Series or parallel for my off the grid system?

Beginnersolar

I need advice for wiring my solar panels. I'm putting together a 24v off the grid system for my RV. So far I have 5 140w 12v 8a panels, Outback 80 mppt charge controller, 4 12v agm batteries, and a 24v power inverter. I was thinking I would wire the panels in series. Would series work in my application with 5 panels? Do I need an even number of panels? Thanks for your input.

Cariboocoot

Re: Series or parallel for my off the grid system?

Welcome to the forum.

Yes, you are going to need an even number of panels. The reason being the panels are "12 Volt" and the system is "24 Volt". This means you automatically must put at least two in a series string in order to have enough Voltage to charge with.

Would wiring all five in series work? Well let's see: first, multiply the Voc, that's probably 22, by 5. Result: 110. Now, how cold does it get? Because under very cold conditions the Voc could get very close to the maximum input Voltage of the FM80.

The other thing is that the Vmp of the array would be around 87.5, which is much higher than the system Voltage which means the efficiency of the controller will be a bit less than optimum.

BTW, six of these 140 Watt panels would be an 840 Watt array, which could supply enough power to recharge roughly 270 Amp hours @ 24 Volts. So how big is the proposed battery bank?

Beginnersolar

Re: Series or parallel for my off the grid system?

Thanks for helping. I have 4 12v agm 150 Ah batteries.

Beginnersolar

Re: Series or parallel for my off the grid system?

I live in Texas and it might get down to the 30s but not for very long.

Cariboocoot

Re: Series or parallel for my off the grid system?

Okay, so you will have 300 Amp hours @ 24 Volts. Now what we'd like to see on that is 30 Amps peak charge current @ 24 Volts (720 Watts) factored for typical efficiency (77%) or a 935 Watt array.

With six of the 140 Watt panels you're looking at (840 Watts * 0.77 / 24) 27 Amps. That would be a 9% peak charge rate which is still acceptable. With only 5 panels you'd get 22 Amps, or 7% which gets a bit "iffy" because loads running will bring that down further and my compromise charging.

Since you're going to use an MPPT type charge controller you might want to consider some higher Wattage panels that would result in fewer over-all. For example four 240 Watt Trinas would give you 960 Watts and could be wired as two parallel strings of two in series. Less array Voltage than five or six of the small ones all in series so the controller efficiency would be maintained and at the same time fewer connections to be made and no need for fuses/breakers on the panels. The downside is the physical size of the larger panels may not fit in the space available.

Beginnersolar

Re: Series or parallel for my off the grid system?

I might be able to squeeze in another panel for a total of six. I just need to power a small fridge, 45w tv, and some led lights. I hope it will work for my needs. So I need to wire them in series and parallel to end up with 24v to the controller. Thanks for your help.

Cariboocoot

Re: Series or parallel for my off the grid system?

Beginnersolar wrote: »

I might be able to squeeze in another panel for a total of six. I just need to power a small fridge, 45w tv, and some led lights. I hope it will work for my needs. So I need to wire them in series and parallel to end up with 24v to the controller. Thanks for your help.

Actually with the six panels you could make two parallel strings of three.
Here's a bit about array configurations which is similar to your situation, except that in your case you need a minimum two in series to provide sufficient charge Voltage: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

Beginnersolar

Re: Series or parallel for my off the grid system?

That helps a lot. Thanks for the advice.

jcheil

Re: Series or parallel for my off the grid system?

Beginnersolar wrote: »

...I just need to power a small fridge...

You may be surprised how much power that "small" fridge will consume. There are several other discussions about these inefficient small frdges going on right now. Also, if it is a true RV fridge, which can also run on propane, a lot of them are designed so that when you run them on electricity, they are actually a heating element (instead of a compressor) that heats the element that the propane would heat, which consumes a TON of power.

ggunn

Re: Series or parallel for my off the grid system?

Beginnersolar wrote: »

I live in Texas and it might get down to the 30s but not for very long.

How long it stays cold doesn't matter. At dawn when first light hits your modules they are as cold as they got all night. Since the voltage of an array is at Voc when the inverter or charge controller is off, at that time the system voltage is Voc corrected for the lowest temp of the night, and when your inverter or CC tries to turn on, that voltage hits the input. If it is above the voltage tolerance of the device, damage can ensue that your equipment warranty will not cover. The value of Voc corrected for cold temperature is the most unforgiving number in the design of any PV system. It is best to be conservative with it.

Beginnersolar

Re: Series or parallel for my off the grid system?

Can you draw me a wiring diagram for 6 12v panels for this system?

Cariboocoot

Re: Series or parallel for my off the grid system?

It's as simple as this: two strings connected in parallel, each made up of three panels connected in series.