SOC transition point..

.2C (20% of Ah capacity in charge current)
100Ah Lifeline AGM Charged From 50% SOC Attains Absorption in 1:16:00 or at approx 77.4% SOC

.4C (40% of Ah capacity in charge current)
100Ah Lifeline AGM Charged From 50% SOC Attains Absorption in 19 minutes or at approx 63.3% SOC


.2C for a marine solar array would be massive and this is why the absorption to SOC transition point is usually considerably higher than the +/- 80% SOC folks often assume it is when charging from solar. We also can't forget vessel loads which further reduce the energy actually charging the battery. It is not uncommon for boats with solar to not attain absorption until well into the mid to upper 90's as a % SOC making the MPPT gains a much broader range then many often assume it is...

[Maine Sail]

Originally Posted by Adelie

Also if you have less than about 200w of panels the low end efficiency advantage of PWM equals or outweighs the voltage to wattage conversion advantage of MPPT.

I have done a lot of head to head PWM to MPPT testing, all the way down to 20W and under panels, and have yet to see a PWM controller beat MPPT even with some horribly slow Chinese controllers that use a P&O algorithm.

Cost wise MPPT does not always make sense, especially if you have real estate and PV wattage is cheap, but they have come down in price dramatically. Good quality PWM controllers have stayed rather stable making the price difference less drastic than 10 years ago. Generally putting money into wattage first, then the controller, is the best practice.... Some applications simply don't need MPPT and some can certainly benefit...

[hellosailor]

It is too easy to forget: An MPPT controller *is* a PWM controller.

It is a PWM controller with higher efficiency and higher cost because it has more brains built into it, with a microprocessor and logic table that are telling it how to use some extra circuitry to squeeze more power out of the daylight hours.

You are looking at a Corvette versus a Kia. Both will take you to the 7-11. If you live in the boonies of Texas and need to make that trip at very high speeds, you might need the 'vette. If you live in suburbia, the Kia (plain cheap PWM) is all you need.

Whether gaining possibly 20% from the expensive controller will be cheaper than buying solar panels, that's a whole other issue, but not part of the OP's original question, was it?

[sailorboy1]

"Cheaper" isn't really the goal of a solar installation is it?

[socaldmax]

Originally Posted by Adelie

As the OP stated, if you are only using 20% of the bank capacity, there is no difference in performance. During absorption charging MPPT controllers work the same as PWM.

Also if you have less than about 200w of panels the low end efficiency advantage of PWM equals or outweighs the voltage to wattage conversion advantage of MPPT.

Did you even read my post? You cannot make blanket statements like that, you need to specify the bank size and solar panel size. In the example I gave, the solar controller would never get to absorption mode, it would be forever stuck in bulk mode because it was horribly undersized. So NO, his assertion was wrong.

PWM isn't more efficient than MPPT. Please post testing data that proves that fallacy.

Originally Posted by hellosailor

It is too easy to forget: An MPPT controller *is* a PWM controller.

It is a PWM controller with higher efficiency and higher cost because it has more brains built into it, with a microprocessor and logic table that are telling it how to use some extra circuitry to squeeze more power out of the daylight hours.

You are looking at a Corvette versus a Kia. Both will take you to the 7-11. If you live in the boonies of Texas and need to make that trip at very high speeds, you might need the 'vette. If you live in suburbia, the Kia (plain cheap PWM) is all you need.

Whether gaining possibly 20% from the expensive controller will be cheaper than buying solar panels, that's a whole other issue, but not part of the OP's original question, was it?

I think the difference between PWM and MPPT is one of efficiency, not speed, so the comparison might be more like a Kia that gets 30 mpg vs a Prius that gets 50 mpg. Why not get the extra 20 mpg, especially if the price of the Prius is only 20% more in initial purchase price? Especially in this case where the total cost of the solar array includes the solar panels, controller, wiring, mounting brackets and possibly an arch.

So you spend hundreds on the solar panels, probably over $100 on cables, possibly thousands on the arch, yet skimp on an extra $50 for the controller. That sounds like an extreme case of penny wise and pound foolish.

[Paul Elliott]

Originally Posted by hellosailor

It is too easy to forget: An MPPT controller *is* a PWM controller.

It is a PWM controller with higher efficiency and higher cost because it has more brains built into it, with a microprocessor and logic table that are telling it how to use some extra circuitry to squeeze more power out of the daylight hours.

I'm going to disagree with your premise here: An MPPT controller is NOT a PWM controller.

PWM gives you an on/off connection between the solar panel and the battery. This on/off can be done slowly and simply with a relay (this is not usually called PWM, but it's close enough), or faster and smarter with a switching transistor. The smart ones can do a more efficient multi-stage charge, but it's still an on/off switch and it will never operate the panel at it's maximum power point.

MPPT transforms the panel voltage and current using switching regulator technology to extract the maximum possible power from the panel. Of course this can be done well, or poorly, but the concept and technique is definitely not PWM.

The methods/circuitry are completely different.

[conachair]

Just to add another angle, I'm just about to order a victron 75/15 to replace a useless PWM controller for about €50 more than the PWM cost. Not so much for the MPPT side but as the quality looks much better and the programmability much better, not locked into going to float far too early and looks like excellent data recording facilities as well.
So hard to see why to bother with PWM when there are high quality MPPT controllers available at a reasonable price.
The PWM controller actually won't let about half the amps through even below 14v.