The HID++ 1.0 Illuminated Keyboard K800 does not support the d7 register, instead is uses 07. Its observed behaviour is documented in the ltunify repository[1].
[1]: https://git.lekensteyn.nl/ltunify/tree/registers.txt
d58a79c hidpp: support battery register 0x07
src/linux/hidpp-device.c | 56 ++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 54 insertions(+), 2 deletions(-)
Upstream: cgit.freedesktop.org