The external power connection on flashes like this is generally not a simple DC input you can wire up from an arbitrary supply. Community replies indicate the flash charging system steps voltage up to around 300V and uses control circuitry to monitor and manage charging. That means building your own pack safely would be difficult and risky unless you fully understand the pinout and charging design.

A safer approach is to use the flash with rechargeable NiMH AA cells, or use a compatible external flash battery pack designed for this type of connector, such as CP-E4-style packs or similar third-party options. These are made to provide the correct output and recycle performance.

So, the practical answer is: don’t assume the 3-pin port is a simple low-voltage input, and don’t DIY a supply without proper technical documentation. Use compatible AAs or a purpose-built external flash power pack instead.