For the curious, the filament of the light bulb turns incandescent starting from only 0.5V, as a barely visible red dot. For charged batteries, the light is clearly visible. The funny thing is that the filament is just right for testing AA/AAA batteries, either alkaline or rechargeable.
With some practice, it is possible to distinguish between fully charged and halfway charged batteries.
Connecting the batteries is much easier and faster than when measuring with DMM probes. I've tested today all the NiMH pile in no time. The biggest delay is that you have to let the bulb shine for at least 2-3 seconds at each battery, because empty batteries after a long rest can still deliver current for a couple of seconds.
The logging setup from yesterday when I was measuring capacitors leakage was still on the workbench, so out of curiosity connected the light bulb instead, and measured the filament current with voltage:
The light and the color temperature changes a lot between 1-1.5V, so I guess one could measure battery capacity, C (mAh), by simply connecting the light bulb then filming the light until it goes off.
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