Few flashlights are so excellent as a Romisen RC-E4 with a drop-in XML T6 LED upgrade. The bright mode is quite bright. The medium is nicely mediumy. And the low mode is an energy-saving 60ish lumens, just right for your journey to the center of the Earth. With a good Redilast 3100 mAh battery inside, it might work on low mode for 3 continuous days.
I took out the original Q2 LED from my RC-E4 and put in a drop-in replacement XML T6 LED meant for an Ultrafire WF-501B/502B flashlight.
Note that while these Ultrafire flashlight bulbs will fit into the Romisen RC-E4, the reverse is not true. The Romisen bulb will NOT fit into the Ultrafire flashlight.
Yes, all things considered, this upgraded Romisen RC-E4 might be my favorite flashlight.
Battery Endurance Test.
Flashlight: Romisen RC-E4 with XML T6 LED (low mode).
Battery: AW 3100 mAh 18650, bought from Lighthound.
Starting voltage: 4.21V
19:05 > 3.78V
24:20 > 3.69V
38:00 > 3.56V
42:00 > 3.52V
46:00 > 3.48V
50:00 > 3.44V
58:34 > 3.33V
61:10 > 3.31V
64:10 > 3.29V
68:00 > 3.21V
70:00 > 3.12V
72:28 > 3.01V
At about 74:30, the LED shifted by itself into bright mode and would not change out of it. The beam was still somewhat dim though. The battery was on its last legs.
75:00 > 2.71V (end of test)
Time in hours:minutes, y'all.
Battery Endurance Test.
Flashlight: Romisen RC-E4 with XML T6 LED (low mode).
Battery: Redilast 3100 mAh 18650, bought from Lighthound.
Starting voltage: 4.23V
13:15 > 3.89V
16:23 > 3.85V
24:00 > 3.72V
32:25 > 3.62V
36:18 > 3.58V
40:00 > 3.55V
46:30 > 3.50V
49:00 > 3.48V
54:00 > 3.42V
57:15 > 3.38V
60:00 > 3.34V
62:30 > 3.32V
66:40 > 3.30V
70:00 > 3.26V
72:22 > 3.17V
75:00 > 3.04V
77:45 > 2.89V
Test in progress.
Panasonic Industrial 3100 mAh.
Starting voltage: 4.20V
10:00 > 3.91V
16:10 > 3.81V
25:40 > 3.65V
30:30 > 3.60V
32:45 > 3.58V
36:00 > 3.56V