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So to me "USB charging" could mean a variety of things.
Personally I don't like the idea of extra ports or openings on a flashlight. For convenience, having the micro USB built into a cell can be helpful by reducing needed equipment or accessories while traveling, but eats up volume within the cell that could otherwise be occupied by the cell chemistry. The USB powered charger Prometheus Lights sells is convenient and handy, I just wish there was a way to control charge current and at what voltage the charger changes from constant current to constant voltage. For example, I don't take my cells up to 4.2 volts anymore. My charger goes into constant voltage at 4.1 volts for the sake of the cells.
Yes, I don't care for the the extra openings that always end up unsealed over time.
Lion cells with a USB port are convenient, do serve a purpose for many, but do take away from the available space for cell capacity. I have a few, but haven't needed them. It's nice to have that option available in the event I did need to use them.
Chargers are a big thing with Lion cells. I think a big factor to look for in a Lion charger is end of charge termination where the charger cuts off all power to the cell when full. As Frodo says, constant trickle charging a Lion cell will diminish it's useful life cycles and eventually ruin the cell.
There are new AA/AAA Lion powered 1000 cycle micro USB chargeable cells available. They have ~90% capacity standard NiMh cells.
I think it's good to hear everyone's thoughts on all aspects of USB charging. Foursevens has a magnetic Lion USB charger.
There are two MKII Foursevens lights needing to be worked on, the Regen MMR-X and MMU-X3R that had
a Micro USB charging port.