Was able to narrow down the issue to the board using a known good 18650 battery. Also have seen a couple forum posts saying it’s the board that goes bad on some of these units. Now I’m starting to wonder if it’s just a bad capacitor or something replaceable with a standard soldering iron…

Could you please provide a source / part # for the replacement battery?

Is it just two 18650 style 3.7 Volt lithium batteries in series or parallel?