I built a battery power supply that can plug into an Arduino board

This tiny board allows you to bring the power to your project, and not the other way around. Bring your micro to the sensor without running wires! A single AA battery is used to provide breadboard power of 5V or 3.3V (or other voltages by tuning the feedback resistors). A boost regulator provides the voltage.

BooSTick provides extremely small and convenient portable regulated power to Arduino, wearables, DIY, and other projects. The entire package is about the size of a single AA and can drive Arduino projects for many hours at a time. When the battery is used up, pop another in for continued use. Or use NiMH rechargeable batteries to reduce your footprint.

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Are the rest of the I/O's hidden/blocked under your battery board then? And not broken out/through?

How does one access all those I/O pins when using this?