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LED power consumption


jbs
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Hello all, I'm trying to find out how to work out what the power consumption will be from these 12 V LED light strips It's for use when camping and I will use the power from a vehicle battery : http://cgi.ebay.co.u...#ht_4535wt_1137 it's the amps that will be used that I'm interested in so that i can work out how long i can go without having to charge the battery

I make no bones about the fact that I really have'nt got a clue how to work this out hence why I asked on here, as someone will know what I'm asking about

Thank you

John

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they dont really give much information unfortunately so its hard to be particularly accurate.

A standard LED might draw 10-20miliamps, usually with a forward voltage of only 1-2v. Depending on how they're wired will also affect it, but if you say 48 LEDs in parallel being fed 12v and drawing 20ma each, thats just shy of 1 amp, or 12w (ie a little bit more than two sidelight bulbs)

They might have batched them together in series to remove the need for resistors, say 4 batches of 12, which i think im right in saying would mean you'd have only 4x20ma, or under 1 watt.

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Unfortunately, the advert isn't very specific about electrical details so it's impossible to tell until they turn up.

As a shot in the dark though, a white LED normally drops about 3.6v at about 20mA, so each LED is 3.6 x 0.02 = 0.072W. As you have 48 of them: 48 x 0.072 = 3.5W (300mA @ 12v).

That said, I doubt there is an LED driver for that price. So, most likely bundled into 4 groups so you'd actually have 12 groups of 4 LEDs in series. That'll drop each LED to 3v, giving a total consumption of just under 3W (250mA).

Hopefully those numbers should be somewhere in the ball park.

Edited as Aragorn got there first: My first numbers above assume the use of an LED driver. Aragorns first lot of numbers assume the use of resistors - I personally doubt they'd control the voltage with resistors.

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James, does this mean if I had a 110 AH battery, would that allow me to use the led strip of 48 led's for 440 hours?, the working out I've just done is as follows I took the 110 divided it by 0.250 that where I got the 440 from, However I'm not sure I've worked it out right so please correct me if nessacary

thanks

John

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Yes, your logic is sound, however this is rarely the case.

Battery manufacturers rate their batteries in some interesting ways. The capacity of a battery will change considerably with load and temperature. The rating on the battery will have been generated using a method to get the most favourable figures.

While you might get 440 hours of life from your LED bulbs (they are a small drain, so your battery might actually last this long), you will kill the battery in the process by discharging it too far.

In practice - 1/2 the theoretical figure will get something vaguely akin to the real world for a small load. 1/4 theoretical for a large load. You will also need to bear in mind there will also be a small internal leakage current that will naturally discharge your battery - normally about 20-30mA for a lead acid I think.

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James, does this mean if I had a 110 AH battery, would that allow me to use the led strip of 48 led's for 440 hours?, the working out I've just done is as follows I took the 110 divided it by 0.250 that where I got the 440 from, However I'm not sure I've worked it out right so please correct me if nessacary

thanks

John

No you can only discharge a car battery to 55-60% before you have issues with it recharging properly.

There are graphs about somewhere, but for arguments sake 11.2 v is probably as low as you want to go.

It all depends on how flat your battery is after a certain time with a constant load.

Even deep cycle batteries only do the full deep cycle bit 50-60 times before they are scrapped.

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Pete and James, Thanks I would'nt run the battery down that far, I was thinking aloud for a better word about how long I could possibly run the led light strip, I would re-charge the battery during the day with a solar Panel that I have as I would'nt necessary want to run the engine of my 90 to charge the battery that I was going to use for running the lights and possibly one or two other things aswell like a small radio to listen to in the mornings whilst I eat breakfast etc, as I would be mostly static during the day anyway

This is what I love about this forum the info that anyone asks for always gets a decent informative answer

John

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I've got a load of these as interior lights in the back and front of the 11o, they're self adhesive strips from maplins.

the chap there said they use 1 watt of power, so however you use them they're unlikely to have any effect on a decent battery if the vehcle being used every few days.

i've had my led's on all day and left them on overnight several times and have never had any issues with battery drainage.

HTH

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