Being able to measure open circuit battery voltage is great, but it would also be useful to measure current while the light is running. That in turn can map to battery runtime and/or lumens. I wonder if any existing boards can do this, e.g. if they have adc pins connected to the relevant sense resistors. If not, it could be a cool feature in future boards.
Only as long as you use a clamp meter.
Hmm is that to avoid voltage sag at high current? I think I’m satisfied with just low current, say below 0.5 amp at 1.2v (SP10 Pro on Eneloop) if I can do that with a DMM. I think a DC clamp meter that goes that low is quite expensive. Maybe someday.