Solar 12 volt panels, is it better to wire two 100 watt panels in series or in parallel? (1 Viewer)

[Fisher23]

I have room for 2 solar 100 watt 10 amp panels on my roof rack.

Is it better to wire two 100 watt panels in series or in parallel?

I am debating hooking these up to a redarc dc charger .. or getting a mppt solar controller, which would be better?

Powering my fridge/freezer while stopped camping in the heat.

 

[Dharma Dude]

Depends. In series, if one panel is shaded, power output will drop more. OTOH, in series, the voltage output is higher, current is lower which might be better for some inverters. In parallel, you get more shade resistance but panel's output voltage in lower light conditions might be below the operating point of your inverter.

 

[e9999]

aside from shading issues, which are indeed critical for some (not all) panels -check the bypass diodes configuration for ex to find out-, some points to consider are what the max voltage and current capabilities of your controller are (likely not a big issue if those are "12V" panels as I assume they would be) -but worth thinking about for later flexibility- and the size of the wiring if need to handle higher current in series (you may need bigger wires, fuses etc). Of course, you'll want to make sure that the controller can handle the 200W. For example, the Victron 75/15 which has been discussed a lot here, will be able to handle 75V in, fine for 2 (or 3) "12V" panels, but is limited to 15A output so about 220W with one 12V battery, so you would be close to maxing out the controller with 200W and would not be able to add usefully another panel later on with that controller and the one battery.

I don't know what a Redarc DC charger does, but I got an MPPT controller, not a PWM. More energy that way, but debatable whether that is worth the extra cost, especially since 200W on your roof is a lot for typical camping needs so you likely won't be hurting for power in the summer. (Are you sure you need 200W?) But also more forgiving (I think) of incoming voltage fluctuations due to insolation changes, shading etc.

There are also other issues relating to the controller, like the charging schemes (various phases), ability to set up custom settings, deal with various battery types, etc. The means of programmability of the controller are also important, like do you need a computer or a phone, etc?

The good thing is that if you get a reasonable charge controller and good wires you can go easily go from series to parallel and find out for yourself what works best for you. It's not like you are locked in forever in one mode. Very easy to do (and interesting for us nerdy types).

 

[Fisher23]

The main advantage I see in the redarc DC (direct current) is that it allows to to isolate and recharge you batteries at 25, 40 or 50 amps depending on which module you have. So with an isolated lithium battery you can recharge in a fraction of a time vs. 6 hours it would take just hooked to an alternator battery parallel system. Down side is the crazy high cost. When primarily camping just a short trip is sufficient for another day's of power, even without the solar.

I have an ARB 63 and in a hot vehicle it sucks amps.

I didn't even think of minimum voltage in shade or cloudy conditions so thanks for that Dharma!

Thanks e9999 for pointing out that best to try both. This way I can put the connectors where I can switch if needed or is some panel gets broken off road I can deal with it easily.

 

[e9999]

^ well, if you have 200W of panel, there is no way you are going to get more than about 14A or so for a 12V battery charging at 14V, regardless of what gizmo you buy. Even the really expensive ones. Just not possible. Conservation of energy and all that, unfortunately.

 

[Fisher23]

The redarc DC runs off the vehicle alternator .. putting 25, 40 or 50 amps directly to the isolated battery only when the vehicle is running. It also works as a solar controller putting what ever you get from the panels to the isolated batteries when not running the engine.

My panels I was going to flat mount so I'm not sure exactly what watts I'll get until I mount them. but I was thinking maybe 60%, since they will not be directly facing the sun. so maybe 120 watts with both? Maybe 10 amps is what I was thinking.

 

[e9999]

Sure, if you use the alternator as another source of power you can get more charging current than what you'd get with the panel alone. If the battery allows it, of course. But the whole point of sizing the solar panels correctly is to not have to use the alternator at all or very rarely, I would think. Otherwise, no need to get a solar panel at all, just use the alternator. And 15A is a pretty good charging current already.
It is very true that if you have 200W nominal, you will get that much only on very rare occasions (midday, and summer, clear sky, orientation just right, cold, etc). But your fridge probably only needs 5A or so, and only intermittently at that. So if you'd get 5A average (or 70W average) over the course of the day, you should get plenty of power to run your fridge.
Remember that if you have a self-contained solar and battery system, you can always tap off the main battery/alternator with just 2 wires in an emergency for a bit of charging.

 

[OSS]

Definitely wire parallel (with 10AWG duplex marine grade wire) to have two independent redundant systems.
That's what I did for 25 years. I had two 75 watt panels I mounted flat on my roof rack when at a camp spot.
While 150 watts was overkill for my needs (even when mounted flat) 75 watts wasn't enough when mounted flat.

A single 100 watt panel when mounted flat on a roof rack (no sun tracking) will barely be able to keep a high performance refrigerator cold day after day in the heat. And that's during perfectly clear sunny days.
200 watts will be more than enough for anything when mounted flat (no tracking).

 

[Fisher23]

would I be completely out of my mind to think about a crate diesel motor straight from Cummins in situations like these?? I know may not be a direct fit, but what if it could be done?? like I said, i may be outta my mind.......

Definitely wire parallel (with 10AWG duplex marine grade wire) to have two independent redundant systems.
That's what I did for 25 years. I had two 75 watt panels I mounted flat on my roof rack when at a camp spot.
While 150 watts was overkill for my needs (even when mounted flat) 75 watts wasn't enough when mounted flat.

A single 100 watt panel when mounted flat on a roof rack (no sun tracking) will barely be able to keep a high performance refrigerator cold day after day in the heat. And that's during perfectly clear sunny days.
200 watts will be more than enough for anything when mounted flat (no tracking).

Thanks !!

 

[e9999]

One more aspect to this: if you wire in series you will get fewer ohmic losses in the cables cuz of the higher voltage. So more power at the battery. Not critical for short distances or if you have excess potential power, of course. And you also won't need some fancy Y-connectors or soldering shenanigans, just a regular MC4 extension cable.

But all those subleties pale in comparison to shading effects. It is astonishing how even supposedly high-quality panels will succomb to shading effects. For many, it takes only a very small fraction of the panel being shaded to basically bring the panel to little or no power production.

So, I would say that an essential for a trip in a hot climate is a long extension cable so you can have your panels in the brutal sunlight out there while you are able to keep the truck in the shade, if any. You don't want to be in a situation where you have to choose between the truck being too hot or having no power, if you have a choice. Of course, if your panels are fixed on the roof, well, you will have to keep the truck in the sun. (Which will make the fridge hotter unfortunately...)

 

[Fisher23]

I ordered 2 of these, as they came in black.

Most of the time we're camping at the beach or water front, with no trees. Shade is our Awning.


Amazon product ASIN B07Q6L8HQ9

 

[e9999]

well, no shading issue then if you watch your other roof rack loads.
It might be interesting to see if you could maintain the fridge with only one panel. Let us know if you try that.
Enjoy the cool brews!

 

[PeterTheBeater]

Conventionally solar panel are wired in series.
Source: built off grid portable solar arrays

 

[e9999]

OP, you probably know this already, but if you put the 12V panels in series you need a MPPT controller, not a PWM, if your battery is 12V as well. The PWM will not use something like half of the power available.

 

[PeterTheBeater]

OP, you probably know this already, but if you put 12V panels in series you need a MPPT controller, not a PWM, if your battery is 12V as well. The PWM will not use something like half of the power available.

most PWM chargers can handle 12-24V, my 10$ renology has an option for choose batteries bank voltage. 12V solar panel don’t actually run at 12V, they can be anywhere from 0 to like 25 or something

 

[Fisher23]

well, no shading issue then if you watch your other roof rack loads.
It might be interesting to see if you could maintain the fridge with only one panel. Let us know if you try that.
Enjoy the cool brews!

Installed the one panel and it's maintaining the fridge. Says it's running at 63% of the panel and maintaining.

Has been running it for a week now.

 

[george_tlc]

most PWM chargers can handle 12-24V, my 10$ renology has an option for choose batteries bank voltage. 12V solar panel don’t actually run at 12V, they can be anywhere from 0 to like 25 or something

A typical '12V' solar panel will typically be around 18V open circuit. This provide some headroom for wiring losses and is intended to have its maximum power point around 14 to 15V which ideal for a dumb PWM charger. Putting a second such panel in series is a waste of time since you do not get double the current or power with a PWM charger. In fact you would hardly see any difference in this case using a single panel or two in series. The reason the PWM charger would be spec'd for 24V is to be used with a 24V battery configuration and then it would make sense to series connect to panels.

To use a PWM charger in the 12V case, you would need to put the two panels in parallel, as long as the maximum current of the two panels in parallel is within the capability of the PWM charger to handle.

With an MPPT charger, there is a DC : DC converter as part of the tracking hardware and it will convert the higher voltage of the two series panels down to the nominal 14V for charging the battery. The input side will track the maximum power point and appropriately draw current as needed to achieve maximum power transfer (while charging).

cheers,
george.

 

[Fisher23]

I am using an MPPT controller. The single panel said it was putting out 13.x volts.

Was also thinking if one panel got broken (off roading or hurricane ) if they are in parallel be easy to unplug one vs rewire.

I used the renolgy mppt with the blue tooth since I had renolgy panels.

When I add the inverter I'll likely be adding the 2nd panel provided I can get it to fit nice.

 

[george_tlc]

^ 13.x volts is because the MPPT controller is loading the panel down. You can google for something like 'solar panel voltage vs current' and you'll see the typical V versus I curve and the product of the two curves gives you the Power curve. What you'll see is that the power curve has a very distinct peak and that is the panel's maximum power point and that is what the MPPT controller tries to keep the panel running at (while charging at full bore).

cheers,
george.

 

[e9999]

F, the specs for the panel should tell you at what voltage the maximum power point lies but that is only for the nominal 1,000 W/m2 insolation. At lower insolations it will move towards lower voltage, but only a little bit though.
Normally, 13V would seem pretty low to me actually for MPP for a modern panel if you are seriously charging. Maybe that was when the battery was already fully charged? From memory, my Renogy Eclipse "12V" mono is about 21 or 22V open circuit and 18V MPP at nominal insolation. And I remember my Victron keeping it right around 18V indeed when it was sunny. (And in fact, it better be up there somewhat cuz my controller won't even start working if the panel doesn't give at least 5V over the battery or about 17+V typically. Fortunately, hardly any insolation at all will bring the panel up to close to nominal Voc so that's not exactly a daunting proposition. ) But anyway your specs will tell (mostly) all...