Brightness when using 3.7v battery for powering 6 LEDs rated at 5v in a series

Question

I am using 6 LEDs in a series (APA102C spec) these specifically powered by some atmega chip, and am having trouble finding a sufficiently small battery that will allow me to run the LEDs at maximum brightness.

My current working design is here, based on adding 3 E90 batteries in the square block as pictured below:

quite ugly, and does not fit the form factor of the rest of the device.

My question is what is the minimum voltage I can use to get the greatest brightness out of my 6 LEDs, and, how much worse off (in terms of brightness) would I be with a 3.7v? (the 3.7v are very small, which makes them attractive for my project). Also, what is the minimum mAh count I can target to get the LEDs to run oh, say, 30 minutes?

Suggestions as to which batteries may be a good fit in my "ring" would be further appreciated as well!

Thank you!

Answer 1

More details about your setup, including what chip you are using would be beneficial. But here are some things to consider.

I am assuming the specs in this datasheet are similar to lights you are using (same model, different manufacturer). The current ranges from 22.5 mA to 26.5 mA, which at 5 V results in 112.5 mW and 132.5 mW.

The first thing you can do to maximize the brightness is to use the designated power supply voltage for the LEDs. There is an internal voltage regulator that regulates the internal voltage to 4.3-4.7 V. It is important to note that these devices do not consist only of an LED light, but the chip that controls the color and uses SPI communication. A proper 5 V input is required to ensure proper communication.

I recommend using whichever setup allows you to provide 5 V to your system for the lowest volume. This Adafruit buck-boost converter will allow you to boost the 3.7 V from the battery you have now to 5V.

As for how long the battery will last, a quick search will lead you to the right equations and even some calculators already set up.

However, study up on the effects that current draw has on battery life. Draining large currents from a small battery will significantly reduce its life. Instead, whenever possible it is preferable to use a higher voltage battery and use a buck converter to reduce the voltage. This will allow a lower current to be drawn from the battery for the same power due to the higher voltage.

Answer 2

As A.S. said, it is technically unsafe to run the SmartLEDs from 3.7V, they need 5V.

Now, since each LED takes 150-200mW, 6 of them will run above 1W. For 1/2 hour it takes a 500 mW-hr battery. Let say with converter losses, a 1 W-hr battery will run your LEDs for 30-40 minutes. If Li-Po is used (3.7 V nameplate voltage), a small 300 mAh battery will do the job. If you get a RC-grade medium-discharge pouch cell (from a RC toy shop), it should run the Adafruit buck-boost module just fine.