I would have got out my spare set of batteries and said "I'm going to put these in" and if the examiner said "well those are flat as well" I would abandon the test there and then and ask for the examiners number and complained to the CAA immediately.
Mind you if he was in the air "rule of thumb" may have got him through.
thankyou for the tip off though, forewarned is forearmed and all that.