• As @Robin says, checking the schematic is a good bet.

    'Normal' voltage regulators have a quiescent current (the power drawn by them even when not supplying any power) that's quite high, often 1mA or more. The MCP1703T used in the MDBT42 breakout is a bit pricier but provides a decent amount of power while also having low quiescent current.

    In terms of charging, there are a bunch of chips you can use - generally now they're all pretty straightforward to use - just the chip and some capacitors. I've used MAX1551 in the past as it's easy to get hold of.

    When you use USB it's also good practice to add a 500mA PTC (self resetting) fuse near the USB plug - just to ensure that if there was any kind of short you don't end up damaging your PC :)


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