Thinking about those batteries in the context of a really big LED tail light for a bike leads to wondering about the variation in LED forward voltages; it’s possible to drive LEDs in parallel if they’re well-matched for forward voltage. A quick-and-dirty test is in order to get some first-pass numbers… and I have bags of nominally identical red and amber LEDs.
Applying a fixed voltage that produces 20 mA through 14 randomly chosen LEDs of each color, then measuring the voltage across each diode:
| LED | Red V | Amber V |
| 1 | 1.895 | 1.939 |
| 2 | 1.893 | 1.921 |
| 3 | 1.903 | 1.918 |
| 4 | 1.895 | 1.921 |
| 5 | 1.891 | 1.918 |
| 6 | 1.935 | 1.906 |
| 7 | 1.891 | 1.926 |
| 8 | 1.904 | 1.930 |
| 9 | 1.901 | 1.923 |
| 10 | 1.894 | 1.927 |
| 11 | 1.901 | 1.914 |
| 12 | 1.894 | 1.939 |
| 13 | 1.901 | 1.933 |
| 14 | 1.903 | 1.925 |
| Minimum | 1.891 | 1.906 |
| Average | 1.900 | 1.924 |
| Maximum | 1.935 | 1.939 |
Pushing 20 mA through the five lowest voltage red LEDs requires 9.54 V. Applying that voltage to the five highest red LEDs produces 18.2 mA.
Putting those two strings-of-five in parallel with 9.52 V produces 40 mA total: 16.6 mA in the low string and 19.9 mA in the high string, all measured with a fancy Tek Hall effect probe. No, those aren’t reversed and, yes, I did check twice: it makes no sense at all.
Temperature matters a lot in such measurements and I wasn’t controlling for that, plus I didn’t have a constant-current supply. Better numbers await better instrumentation, but I think binning a couple bags of 100 LEDs based on forward current should be straightforward.
The Smell of Molten Projects in the Morning 