Solar Array Wiring and Requirements

Re: Solar Array Wiring and Requirements

quique wrote: »

1) My dad has 2 solar panels, 280W panels of 36V & 7.7Amps. A friend told me it was ok to use a 36V solar panel to run through a charge controller and charge a 12V battery bank. I have another 12V-30A CC, but im afraid 36V solar panel might FRY my 12V charge controller, because its a 12V CC which would be getting charge from a 36V array. Would this be the case?

It depends on the charge controller... You need to check the input specifications.

If it is a PWM controller, you are wasting a tremendous amount of power from the panels. You are probably in the efficiency range of:

• 14.4 volts battery charger / 36 volts panel = 0.4 or 40%

If this is a properly rated MPPT type charge controller (more expensive type charge controller)--then you are just fine.

2) Another option is that I have 2 extra 210W panels, same as the 2 currently in use. They are 12V & 17.5A solar panels, in parallel running thru a 12V-30A CController. If i connect my 2 new 210W panels in parallel, i get 12V & 35A again, like before. I have the extra CC fo 12-30A which should work fine as the existing setup. But how would i take current from the new CC to the existing battery bank? Currently my wiring is 4 pairs of batteries for a 12V set. The terminals from the CC go to the 1st and 3rd pair of batteries and the terminals from the inverter are on the same 1st and 3rd set. Would i want to wire the new terminals from the new CC to the 2nd and 4th pairs of batteries?

Just connect the 2nd charge controller with relatively heavy wiring directly to the battery bank at the same point as the first charge controller (in parallel). Each controller should have its own properly rated fuse/breaker to protect against shorts.

When you parallel connect batteries--It is important that they all have the same length/gauge wiring to the +/- bus or common connections so they properly share charge/discharge current.

Battery Bank Wiring

3) This all seems complicated, 2 charge controllers. Id like to have a larger charge controller. However as much as i can find online is 60A. Why is it so uncommon to have larger CC? What if eventually i want to have like 12 solar panels to run my complete house and stuff!? I would need 1 CC per 2-solar panels, id need 6! That just seems inefficient. Furthermore, the battery bank will still be 12V. My inverter is rated at 3kW and 15A cont-30A startup. If i am using 2 CC @ 30A each to load my batteries, thats separate from the amperage rating of my inverter right? My inverter amperage rating just means how much Amps it can put out, regardless of my battery bank and CC, right? So i guess i would also need to get a larger inverter and again i find 3kW and maybe some 5kW inverters max! So once more, if i have 12 solar panels and 6 CC, would i need 6 inverters as well?

It does get complicated and expensive to build out a solar RE power system... Somethings that help:
As your power needs (and charging currents) get larger, it is usually better to go with a higher voltage battery bank... Look at 24 or 48 vdc bank. This reduces the current to 1/2 or 1/4 that of a 12 volt system.

Charge controllers (typically larger MPPT type) are rated in amps out... At higher voltages, the charge controllers can manage 2x or 4x the panels at higher voltages:

• 15 volts * 60 amps = 900 watts of panels
• 30 volts * 60 amps = 1,800 watts of panels
• 60 volts * 60 amps = 3,600 watts of panels

Modern MPPT type charge controllers are a bit on the complex side--so you have to read the specifications in detail to insure that you connect them correctly.

In general, you don't want to pull much more than 1,200 watts or so from 12 volt system (inverters, direct DC loads, or charge controllers)... Or more than ~2,400 watts from a 24 volt system. If you go larger, the currents become very large--requiring lots of copper for wiring and very large/expensive fuses/breakers. For example, at 5,000 watts:

• 5,000 watts * 1/10.5 volts cutoff * 1/0.80 inv efficiency * 1.25 NEC safety factor = 744 amp circuit minimum

Plus issues of voltage drop and wiring losses make it very difficult to send 12 volts very far vs a similar 48 volt system.

Because of these, and other factors, it is actually quite difficult to "grow" an off-grid solar RE power system. Many of the components have to be swapped out as you increase system size/rating. Higher voltage battery bank, higher voltage inverters (and sometimes new charge controllers and even solar panels to meet the new voltage requirements).

-Bill

Re: Solar Array Wiring and Requirements

quique wrote: »

2) If the largest CC is 80A, than it means that when wiring more than 2 panels, you always need to use more CC?

Realistically, you could justify on a 12 volt battery bank and 80 amp controller:

• 14.4 volts * 80 amps * 1/0.77 typical derating = 1,496 watts aprox. of solar panels

So, for 210 watt panels:

• 1,496 watts / 210 watts of panels = 7 panels in series or series / parallel

You can put two or possibly three panels in series, then stack them up in parallel and support 6 panels cost efficiently with an 80 amp controller on a 12 volt battery bank. (the 77% derating accounts for typical panel and charge controller losses in real life--In cool, very sunny weather, you might do better--But those are few and far between and the MPPT charge controller will simply limit the maximum current to 80 amps even if the panels can provide more power).

3) I have 8 batteries of 6V each, how do you get 900Amps from that? The battery bank as read by the CC is always at around 13.4 when not used and its midday or midafternoon.

I think he is reffering to total Amp*Hour storage capacity of the battery bank. I am not sure where he got 900 AH (from another post/thread of yours?). For a 8x 6volt @ 200 Amp battery--Your bank capacity would be 800 AH (series connections add voltage, parallel connections add current).

4) ...I calculated battery capacity at 200Amps x 6V = 1200W x 8 = 9600Watts, but the inverter lets me use 50% = 4500W.

Energy is in Watt*Hours... So, if you have 8x 6v @ 200 AH batteries setup for 12 volts, then:

• 800 AH * 12 volts = 9,600 Watt*Hours of bank capacity

Normally, you don't want to go much below 50% state of charge (for longer battery life)--so that leaves you with 4,500 WH of useful energy storage.

The inverter may, or may not, be able to accurately warn/turn off at 50% battery capacity. You really should measure the specific gravity of your battery bank (after a few hours of rest) and see what the actual state of charge is... If you do this wrong, you can quickly kill your expensive battery bank.

Normally i run 3 ceiling fans (100W/e), maybe 4 lights (25W/e), 2 tvs (100W/e) which is 600W per hour. This makes my battery bank last approx 8 hours which is right! Then during the day it recharges because currently i have 2 solar panels @ 210W = 420 x 7hrs = 2940W a day. And normally i let it rest for a day in between. So by the next day its ready to go again! Which is why i want to up it to 4 solar panels, so i can use it everyday.

• 600 watts * 8 hours = 4,800 Watt*Hours

Note, that Amp*Hour ratings are frequently in at the 20 Hour Rate... If you discharge your bank faster, then the apparent storage capacity will be less (check your battery spec. sheet for 10 hour, 8 hour, 2 hour, etc. rates).

ps i didnt understand the calculations you made for the 5% and 10%?

Typical "optimum charging" for a battery bank is 5%-13% of the 20 Hour Bank capacity... Too low, you may have problems equalizing or getting the batteries fully charged. Too high, a waste of money and you could overheat the battery bank on charging (rough rules of thumbs, details do matter and change things).

2) So it wouldnt be more efficient to have the 2CC leads go into a different terminal battery pair? Its more a matter of how many leads you can actually get into the same terminal without it getting unconfmortable if not impossible. And should the leads from the CC always go to the terminals where the leads from the inverter go out from?

Read how to wire a battery bank link I provided... In general, your "common bus point" should be able to handle 125% of your maximum current loads. Picking charging/loading points on different points of the battery bank just makes things confusing and difficult to see if there are design/safety problems.

3) Finally, if i plan to go further than 4 solar panels of 210W each (840W), then you recommend i up the battery bank by making 2 battery quartets, making the system 24V? And so i would need a 24V inverter and 24V CC? Or 48 if it should be the case?

Larger MPPT charge controllers can typically be programmed/setup to charge any battery bank between 12 and 48 volts... So you can migrate those to a different battery bank voltage/setup.

And, yes, you would need new inverter(s) if you change the battery bank voltage.

Regarding wiring the bank--read the thread link. And for safety, you need to protect all of your wiring with proper fuses/breakers--Car sized batteries can output 1,000's of amps into a dead short. Bigger cells and parallel batteries can output much more.

-Bill