There's a guy called AW on candlepower forums who's selling protected 17500-size lithium ion cells (3.6-4.2 volts) for around $10 each. Two of them will substitute for three CR123 and should fit in the C3. Runtime will be ever so slightly less than the 123's, but you can recharge them. Brightness will be about the same despite the nominally lower voltage, because of less voltage sag.