Motivation
2 Questions:
When powering 5V LED strips directly from 3.7-4.2V of LiPo/Li-Ion batteries, how much of a difference in brightness is there?
Which is brighter, 144 LED/m strips at 3.7-4.2V or 60 LED/m strips at 5V?
While some quick searching showed that 3.7V will work, I did not find any visual comparisons.
Tests
| 60 LED/meter Strip | 144 LED/meter Strip | |||||
| 1 Color | 2 Colors | White | 1 Color | 2 Colors | White | |
| 5V | 1.09A | 2.11A | 2.73A | 2.61A | Unable to Test | Unable to test |
| 4.5V | 1.84A | 2.27A | 2.55A | 3.89A(at 4V) | 5.25A (at 4.3V) | |
| 4.2V | .79A | 1.55A | 1.9A | 2.48A | ||
| ~4V Battery* | 2.05A* | 3.55A* | 4.1A* | |||
| 3.5V | .48A | 1.03A | 1.35A | 1.57A | ||
Test Notes:
- Tests were done using APA102 battery, WS2811/WS2812B should have similar results.
- By wiring the power supply to the opposite sides of the LED strip, current is distributed evenly.
- Batteries used were not PCB protected. Manual precautions were taken to avoid over discharge.
- The brightness differences were small and are harder to see in photos.
- All voltages were tested using the output of the boost converter, with exception of tests denoted with * , which were connected directly to the battery.
- Presumably, 4.2V output of the boost converter would be the same as a freshly charged 4.2V battery, and so forth for 3.5V
- Due to limitations of the boost converter used, 5V for 144 LEDs for 2colors and white was untested. But these results can be estimated judging by the 60 LEDs tests. ~5A for 2 colors, and ~6.5A for white.
Notes
Although current is indicative of a LEDs brightness, brightness is a nonlinear function of current at a decreasing rate. i.e the relative brightness increment from the 2nd 10mA may not be as noticeable as the first 10mA. (This fact can be used to deduce that 144 LED/m strips at 3.7-4.2V is brighter than 60 LED/m strips at 5V)
Notice that there is a 4.5V voltage regulator on APA102 LEDs according to the datasheet, which likely means that additional power is dissipated by a LDO. 4.5V may be all that is necessary to achieve maximum brightness.
4.5V/5V boost converter may be preferable to keep forward current, and brightness, constant even as the battery voltage decreases over discharge time. This is noticeably less troublesome for 60 LED strips, since current draw is lower and heat constraints is not an issue.
Tests were to determine if lower voltages powering the LEDs makes a difference in brightness, 5V data lines are still necessary.
Conclusion
There is a small but noticeable difference between 4.2V and 3.5V. The difference between 5V and 4.2V is not significant.
Good results can be achieved for both 144 LED strips and 60 LED strips with direct connections to a LiPo/Li-Ion battery.
144 LEDs with a direct 3.7V-4.2V battery connection is brighter than a 60 LED strip with a 4.2/5V boost converter.
LED Strip Lightsaber Blades
In regards to 2-sided LED strip lightsaber blades, what battery capacity and discharge rating is necessary for good results? Although further testing will be needed to provide more reliable indication. From these tests we can reasonably estimate, assuming the typical use case is between 1 and 2 colors: 1 high discharge 18650 battery will be sufficient for a 2 sided 60 LED/meter strip blade. 1 high discharge 18650 batteries would be sufficient for a 2 sided 144 LED/m strip blade, although 2 batteries would most likely be preferred for a ~1 hour run duration.