I found a LiPo (11.1V / 3.6 Ah) which fits into the internal battery case of the FT-817. First used yesterday on a cold summit without problems. It can be found on ebay (.de) with a search for “LiPo Sanwa M11” (RC TX), and is manufacturer labeled “LRP”. The LiPo discharg purpose is only for low current, but the connection cables are still thicker than those in the 817, and LRP mention a charge with 1C which is also more then the 817 need.
73 Jörg DL1DLF