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Electrical Engineering/Compatibility of 12V battery with LED floodlights

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DC 12V LED floodlight
DC 12V LED floodlight  
Hi there,

I'm setting up an outdoor lighting system using LED floodlights identical in appearance to the image attached.  On the local market I'm able to buy different wattage floodlights powered by DC 12/24V or AC 220V (I'm in the Middle East with AC 220V).  I will probably opt for the 50W model, which requires either DC 24V or AC 220V, but I may want to use some 30W floodlights which are either DC 12V or AC 220V (the stores sell with or without AC/DC LED drivers.

I will be setting up at least 20 such floodlights, placed in various locations in a compound measuring approximately 150m x 150m (500ft x 500ft).  The lights need to be always-on at night; i.e. when the city power cuts out, there's a 2-3 minute delay before the generator kicks in, but the floodlights need to remain on.

I happen to have two brand new Studer Xtender XTM 1500-12 charger-inverters on my shelf (tech specs: http://www.studer-inno.com/?cat=sine_wave_inverter-chargers&id=432&tab=5).  I also have a Victron 12V charger (not inverter) which, if I'm not mistaken, would allow me to connect DC LED floodlights to batteries, batteries to charger, charger to mains.  12V 90Ah batteries are available locally.

From a user-friendliness perspective, I would ideally like to have all floodlights powered from one central location, but I imagine that may not be ideal on a technical level.  Given the size and layout of the compound, cable runs from a central battery bank would be as long as 60-70 metres.

If I opt for AC-powered floodlights connected to an inverter and battery bank, would such long cable lengths pose any problems in terms of voltage drop?  And how about if I install the DC-powered floodlights - do the same rules regarding cable length apply as for AC?

Is there a maximum number of LED floodlights that I should put on a single string?  I can get good quality solid-core or stranded cables up to 150mm2 cross-section, but I'm not sure how to calculate which size I would need for this application.

Lastly, in a perfect world I would be able to set up a photocell sensor in a tube on the roof to sense when the sun is going down and automatically switch on the floodlights.  I can't find a purpose-built sensor on the local market, but CdS photocells are available, as well as other common semiconductors.  I have a basic knowledge of electronics and relatively good soldering skills, but searching online I haven't found any straightforward DIY instructions to make my own light-sensor switch for my floodlight system.  Ordering parts from other countries is sadly not an option.  Any tips would be highly appreciated.

If you feel my questions are more concerned with power than electrical engineering, just let me now and I'll redirect them to another expert.

Many thanks in advance!
Chris

Answer
First, I would caution you from using a charger to power the LED lamps.  Why? Because chargers have built in current tapering circuit to keep the battery from over charging. Further, they usually start at a higher voltage than the battery being charged in order to hasten the charging time.  Therefore you must study that very carefully.

For the 220v units (or the 12v units, as well) you must know the actual wattage consumption and then calculate the current being drawn by each LED.  You can do this using the Power Law where Power equals voltage times current.  For example is an LED actually consumes 22 watts then the current is 22w/220v = 0.1amp or 100 ma.

Then, considering the total draw of current you can find out which gauge wire to use from such tables as found online of which there are many.  Here is one I like and may work for your application:

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/wirega.html

For night switching there are many such devices that I think you can probably find in your country;  they are cheap and very reliable, I think.

https://www.google.com/#q=night+switch+diagram

But if you cannot find them on your local online resources the URL above may give you some ideas for the design you need.

It would be much better if you could run all LED lamps off the same voltage ratings. Mixing the 220v ac and 12/24v DC is a complicating factor I think.

In any event, I wish you well and hope this has helped you with a suitable solution in the long run.  

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