Charge Controller Sizing / Panel / Array wiring

tr0y · February 2018

Hi All -

I am new to the 12V side of solar for my RV, cant stand the generator. So what I have so far is four (4) 275w panels ( 38.6 Voc, 31.50 Vmp, 8.74 Ipm), a 2000w Inverter charger, and three (3) 150ah / 12V AGM telecom batteries ( I have the ability to get more should I want) and of course all of the necessary wiring etc ... My last piece of the puzzle is a charge controller and how to wire up the panels.

I have been looking towards the Midnite Solar charge controllers as I think @ 12v I'm pushing into high wattage territory. I have outback controllers in the house and like them but am thinking the Midnite Solar may fit this use case better.

Are there any charge controllers I am overlooking ?

Does anyone have any thoughts ? Insight would be greatly appreciated.

Estragon · February 2018

I have my 12v system (4x250w panels) wired as 2 series strings of 2 into a classic 150 which works well.

Photowhit · February 2018

At 4 x 275watt panels, you seam to be close to maxing out any charge controller at 12 volts, the math may seem odd.

1100 watts of panels will only put out about 75% of their panel rating in normal every day conditions. If you look for a spec sheet .pdf for your panels look for the Normal Operating Cell Temperature Values, NOCT values. These will be roughly 75% of the name plate value. Also even if you allow your battery to go down to 50% state of charge, or about 12 volts, you need a voltage about 10% higher than that to effectively charge the battery bank.

So the math looks more like 1100 watts x 75% to arrive at NOCT value = 825 watts / 13 volts = @63.5 amps

While I like the Midnite classic and use them at home, they do have internal fans to cool the electronics. For a Camper, I'd suggest a Morning Star TS 60 MPPT. It has no internal fan and used large cooling fins instead. You might be slightly over paneled for it, but that is not bad at all.

https://www.solar-electric.com/motr60ampmps.html

SupraLance · February 2018

Don't forget that you can use two charge controllers, with two panels on each, into the same battery bank. In addition to saving you money in some cases, this also allows you to tilt the panels differently or better account for partial shading.

myocardia · February 2018

SupraLance said:

Don't forget that you can use two charge controllers, with two panels on each, into the same battery bank. In addition to saving you money in some cases, this also allows you to tilt the panels differently or better account for partial shading.

Remember that this not possible with most MPPT controllers. It's possible with as many PWM as you want to use together, or one MPPT plus as many PWM as you want, but only the best MPPT controllers will let you use two of them together, as long as you put them in a master-slave configuration. The Midnite controllers, the better Outback controllers, and a few of the Schneider controllers are the only MPPTs, as far as I know. I may very well have forgotten a manufacturer, though. I'm a wee bit forgetful in my old age!

edit: I'm going to go make sure, but I may have forgotten some of the better/more expensive Morningstars.

mcgivor · February 2018

myocardia said:

SupraLance said:

Don't forget that you can use two charge controllers, with two panels on each, into the same battery bank. In addition to saving you money in some cases, this also allows you to tilt the panels differently or better account for partial shading.

Remember that this not possible with most MPPT controllers. It's possible with as many PWM as you want to use together, or one MPPT plus as many PWM as you want, but only the best MPPT controllers will let you use two of them together, as long as you put them in a master-slave configuration. The Midnite controllers, the better Outback controllers, and a few of the Schneider controllers are the only MPPTs, as far as I know. I may very well have forgotten a manufacturer, though. I'm a wee bit forgetful in my old age!

edit: I'm going to go make sure, but I may have forgotten some of the better/more expensive Morningstars.

Two MPPT controllers cannot share the same array, but can feed into a common battery as long as the settings are the same and wired/fused independently to prevent conflict. Even two controllers from different manufacturers would work, again settings the same, or very close. Some systems as you say can communicate which is a good feature but not absolutely necessary.

https://www.morningstarcorp.com/parallel-charging-using-multiple-controllers-separate-pv-arrays/

Estragon · February 2018

I have two classic mppts and a morningstar pwm on 48v bank. The classics are master/slave, but the MS isn't. Each controller has its own array. Works fine.

mike95490 · February 2018

As long as each CC (PWM or MPPT) has it's own PV array, you can several feed a battery. SOMETIMES a noisy PWM can disturb a MPPT sensing and it does not charge well, but thats a case by case basis. Most good PWM have a Low Noise selector switch

tr0y · March 2018

Hi All -

Thanks for the input. I ended up going with the Midnite Solar 150 charge controller, thanks NAWS for great service, it seems to handle everything I need and actually then some. Should I want another panel, overkill yes but with super cheap panels and a big roof why not.

Next is thoughts on panel wiring, thoughts anyone ? I was thinking series, is there any reason, maybe redundancy, I should not just wire all four panels in one string ?

mike95490 · March 2018

simulate your proposed configuration in the midnight sizer

http://www.midnitesolar.com/sizingTool/displaySizing.php

See if 4 panels will over-volt and void your warranty

tr0y · March 2018

Thanks Mike, according to Midnite and NAWS the Classic 150 will deal with 1400W @12v so I have a decent amount of headroom on the controller. Thoughts on panel configuration ?

Photowhit · March 2018

tr0y said:

Thanks Mike, according to Midnite and NAWS the Classic 150 will deal with 1400W @12v so I have a decent amount of headroom on the controller. Thoughts on panel configuration ?

That is the wattage it will handle, in series the voltage adds, in a 12 volt system the Classic 150 has a max incoming working voltage of 150 volts. It has a max incoming voltage until the voltage can damage the controller of 162, in a 12 volt setup.

Generally the higher the incoming voltage the harder the controller must work. Usually the sweet spot is about 2x the system voltage. If you have more than 2 strings you would normally want a combiner box. I would do 4 panels as 2 sets of 2 in series, and use branch connectors to combine them on the roof and just bring 2 wires to the charge controller.

mike95490 · March 2018

tr0y said:

.......
Next is thoughts on panel wiring, thoughts anyone ? I was thinking series, is there any reason, maybe redundancy, I should not just wire all four panels in one string ?

1) 4 panels in series, puts your Voc @ 154.4V @ 70F. Not good.

2) more than 2 or 3x your battery voltage, and you dissipate a lot of heat in the controller

If you want to use 4 panels in your system, wired 2s2p would be the best

I have so far is four (4) 275w panels ( 38.6 Voc, 31.50 Vmp, 8.74 Ipm)

1100watts / 12V = 91A, right at the limit! but hopefully you won't be hitting that very often, but you do need to use some really heavy wire between the controller and the batteries. Find out what the controller terminals will accept (it was not real good in the early versions, they may have beefed it up some now.).

Estragon · March 2018

> @tr0y said:

> Next is thoughts on panel wiring, thoughts anyone ? I was thinking series, is there any reason, maybe redundancy, I should not just wire all four panels in one string ?

Two reasons:

- As mentioned, all in one string could be pushing voltage limitations in the controller.

- Especially in a mobile application, shade could be an issue. With a series string, even minor shading (like a nearby overhead wire) can severely limit output for the entire string. In parallel, only the shaded panels (string) is affected. Also, in a mobile application, distances from pv to controller tend to be short, so the cost of using heavier wire to carry higher currents in parallel tends to be low.

tr0y · March 2018

@mike95490 , maybe its not enough coffee yet or I am just slow please explain 2s2p and what that looks like. Thanks in advance for my ignorance.

BB. · March 2018

Two panels in series. And those 2 strings connected in parallel.

Gives you a solar array with:

2xVmp
2xImp

Bill

scrubjaysnest · March 2018

I run my panels in parallel because of shading when camping. Out west series is fine.

ScoobyMike · March 2018

mike95490 said:

Find out what the controller terminals will accept (it was not real good in the early versions, they may have beefed it up some now.).

I just downloaded read the Classic manual, they now can take up to 4 AWG.

mike95490 · March 2018

only 4 ga, that's about 80A wire for a 90+ amp controller. I was hoping that had been improved

mcgivor · March 2018

Sometimes the maximum size of the wiring is determined by the volume of the wiring compartment rather than the actual capacity of the terminal, in order to comply with the electrical code regarding wire fill, Morningstar is very generous, Schneider very tight, conductor bend radius may also play a part, I've no idea what Midnight controllers are like, but perhaps this is a factor. Can anyone comment?

Estragon · March 2018

Midnite is okay for space in the wiring compartment for pv in and battery out (a bit less than my morningstar pwms) with knockouts located so tight radius turns inside are avoidable. My classics (c.2013) terminals are definitely max 4ga (4ga solid, stranded 4ga is really tight). I only use short lengths of it to DC buss bars.

Photowhit · March 2018

I have 4 gauge and a tight bend and even the older/original type which has a fixed cover. The newer ones pop off to make installation easier. It was tight but manageable. I believe 4 gauge is rated for 95 amps (THHN) in conduit, up to 90°c.

Most people will have very short runs out of the charge controller to a breaker, then a short run to a bus bar attached to a heavier cable headed to their battery bank.

One of the reasons I like to suggest Morningstar for RV's is they don't use fans for cooling.

Charge Controller Sizing / Panel / Array wiring

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