unfortunately I cannot remember how my old NHS hearing aid used to warn me of low battery as it is such a long time ago that I had one but Graham has come to your rescue with that question. With the ones I had a voice came on to say BATTERY!! 
when I had about fifteen minutes left then if I ignored the warning it would later say SHUT DOWN!! and it did!!!! 
Pleased to hear that you are chuffed with it though can understand you being resentful at having to wear it. I feel exactly the same, in fact I often feel very self conscious when wearing a hearing aid and often you are treated differently by those who know you have deafness. Many people will have tolerence for those who have sight related problems but for those who are deaf they can become quite intolerable in so much as becoming fed up through having to repeat what they have already said then having to say it all again when you say pardon?!!
I suffered a lot of harrasement at work due to my deafness and at one point it got so bad especially from another colleague that my doctor signed me off work through stress and anxiety for a couple of months, that is when Babs made an appointment for me at a hearing dispenser and I ended up forking out four grand for the hearing aids, it was either that or face losing my job!! Though things improved greatly at work after buying them I still blame all the harrasement on my heart attack that I suffered later on. Still, that is now all behind me since I took redundancy and am really enjoying my early retirement.
Anyway, going to have to sign off for the mo as my first hour is already up here at the library so speak again later or even tomorrow.
Nigel.