series or parallel configuration

squarebob

I am planning on putting 2  ~300w panels on my boat to charge a 24v battery bank of 4, 6v gc2 batteries in series. The panels I am looking at are about vmp 32.5 and about imp 9.33. I will be using an mppt type controller not yet purchased. 10awg cable from panels to mppt. In parallel connection I should have 32.5v and 18.66 amps. In series I should have 65v and 9.33 amps. Question is what would be more efficient, having the panels in series or parallel. Looking at an mppt controller with150V 20 amp input specs so it should handle either connection setup.

Photowhit

You will need them to be in series so that the MTTP charge controller can work properly, They need about 30% above the charging voltage to work properly. 24v batteries charge at around 29 volts.

MysteriousFoundation

I have a related series/parallel question.

If you wire panels in series, the current stays at a minimum. How does this affect what size MPPT controller you need? I have a feeling it doesn't help that the input is low amperage because it will be converted to high amperage going to your batteries, and the controller still has to be able to handle that.

For example, I'm looking at 3x 175 W "12 V" panels. Vmp = 19.06V. Imp = 9.18A. If I string three in series, that's 57.18V and 9.18A. In parallel, it's 19.06V and 27.54A. I'm assuming that even in series configuration, I'd need at least a 30A controller because the MPPT controller is going to convert all of that voltage into high amperage going to the batteries. The rating is not just for the input, right?

I've seen a lot of mentions that the maximum current for bulk charging should be C/5. (Which I understand to mean the C20 amp hour rating of the battery divided by 5, in amps) With a 225 Ah battery, that would be 45 amps. I can't reach that with those three panels. Even if they reached their peak in perfect sun, and the controller were 100% efficient at converting it to 14.8V it would only yield 35.5 A. But that still suggests that I need at least a 30 amp controller, and probably more like 45 amp.

Does this all sound about right?

Photowhit

Sort - of...
Charge controllers are rated on output amps, So a 30 amp MPPT type charge controller with your 3 panels would take in max about 175watts x3 panels or 525 watts, Your max output would be around 525 ÷ 14.8 charging voltage = 35 amps in a perfect world. In reality your panels will produce about 75% of their panels rating so you will be around 26 - 27 amps. Most MPPT type can be and even suggest 'over paneling' by as much as 30%.


Speaking to batteries, they vary by type, but flooded lead acid batteries should be comfortable with 13% charge of their C/20 rating. So a 225 amp battery should be fine with a 225 x .13 = 29.25 amps charging. Check with your battery manufacturer.

Estragon

To add a bit to what @Photowhit noted, one input number you do need to pay attention to is the maximum input voltage of the controller, at the maximum temperature adjusted Voc (not Vmp) of the string of panels, for the record low temp at your location.

Three nominal 12v panels in a string (Voc ~70v) should be fine on most controllers in that amp range, but something to check.

mike95490

Many inexpensive controllers are limited to 50-100V PV string input. Larger controllers can accept up to 150V string input.  A few go to 600V inputs.  Lots of spec sheets to read and compare - panels vs controllers

MysteriousFoundation

I was just looking at the MorningStar TS-45 spec, which also covers the TS-60, and I found this quote:

Ratings to 60A at 48VDC will handle solar arrays up to 4kW.

4 kilowatts? How is that possible?

https://www.marlec.co.uk/wp-content/uploads/2015/10/TriStar-Datasheet-English.pdf

At any rate, someone in the comments said they've been using it for 17 months with 600W of panels and 6x T-105 batteries. That's very close to what I had in mind, except I was only planning on 4 T-105's. Six is a heck of a lot with only 600W.

Is there any advantage running a PWM controller with your panels in series like there is with MPPT? The Voc Max is 125V, so it seems like you could do it. I'd think the higher voltage is good if your wires are long and you didn't want them to be huge.

BB.

Configuring a good quality MPPT charge controller, you can use a 77% or 75% derating of panels (Vmp depression as panels become hot on bright/sunny days, with a bit of dust on panels, and controller losses) and very easily have a nicely working system with an array of:

• 60 Amps * 59 volts batt charging voltage * 1/0.77 panel+controller deratings = 4,597 Watt array "typical cost effective maximum array"

At this level (77% deraring), only on cool/clear days around solar noon, will the controller "clip" maximum current output to 60 Amps (for a few tens of minutes.

A major reason for MPPT on large systems... >200 Watt (Vmp~30+ volts) panels are 1/2 the price vs 140 Watt (Vmp~18 volt) panels (Vmp~18 volts are the "optimum voltage" for charging a 12 volt battery bank with an PWM controller). (on the negative side, MPPT controllers are much more expensive than PWM controllers).

And there is that you can use (for example) a Vmp-array~100 VDC and much smaller diameter wire (and longer distance runs from Array to battery shed) vs trying to make an "18 volt" Vmp-array with a PWM controller charging a 12 volt battery bank.

That an MPPT controller can "get more power" from solar panels--For properly designed PWM vs MPPT systems, MPPT controllers only harvest substantially more power from solar installations with lots of cold (typically sub freezing) weather (when Vmp rises due to cold cells).

Another "nice" thing about MPPT controllers, you can run a "higher voltage" array in hot climates and fully/quickly charge the battery to higher voltages (and higher Equalizing charging voltages--When Vmp-hot have lower array voltages).

Also, high end MPPT controllers have more options (like communications/Internet) and more programmability/charging configurations than PWM (less expensive) controllers.

-Bill

Photowhit

MysteriousFoundation said:

Is there any advantage running a PWM controller with your panels in series like there is with MPPT? The Voc Max is 125V, so it seems like you could do it. I'd think the higher voltage is good if your wires are long and you didn't want them to be huge.

PWM (Pulse Width Modified) does NOT down convert DC to DC. Any voltage above the charging voltage is just lost.

So if you have panels that are 17.5 vmp and your charging voltage is 14.8, the voltage above 14.8 volts is simply moderated by Moderating a pulse to the battery. If you had higher voltage coming in you would just lose the higher voltage (if the charge controller can handle it) So a string of 3 panels producing 17.5 volts and 6 amps each, or a string of 52.5 volts and 6 amps would produce a charging current of just 6 amps.

A MPPT type charge controller does down convert DC to DC so you would get a bit more current (amps) than 3x the 6 amps.

MysteriousFoundation said:

I was just looking at the MorningStar TS-45 spec, which also covers the TS-60, and I found this quote:

Ratings to 60A at 48VDC will handle solar arrays up to 4kW.

4 kilowatts? How is that possible?

Again, Charge controllers are rated on Output amps and can handle about 30% over paneling due to the nature of how panels are rated and what they can actually produce when hot.

So at 12 volts charging, a charge controller that can handle 60 amps can handle  60 amps x 14.8 volts = 888 watts, 888 watts x 1.3 = @1150 watts.

As the voltage doubles with a system voltage of 24 volts, the same charge controller can use twice the watts at 24 volts systems charging voltage of 29.6 volts...

And double again at 48 volts!

This is one of the advantages of higher system voltages.

littleharbor2

MysteriousFoundation said:

Is there any advantage running a PWM controller with your panels in series like there is with MPPT? The Voc Max is 125V, so it seems like you could do it. I'd think the higher voltage is good if your wires are long and you didn't want them to be huge.

That max. Voc. rating pertains to a 48 volt nominal array. As explained above the battery will clamp the voltage down to slightly above the actual batteries voltage and everything above that will be lost. PWM controllers, current in = current out. MPPT controllers wattage in = wattage out.

mcgivor

MysteriousFoundation said:

I was just looking at the MorningStar TS-45 spec, which also covers the TS-60, and I found this quote:

Ratings to 60A at 48VDC will handle solar arrays up to 4kW.

4 kilowatts? How is that possible?

https://www.marlec.co.uk/wp-content/uploads/2015/10/TriStar-Datasheet-English.pdf

At any rate, someone in the comments said they've been using it for 17 months with 600W of panels and 6x T-105 batteries. That's very close to what I had in mind, except I was only planning on 4 T-105's. Six is a heck of a lot with only 600W.

Is there any advantage running a PWM controller with your panels in series like there is with MPPT? The Voc Max is 125V, so it seems like you could do it. I'd think the higher voltage is good if your wires are long and you didn't want them to be huge.

The answer lies in the Morningstar string sizing tool, just because the PWM  controller can accept higher voltage doesn't mean it will be of any advantage, in fact you will find it is not recommended. We have however derailed the OP's original questions, perhaps a new thread is in order? 

fratermus

Photowhit said:

...Charge controllers are rated on Output amps and can handle about 30% over paneling due to the nature of how panels are rated and what they can actually produce when hot.

And due to MPPT's ability to move off Vmp to clamp the panel's output.