Braun 6550/5704 shaver battery replacement using two NiMH AA batteries.
The shaver feels like it may run a little bit slower than when new.
It's much better than dead batteries though.
2017/01/04 Update: The shaver is still running with the first pair of
AA Energizers. It may be time to replace the batteries again as it
does not appear to hold its charge as long as it used to. The second
pair of AA Energizers were used for other purposes long ago. I'll most
likely use Eneloop from Costco this time.