Interesting. I never cared about what my Japanese beard trimmer uses as battery, because whenever it dies on me (which is long after prominently lighting a red LED), it works instantly fine connected to the wall to let me finish my cut, and can take however long it wants to recharge after that ¯ \ _ ( ツ ) _ / ¯
You see, some Philips MBA decided to pinch 0.0001 pennies and forgo installing a red LED to warn the user the battery is drained, and a diode to allow it to run from the wall.
