12 v battery to 2x 3 watt led bulbs---12 metre length--what cable size?

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lenten

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non boaty but hope someone can help---also please note i am an electrical idiot----i am currently using candles to light my strawbale building at thegreenescape.co.uk---these give enough light to eat and generally see but i need more light over the hob and kitchen area and as the clocks are soon to go back i am thinking of putting in 2x 3 watt led bulbs---the distance battery to lights is 12 metres-----(please bear with me this might not be correct electrical terminology)----i believe the longer cable run is from the battery the more power i will use to get the current to the lights but that thicker cable will mitigate this---anyone got an idea what size cable-----quick details of the rest of projected system--thinking of 100 ah battery ----bulb use 5-10 hours a week---the reason the battery is so far from the lights is because this is near the sunny spot on the roof and i wiil probably fit a solar panel later on------thanks lentenrose
 
In general you are correct - the greater the power and the longer the cable the thicker it needs to be.

However in this case there should not be an issue. Firstly the current draw of the LEDs is low so there won't be much voltage drop in the cables; and secondly most LEDs are tolerant of a very wide range of input voltages. So pretty much any zip wire/speaker cable and the like will do - look for something with a DC rating of 2-3A or above and it will be fine
 
Running it through my volt-drop spreadsheet, and assuming your battery is at exactly 12v (if it's healthy it'll probably be a bit more), I make it a drop of 1.8% for 1mm2 cable and 1.2% for 1.5mm2 - neither really worth worrying about.

Pete
 
I make the volts drop 0.2 v for 1.0mm² and 0.14 v for 1.5mm² ........ so in agreement with Pete
 
Bedouin suggests speaker wire

13x0.2mm speaker wire is rated at 2.5 amps and has a cross-sectional area of 0.4 mm²

It will give a volts drop of 0.53 V
 
As said cables can be quite small. In fact you don't want to go too small because the cables will then be too susceptible to damage etc.
The current drain will also be quite small so your 100AH battery might be an overkill. You might want to consider recharging the battery. If you use a car for regular travel then you might consider a smaler battery that you can carry to the car when ou go out. Plug it into the car for a recharge while you are driving. Hence a smaller battery would be more manageable.
However solar recharge may be successful. The cheapest way to get lots of solar power is to buy a 40v 200w unit as used for domestic PV to mains systems. Around here they can be go for around 100 squid. You will need an MPPT controller then to convert the 40v to a suitable charge voltage for 12v battery. With that then you can go to a lot more lights and possibly an inverter for other things.
In any case don't forget a fuse at the battery. One for both charge and discharge. The fuse rating must be related to the wire size and the load. A 43 amp fuse might be right.
Lastly Mr Banggood is your friend when it comes to LEDs http://www.banggood.com/buy/5M-LED-Strip-Light.html thse are great for work areas cupboards etc.
good luck olewill
 
A word of caution to those doing sums - the 3W rating of the LED will relate to its output, so it is likely to draw about 1A - unless the driver circuitry is very clever it is likely to draw 1A from the battery as well so requiring 12W - with the additional power being dissipated as heat
 
In general it's always best to oversize cables if you can, for two reasons.

1. It minimises losses
2. It allows for future additions in case you decide to add more lights or other small electrical devices later on. You won't have to run another cable. You never know what you're likely to need or do in the future, so always best to try and think ahead, and cater for many eventualities if possible.

The longer the cable run, the more important and useful it is to oversize because it can be a real PITA to actually install it (and hide it discreetly).
 
bedouin said:
A word of caution to those doing sums - the 3W rating of the LED will relate to its output, so it is likely to draw about 1A - unless the driver circuitry is very clever it is likely to draw 1A from the battery as well so requiring 12W - with the additional power being dissipated as heat
Click to expand...

Bedouin

can you go over that again please. I thought I was beginning to understand LED planning parameters, but where/how does a 3W LED use 12W. Genuine enquiry :)
 
sarabande said:
Bedouin

can you go over that again please. I thought I was beginning to understand LED planning parameters, but where/how does a 3W LED use 12W. Genuine enquiry :)
Click to expand...

Most Ultra Bright LEDs need about 3V across them - so a 3W LED will draw about 1A. I don't know how the 12V driver circuit works but I think it is highly likely it just drops the voltage using something like a big resistor - so it will draw 1A from the 12V supply too.

It is possible to do it more efficiently but I doubt if they do
 
bedouin said:
Most Ultra Bright LEDs need about 3V across them - so a 3W LED will draw about 1A. I don't know how the 12V driver circuit works but I think it is highly likely it just drops the voltage using something like a big resistor - so it will draw 1A from the 12V supply too.

It is possible to do it more efficiently but I doubt if they do
Click to expand...

I think it's unlikely that the driver just throws off the 9 surplus volts as heat. But in any case, if the whole assembly is quoted as 3W then that should be what it uses. The rating of the individual LEDs used to achieve that shouldn't matter.

I guess you then have the risk of Marketing folk getting involved and quoting whatever number sounds best - they might seize upon the LED rating if they thought that sounded better. But they tend to work on the "bigger is better" principle, so in the case you describe I'd expect them to call it a "12 watt" bulb instead.

Pete
 
prv said:
I think it's unlikely that the driver just throws off the 9 surplus volts as heat. But in any case, if the whole assembly is quoted as 3W then that should be what it uses. The rating of the individual LEDs used to achieve that shouldn't matter.

I guess you then have the risk of Marketing folk getting involved and quoting whatever number sounds best - they might seize upon the LED rating if they thought that sounded better. But they tend to work on the "bigger is better" principle, so in the case you describe I'd expect them to call it a "12 watt" bulb instead.

Pete
Click to expand...
No - LED bulbs are quoted in terms of light output so a 3W ultrabright LED will consume 3W in the LED itself - not much point quoting the value if it doesn't relate to light output.

I would imagine the detailed spec will also give the current consumption but the OP doesn't quote that.
 
bedouin said:
LED bulbs are quoted in terms of light output so a 3W ultrabright LED will consume 3W in the LED itself - not much point quoting the value if it doesn't relate to light output.
Click to expand...

I defer to your knowledge of market practice, but quoting watts as a measure of light output seems silly. The last LED lamps I bought quoted light output in lumens and current draw in milliamps, which are both useful figures, but they weren't exactly mass-market.

Pete
 
prv said:
I defer to your knowledge of market practice, but quoting watts as a measure of light output seems silly. The last LED lamps I bought quoted light output in lumens and current draw in milliamps, which are both useful figures, but they weren't exactly mass-market.

Pete
Click to expand...
Yes I agree it is silly but the market is used to measuring bulbs in Watts - and it seems particularly poor for LEDs when there can be vastly different light outputs from LEDs with the same nominal rating. They do at last seem to be moving to quoting Lumens alongside Watts.
 
bedouin said:
No - LED bulbs are quoted in terms of light output so a 3W ultrabright LED will consume 3W in the LED itself - not much point quoting the value if it doesn't relate to light output.

I would imagine the detailed spec will also give the current consumption but the OP doesn't quote that.
Click to expand...

Not what I am finding from the specs i am looking at

3watt 12volt LED .... 239mA at 12.7 volts for example ... light output equivalent to a 30w halogen

http://www.bedazzled.uk.com/12v_LED_Bulb_Replacement.htm#MR16type
 
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bedouin said:
Those aren't single LED lights - they have multiple LEDs - that means they can wire say 3 in series. I would be surprised if you can find a single 3W LED fitting that only draws 240mA (but I could be wrong).
Click to expand...

Similar to this sort of thing then

12 volt, 4W, Single LED. Current consumption 350mA

u16cobww_1.jpg
 
LED can be driven by either a switch mode current controller or a simple series resistor. For a 12v supply then with simple series resitor current control you almost always have 3 LED diodes in series giving as said about 3v to be dropped in the current control resistor. Indeed with the 5050 LED there are 3 diodes in the tiny package so that it is convenient to wire them in series. With the larger down light type bulbs you often see 3 LED in a cluster. In some cases 4 in a cluster but then they are run fromna switch mode power converter from 240vAC. The switch mode power converter even for 12v DC input (often rated at 9v to 30v) actually switches the current to AC then through a transformer to give precise required LED current. This minimises heat power wastage and means that there is an aprox. 95% efficiency of input powewr to actual power to the LEDs.
So as said power rating for LED bulbs is AFAIK actual DC input. However LED diodes are often described by their power rating. So 3X 3w LEDs may inded draw 1 amp if it is rsisitor current limited but more like 10watts if it is electronically controlled.
( I have a real language problem on this forum differenciating between an LED bulb which may have many LED (actual individual diodes). You might think as once was that the Diode of "LED' meant an actual diode but that has been hijacked by popular use to mean any device with LED installed. (a bulb) but then people often think when you speak of LED bulb that you mean a tiny diode.)
 
thanks everyone for replies----i think i will use 1 mm cable ---its fairly cheap and robust----the bulbs i have are shaped like a normal bulb ---opaque with 6 leds inside---they are supposedly equivalent to a 25 watt incandescent----i don t want a lot of light as the building is normally lit with 18 candles and 20+ night lights---and 30 metres of warm white fairy lights---it gives it a nice oldfashioned glow----will i need to put a fuse in the system?----regards lentenrose----------------ps thanks for the bangood link-----i thought about led strip lighting but was worried about safety/fire risk
 
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