123s are nominally 3v but sag in voltage under load. A single 16650 can deliver more current and will have less voltage sag.
You can, but it'll run much dimmer. To my eyes about half as bright, which sounds about right.Why/how can we run 16650 which is rated at 3.7v?
This is helpful, I did fail to take sag (voltage drop under load) into account. BUT, I don't see a curve showing 2xCR123 drop vs. the 16650. ITSM that 2xCR123 starts with a higher voltage and has more "room" to drop. OTOH a single rechargeable cell is more attractive than feeding CR123 batteries.Not really. What you're missing is voltage sag. CR123A only test ~3.24V new when not under load. They can't actually put out that much voltage under a P60 lamp's 1.13A, more like 2.7V—2.5V (or two of them, 5.4V—5V), and as their capacity is depleted, their combined voltage under load drops below 4V. A Li-ion is nominally 3.7V under load, but fully charged under 1A load will put out 4.2V for a short time, but as capacity is depleted voltage, again, drops.
Look at the discharge curve for 1A (in blue), voltage stays above 4V for 20 minutes:
source: HKJ Test/Review of Sanyo UR16650ZTA 2500mAh
Here is a CR123A (look at what 1A does. It barely holds 2.5V for 30 mins, remaining 60 mins is below 2.5V):
source: HKJ Test/Review Panasonic Lithium Industrial CR123A