LED Power consumption and battery life

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blackcoffee17's picture
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Joined: 02/22/2021 - 18:41
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Location: UK
LED Power consumption and battery life


I am thinking about designing a LED lamp. It’s a long term project and an electrical designer will do the work.
For now i am just trying to get informed and learn as much as possible.

I have a question about how to calculate the power consumption of a LED and the battery lifetime.

Let’s say i have a Cree XM-L3 and a 3.7V 1200 mAh lithium battery.
The max light output of the Cree XM-L3 is 1860 lm and maximum efficacy of 187 lm/W.
(“Cree XM-L3 Specs”: https://www.cree.com/led-components/products/xlamp-leds-discrete/xlamp-x... )

How can i calculate the most efficient modes for this LED and approximate battery life?
(I know that the rest of the electronics also needs some power)

Let’s say i want the following modes in my lamp: 400lm, 250lm, 150lm and maybe a very low setting of 50lm.

I imagine a very high power LED like the XM-L3 is also efficient and can provide long battery life at lower lumen levels.

Essentially i want long battery life at good luminosity and good thermals (it’s a small lamp with small space inside for cooling).

Or how to find the most efficient LED?

Sorry if this is too many questions but any input is welcome Smile

Last seen: 8 hours 36 min ago
Joined: 10/17/2020 - 21:25
Posts: 22
Location: UK

I’d start by comparing the LED tests on this forum. By checking the current and Vf at a certain output you can find the most efficient LED for your needs. After that you could look into a high efficiency driver; a buck driver may well be more efficient at lower levels.

You can find a test for the XML3 here:

Last seen: 8 hours 28 min ago
Joined: 02/25/2016 - 17:38
Posts: 577
Location: Uruguay

By lamp do you mean a lantern?

First you need to figure out color temperature and which CRI (Color Rendering Index ) you need. If you need a color temperature below 5000K or more than 70CRI you can discard many LEDs. If you need 90+ CRI you may just stay of CREE, they typically don’t do High CRI well, in that case you are better off with Nichia 219C, Samsung LH351D or SST-20.

Start here if you are going to use CREE LEDs. Note that the web version (v3) lacks data for some of the new LEDs like XM-L3. The XM-L3 is pretty much like a XP-L2 in a larger format, the increase in output comes from the increase in maximum rated current, the actual difference in efficiency is very small or even none.

LEDs are more efficient at low current, if you want to run it efficiently at high current you need multiple LEDs or a larger LED like a XHP50.2. At 1.2A, the lowest data point for the XHP50.2 (3V version) the output is 643lm vs 557lm for a XM-L3, efficiency is 190lm/W vs. 159lm/W, in both cases two bins below the maximum, actual values will depend on which bins are available. For 150 lm/W I don’t think there is a large difference in efficiency.

You will need either a buck (for 3V LEDs) or a boost driver (for a 6V LED), linear drivers or just resistors are less efficient.