The flash itself uses a high-voltage capacitor internally—around 300V is plausible for the main storage capacitor—but that does not automatically mean the external battery port should be fed 300V directly.

From the community responses, the connector style is the same type used by Canon-compatible external flash battery packs, and one answer confirms roughly 300V exists in the flash circuitry along with a pin diagram. Another response notes these battery packs contain more complex electronics than a simple direct supply.

So the safe takeaway is:

• The flash’s internal capacitor is in the ~300V range.
• The external pack interface is not something you should assume is just a raw 300V input without verifying the exact pinout and pack design.
• DIY powering is risky because of the high voltage and the electronics involved.

If you proceed, treat this as a high-voltage design problem and verify the exact connector wiring from a proven service diagram or by reverse-engineering an original compatible pack. For most users, using a purpose-built external battery pack is the safer option.