MPPT Controllers

[obiwancanoli]

As I continue the learning curve, I'm finding the answers I get always lead to 2 more questions... funny how that is...

I've been working backwards in developing my solar needs, going from panels to mounting to wiring, then controller, and that's where I'm at now... my research shows many different types and brands, some with meters and displays, others with LED's... I'll need to upgrade the 30A PWM to 40+A. I'm thinking of adding another 2 panels 340W, 9.1A ea.

For efficiency and convenience' sake, I like having a display that's easy to use, and thorough in it's functions.

What's your experience? Recommendations? Tips?

Thanks!
Michael

[time2roll]

CA Traveler wrote:
What do over amped PWM controllers do? ie Instead of +25% oversized they are say -15% ? Zap = smoke?

Depends on the controller. Some have protection, others do not. Read the installation manual carefully not just the sales sheet.

[ramsaymike]

With respect to your original question on data display I would like to offer an alternative approach. Rather than purchase an off the shelf solution - usually quite expensive - I suggest you just make your own! The basics - say a shunt on the batteries and a MODBUS data connection into your solar charge controller are available at very low cost (<$50) and offer essentially unlimited capabilities all with no cost tools. I'm not saying it will be easy but it will be cheap and you will end up really knowing what is going on with your system. If you are at all technically inclined I'm sure it is achievable. Something to consider...

[Lwiddis]

“what is the difference between 12V and 24V panels, and why should it matter? ”

Oh boy.

[pianotuna]

StirCrazy wrote:
take the sizing rules with a grain of salt.

Panel sizing for lead acid jars based on acceptance rates at 85% state of charge is about 12.5 amps per 100 amp-hours of storage capacity, or about 150 watts. Since there is a need to equalize the bare minimum is about 60 watts of panel per 100 amp-hours.

It is a bit better with AGM--90% SOC. Equalizing not recommended but "recovery" charging is sometimes needed. I would not design a system for recovery charging--but would do that "deed" on shore power.

With the extremely low impedance of Li chemistries there is not much need to limit sizing.

Li are crazy in that lifespan is reduced if you take them to 100% soc--better to just take them to 90%. They do sometimes need to be "woken up" with high rate charging. Again I would do that on shore power.

SiO2 also need a 'wake up' charge from time to time.

[BFL13]

CA Traveler wrote:
What do over amped PWM controllers do? ie Instead of +25% oversized they are say -15% ? Zap = smoke?

They do the over-amps and get warmer and warmer until some component "fries". They get pretty warm at rated amps, but can handle that with good ventilation (same as MPPT there) per their manuals re installation.

Usual advice is to ensure staying under rated amps when your array might be over-performing such as in colder temps by leaving a margin compared with normal temps.

The percentages are confusing as often used.

Say the panel can do 20 amps max. The controller would be 20 x 1.25 by one rule. = 25 amps. That 5 amps over is 1/4 of the 20 but 1/5 of the 25. So the controller is 25% above panel, but the panel is 20% less than the controller.

This is where the other usual rule of 80% for electric loads gets mixed up in all this, where now you want the load to be 1/5 smaller than the supply. But you want the wires for that to be 1.25 more but the fuse for that wire to be under that so the wire is not the fuse. So what does that make the fuse size wrt the load? And so forth. Only the NEC knows for sure! 🙂

[pianotuna]

CA Traveler wrote:
What do over amped PWM controllers do? ie Instead of +25% oversized they are say -15% ? Zap = smoke?

You might let out the magic blue smoke. Efficiency will suffer. My Blue Sky, run at 17 volts input is 98%. At 33 volts it is 94 or 95%.

When brand new, panels often "out perform" their specifications.

I did get 23 amps from 256 watts of fixed flat panels, location was Rugby North Dakota about June 15. Temperature outside was 97 f The panels were less than one month old. "book" value should be about 15.5 amps and I routinely harvest 17 amps *if* the battery bank is hungry, or if I present a large load to the inverter.

17 amps is not bad for panels from 2005, especially in a flat fixed installation.

[CA_Traveler]

What do over amped PWM controllers do? ie Instead of +25% oversized they are say -15% ? Zap = smoke?

[pianotuna]

obiwancanoli wrote:
OK, this introduced another tangent to the process... what is the difference between 12V and 24V panels, and why should it matter?

The difference is voltage and size. 12 volt panels are made in lower volumes than higher voltage ones. 12 volt panels pretty much always cost more per watt.

Nominally 12 volt panels are in a range from 16 to 18 volts, with most at bout 17. Mine are 16.5.

I deliberately went with series/parallel to have a nominal input voltage of 33. The controller is a Blue Sky 3024 di. The amperage is derated at the higher voltage. It was, at 33 volts input pretty much "on the money" for 125% of panel output.

The higher the input voltage, the less efficient the MPPT controller is.

If I rewired to all parallel I might be able to do one extra panel.

At the time of purchase, Blue Sky had a 50 amp controller. It was bunches more expensive--but I rue the day that I decided on the 30 amp. They no longer make the 50 amp unit.

If I were starting from scratch, I'd move to a 48 volt battery bank.

[3_tons]

As with a RV battery, the term 12v is simply shorthand jargon...Due to a series of voltage drops along the way (connectors, controller, switches, fuses, wire resistance, heat conversion) and battery chemical resistance, it takes a minimum of about 18v useful at the panel output to be able to charge a battery at roughly 14.4v, thus the term ‘12v panel’..(at approx 0.5v per each cell...0.5 x 36 = 18v to controller thingy)... .You’ll likely still get a marginal gain with MPPT, but higher voltage (more cells) equal an even better MPPT enhancement, and can help overcome any existing marginal size wire runs.

3 tons

[pianotuna]

BFL13 wrote:
"Sizing should be 125% of the maximum harvest. So for a 20 amp harvest--the controller needs to be 25 amps. The reason for this is partly "cloud" effect. Essentially the clouds may act as a lens and temporarily increase solar irradiance"

This applies to PWM controllers, which will heat up and eventually fry when doing more than their rated amps for extended times. Brief cloud effect overages should be harmless though.

MPPT controllers can be over-panelled somewhat, as recommended by Morningstar for greater daily AH haul --see their blurb on that. That is because unlike PWMs, MPPTs "clip their amps" to their amp ratings.

It applies to my MPPT controller. In black and white in the owners manual. Over paneling is more about flat fixed installations, than controller type.

[CA_Traveler]

At one time 12V panels were the big game in town. But with the nation wide ramp up in solar the physically larger with more watts now cost less. 12V panels in parallel can use the cheaper PWM controller. 24V panels require a MPPT controller for a 12V battery.

Many MPPT controllers work with 12, 24 or 48V batteries. Of course the panel Vmp has to be adequate. My 3x serial panels run 90V+ for my 12V battery.

There are many factors and one size doesn't fit all. From my prespective some RVers prefer panels mounted close to the roof and often near the evil power robbing power of A/C shadows. But just add more panels.

A recent Phoenix ad had 295W 60 cell panels for $100 and cosmetic blemish 250W 60 cell for $37.

[red31]

obiwancanoli wrote:
OK, this introduced another tangent to the process... what is the difference between 12V and 24V panels, and why should it matter?

12v are usually 36 cell (~.5v/cell or 18v Imp) less than 200w
'24' can be 60 or 72 cell, usually >200w

For a '12v' batt bank, '24v' needs mppt.

You can consider 24v as 2 x 12v in series although not a good analogy when it comes to 60 cell!

[obiwancanoli]

OK, this introduced another tangent to the process... what is the difference between 12V and 24V panels, and why should it matter?

[CA_Traveler]

12V panels have a use. But not for me as the panels are much more expensive than 24V panels per watt and take more space per watt. Even in 2014 my 750W MPPT system was only $100 more with less space and less installation than the equivalent 12V panel system.