My view would be don't mention BPD. I now work full time but have not told my employer about 'BPD.' have disclosed it to Occupational Health but only in the sense that I have this label and find it unhelpful, stigmatising and a way to dismiss my needs. I do talk in terms of long term mental health problems which have included anorexia and self harm. My line manager does know about the self harm but it was only when I trusted they wouldn't get rid of me was I honest in saying I needed a day off sick to get treatment or for time out to prevent a crisis.
The general public mostly haven't hard of BPD. I think that the NHS uses it against us too since it's such a fixed position they adopt about 'taking responsibility' rather than having real empathy.
In your application for voluntary work you can say about what you offer, the question of why are you available to volunteer may well come up later but it's fine to say you have been ill for quite a long time but now working on recovery eventually returning to work, but not for a good while.