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LED 12v DC downlights

PostPosted: Thu Aug 29, 2013 9:18 pm
by Hobie1
I am constructing a new house, and I want to have all lighting where possible as LED’s. The majority will be downlights. Prices are not cheap but I assume a lot of this is because they have a transformer attached. Is there any reason why the lighting cannot be 12v, supplied via batteries (LiFePO4). And if so what LED down lighting is then used? I am having a hard time finding any on the web that do not come with a transformer.

Re: LED 12v DC downlights

PostPosted: Thu Aug 29, 2013 9:41 pm
by davidg
Hobie1 wrote:I am constructing a new house, and I want to have all lighting where possible as LED’s.

good :)

Hobie1 wrote: Prices are not cheap but I assume a lot of this is because they have a transformer attached.

Not really

Hobie1 wrote:Is there any reason why the lighting cannot be 12v, supplied via batteries (LiFePO4). And if so what LED down lighting is then used?

The losses in running 12v for any distance in ceiling will require much heavier wiring than mains does for one thing, the other is not doing it right creates risk of fire, I suggest that is not a good solution for a house that has runs any longer than just a few meters.

If your hell bent on that style of light then look seriously at GU10's instead. they don't require transformer and work a treat, I converted our house to these instead of MR16's took the transformers out saved and then later on I got dimmable versions of the led lights, they are all mains power based. I did a test on them they are using 7.5watts (they say 12w) each for light that have 95% of the same light output as traditional 50w MR16's according to my Lux meter.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 8:12 am
by Hobie1
Thanks for the reply David. I haven't done the calcs on the wire size, but will probably need 4 AWG as "backbone" wire. Depending on the losses I may even consider two power sources, one at each end of the house. Why would there be a greater risk of fire with 12v?
Cheers

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 10:55 am
by LEDMAD
davidg wrote:look seriously at GU10's instead. they don't require transformer

More accurately, they don't require an external transformer. They have a small transformer (driver) included in the globe itself. So the transformer is still there and can still go wrong. And you are still paying for it. Plus having it in the globe, with the heat from the globe, makes it more likely to go wrong.

davidg wrote:I did a test on them they are using 7.5watts (they say 12w) each for light that have 95% of the same light output as traditional 50w MR16's according to my Lux meter.

You can't test overall light output with a Lux meter. Many of these LED lights have a very narrow beam (you can't believe the packaging). So directly under the light might be as bright as a halogen. But at an angle of say 30 degrees, they might only be a third as bright.

Hobie1 wrote:Is there any reason why the lighting cannot be 12v, supplied via batteries (LiFePO4)

Batteries tend not to have a very stable output. A small 8% change in voltage (e.g. 12V to 11V) can have a much bigger impact on the light output, e.g half the light output.

For an example of this have a look at the Cree data sheet page 8 & 9. See how tiny voltage changes lead to massive changes in current and light output. Most LED transformers (known as drivers) are constant current devices to keep the light output steady.

The final problem is that you limited yourself to only using 12V DC LED lights. So you would end up excluding many of the better LED lights available, which normally work at voltages other than 12V.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 11:09 am
by Gordon-Loomberah
As LEDMAD says, going with 12V restricts the variety of LEDs you can choose from, but more of a concern is that you are thinking about 12V and being off-grid at the same time :o
12V really is only good for portable/camping power supplies where you want compactness and ease of transportability. For a house, 24 or 48V is a much better idea.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 2:03 pm
by Hobie1
I have spoken to an electrician who has convinced me not to go off grid. He has quoted $12k to do the 220mt underground cable (I have a ditch witch so will be doing the digging). This compares against an investment of around $30k for batteries alone. Solar size is yet to be determined.

The main reason I was looking at 12v lighting is cost. I am doing the house as an owner builder so I am happy to get my hands “dirty”. While I can’t touch 240v, it seems low voltage is fine. The sparky has quoted $65 per downlight, I supply the light. With over 60 lights, that is a large bill.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 2:27 pm
by Gordon-Loomberah
Hobie1 wrote: $30k for batteries alone.


Wow! :shock:
Are they Gold plated batteries? 21kWh of Lithium, good for at least 3000 70% DOD cycles will only set you back about $10000, so you are talking about an absolutely massive amount of storage for $30K, which would need at least 10kW of solar PV to be a reasonably balanced system. Just how much energy do you use per day?

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 2:36 pm
by LEDMAD
Hobie1 wrote:He has quoted $12k to do the 220mt underground cable (I have a ditch witch so will be doing the digging).


Can you do the digging and laid the cable yourself. Then just get sparky to connect up either end. I assume this is on your land.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 3:29 pm
by Privatteer
220m is a fairly long run for 240v so a lot of the cost will be the cable. Depends on the calculated maximum demand but the cable size has to be increased to prevent too much volt drop.

Re: LED 12v DC downlights

PostPosted: Fri Aug 30, 2013 7:17 pm
by Hobie1
Gordon-Loomberah. Agree that 21kWh would be around the $10k mark. My current usage according to my electricity bill is an average of 17 kWh per day, excluding off peak hot water. Admittedly we are all electricity, reverse cycle heating etc, except gas cook top. And there is someone home all day on most days. But there are many times when it is overcast/raining/fog here that would not put much charge into the batteries from solar. So to have 3 days usage in the batteries would be about $30k worth of batts.

LEDMAD. Possibly could lay the cable as well but doubt it would save me much. My guess is the sparky would get the cable a lot cheaper than me to compensate any saving. The cable will be 3 phase.