Here are some thoughts on the PWM article from Bogart’s FAQ, with a focus on solar power for boondockers. Ralph (owner of Bogart and author of the text below) has a background in off-grid housing. He is a good engineer and is well-respected in the solar community. Below I will suggest his housing-centric observations are not completely in line with the needs of vandwellers and other mobile off-grid folks.
I’ve added emphasis in some of the quotes.
"**...Why did you choose PWM technology instead of MPPT for your SC-2030 Solar Charger**?" The “good” for PWM: It is simpler and lower cost technology.
Absolutely. I use PWM, MPPT, and shunt controllers where each is best suited.
Under some common circumstances–it can actually deliver more amps to the battery. That could be when:
(1)**days are moderate or warm**, with few clouds.
Temperature is an important factor.
MPPT’s DC-DC losses (typically 5%) come into play when temperature derating drops the voltage at which the panel makes max power (Vmp) down near the present charging setpoint. This is most apparent at absorption voltage (Vabs) since it is the highest normal charging setpoint.
The reason that PWM is generally not affected by higher temps is because the losses are happening at Vmp, which is usually much higher than the Vbatt where PWM runs the panels. You can’t lose what you never had!
Note: Even when PWM efficiency beats MPPT at Vabs, PPT would still continue to outperform PWM in Bulk and normal Float because Vfloat < Vabs. For examples, see the Float voltage sections of the tables below.
(2) batteries are charging at over 13 volts, (in a 12 battery system) **which they almost always are when actually CHARGING**.
This is the point at which the author’s experience does not line up well with boondocking. It is typical for boondockers’ lead-acid banks to be depleted to 50% Depth of Discharge (DoD) each morning before the sun rises. 12.1v - 12.2v, depending on how the LVD is set. Since PWM is hamstrung when battery voltage (Vbatt) is low it may not rise to 13v for hours under challenging conditions. This is a significant issue for boondockers.
(3) Panel voltage is properly matched to the battery voltage, for example “12V” panels are being used with a 12V system.
PWM is actually more “power efficient” than MPPT–which means less total power loss in the controller itself.
Missing in most analysis of MPPT is that there is always a conversion loss with MPPT, which tends to be higher the greater the voltage difference between battery and panels.
The DC-DC conversion losses of MPPT controllers are well known, and typically shown in the spec sheet. It’s not a big deal for most folks because it typically comes into play only in edge cases for vandwellers.
Some places that analyze MPPT assume that panels with 30V open circuit voltage are being used in a 12V system. Any good MPPT system will easily provide better performance in that case.
He’s talking about higher voltage panels, nominal 20v/24v.
I haven’t seen any analysis that compares high voltage panels on MPPT vs. PWM on 12v nominal banks. It’s not fair to PWM, which cannot access those higher voltages. If they did, you’d see claims like “75% moar power gainz from MPPT!!!” which no one is making.
They also may assume batteries are charging at 12 or even 11 volts, which is unrealistic.
What? I’ve never seen that. But now that he brings it up, it could be a factor if folks were charging non-LFP 3S lipo banks.
Lead acid batteries are typically below 13 volts only when discharging...
I think this is backwards. Lead-acid batteries are typically above 13v only when charging. Same information, different emphasis.
The benefit for MPPT becomes apparent if you use panels not voltage matched for the battery. If they are not, MPPT will utilize more of the potential energy of the panels.
Well, more benefit.
For example, if you use 24 volt panels to charge a 12 volt battery system **you must use MPPT**, otherwise you would be using your panels very inefficiently. If you are trying to use PWM in that case, you are misusing the PWM technology.
I agree that PWM is not suited for that application.
Howevah, if you have a higher voltage array and your MPPT dies, you can slap a PWM in there (assuming it’s not above the controller’s Vmax and you connect the controller to the batteries before the panels). You’ll get about half the temp-derated power you would get with MPPT. PWM are cheap enough you can carry a spare for just this purpose. Inefficiency in this case doesn’t matter as much as getting something into your banks in an emergency.
Note: remember, as always, to stay under any controller’s max voltage input.
Another potential benefit with MPPT is that if distance between panels and batteries is far, smaller wire can be utilized by running panels at higher voltage to the batteries. Running at twice the voltage reduces wire size to 1/4, which for a long run can be a significant saving in copper wire.
True, but not that important for vanfolk. Our installs tend to be compact.
There can be theoretically optimal situations (that I don’t personally experience where I live) where MPPT could give some advantage: that is when **solar current is present, but the batteries are quite low in charge**–
That scenario is a common and serious issue **for boondockers**. See below.
but because loads are high and even greater than the solar current the batteries are still discharging despite the solar current.
That’s an OR condition not an AND condition. Low Vbatt is the issue. Yes, additional loads would make it worse.
Under these conditions the voltage COULD be at 12.5 volts, or even lower. Again, using data from Kyocera panels, (“Normal Operating Conditions”) there is a theoretical maximum gain over PWM of 20% current assuming NO MPPT conversion loss
The author is referring to Kyocera KD-140 panels, poly with a Vmp of 17.7v. As we laid out the other day, poly is a good fit for pwm. This chart assumes MPPT takes 5% conversion losses, and 12.66% Vmp temp derating at 68F ambient. This is a good situation for PWM to shine.
We can ignore the voltages below 12.1, since we boondockers never run our batts that low. Right? :-)
|Vmp = 17.7||Vbattery||PWM||MPPT||MPPT advantage|
The percentages for the Renogy poly 100w (Vmp == 17.8) will be almost identical. Wattage is different because the Rens are 100w rather than 140w.
|Vmp = 17.8||Vbattery||PWM||MPPT||MPPT advantage|
If we were using the Renogy mono 100w (not recommended, but common) it’d look like this:
|Vmp = 18.9||Vbattery||PWM||MPPT||MPPT advantage|
addition you lower the temperature to below freezing at 28 degrees F (while sun is shining) you might actually get up to a THEORETICAL nearly 30% gain while the batteries are discharging.
Let’s use the original panel at 28F ambient, which increases Vmp by 3.06% so temperature rating is “reversed”. Again we are factoring in 5% MPPT losses:
|Vmp = 17.7||Vbattery||PWM||MPPT||MPPT advantage|
The only REALLY BAD part of MPPT, is all the hype surrounding it–for example one manufacturer advertises “UP TO 30% OR MORE” power harvested from you panels. If you are using solar panels properly matched to the batteries, 30% ain’t gonna happen unless it’s EXTREMELY cold.
I agree that hype (both pro- and anti-MPPT) and can lead people into poor decisions. The reality:
Daily average MPPT advantage over PWM is typically 10-15%.
MPPT advantage is greatest when the delta between Vbatt and Vmp is largest. This can happen when
battery voltage is low
ambient temperatures are low, increasing Vmp.
using higher Vmp panels like mono
MPPT advantage is least or even negative when the delta between Vbatt and Vmp is smallest
battery voltage is high, like during Absorption (particularly if set high like 14.7-14.8v)
using lower Vmp panels like poly and especially amorphous/thin-film panels
ambient temperatures are high, depressing Vmp.
30% advantage is indeed possible at normal temps – it happens in the “trough” when batteries are deeply discharged. Hopefully that condition doesn’t last long, but it’s those desperate situations where PWM is mired and struggles to pull itself out.
And your batteries have to be abnormally low in charging voltage–which tends not to happen when it’s cold (unless you assume the battery is still discharging while solar is happening).
Happens to boondockers on most mornings.
Virtually all the analyses I’ve seen touting MPPT on the Internet ignore the conversion loss, assume really cold temperatures, assume unreasonably low charging voltages, assume no voltage drop in the wires from panels to batteries, use STC conditions for the panels (that the marketing types prefer) rather than more realistic NOCT conditions, and in some cases assume panels not voltage matched to the batteries.
See the tables above where they are not ignored. Except the cold temps one I threw in since you mentioned it.
The other thing that is misleading about MPPT, is that some manufacturers make meters that show **both the solar current and the battery current**. In almost all cases for a well designed MPPT type **the battery current will be greater**.
Right, due to the DC-DC conversion.
The engineers making these know better, but it is implied (by marketing types?) that if you were NOT using MPPT you would be charging your batteries with only the SOLAR current that you read on their meters.
I have never heard anyone make that implication (or inference). The difference between solar current and battery current is one of the defining features of MPPT. It would be weird if that information was hidden from consumers.
That’s not true, because the PWM BATTERY current should always be higher than the MPPT SOLAR current. It is the nature of the MPPT that maximum power occurs when the current is lower than the maximum, so they must operate there to get the maximum power.
This is a weird thing to say. Current out to the battery/loads is apples-apples, and MPPT will, on average, put 10-15% more. There is no conspiracy with MPPT displays to make PWM look bad.
So to properly compare the two you need to compare MPPT with an actual PWM controller in the same circumstances.
See the tables above.
Finally, the reason we went to PWM is that I was anticipating that panel prices were going to drop (which they certainly have over the last 5-10 years!)
True! And it is often cheaper to add panels than “upgrade” to MPPT, especially on a house or ground-based install where space is relatively unconstrained.
and that the small advantage of MPPT (under conditions where the correct panels are used for the batteries) would not justify their additional cost and complexity. So my thinking, for more total benefit per $, put your money in an extra panel rather than a more expensive and complex technology.
Horses for courses. Boondockers with roofspace limitations are more likely to get the full benefits of MPPT. Off-grid cabins with copious room for large banks and panel arrays would likely get more W/$ out of PWM.
I express it this way on the RVwiki:
**PWM is the default choice** in most situations because they get the job done and are inexpensive. PWM controllers can cost half or a third of their MPPT workmates for any given rated output.[16)](http://rvwiki.mousetrap.net/doku.php?id=electrical:solar:charge_controller&s=controllers#fn__16) If more power is needed (and there is physical space) additional PV can be added to match the charging output of an MPPT charger, often at a lower cost. _[There are no prizes for fanciest or most expensive charge controller! Do what is best for you – frater secessus]_ MPPT controllers also tend to consume more power to run themselves than PWM models due to additional processing and electronic components.