How to secure personal data on a factory reset tablet with broken microUSB port?
If the battery lasts long enough, you can use a tool to perform a Secure Wipe. As you've already factory-reset the device, one that cares to sanitize free/empty space should suffice, such as Secure Eraser or the free version of iShredder.

What those apps do is overwriting empty space with random patterns (or just zeros, or just ones) – so all a potential "restorer" can find afterwards is garbage.

If your battery gives up inmidst of the process, you've got at least something sanitized – and cannot do anything about the rest anyway. Still better than nothing I'd say, so I'd give it a try.