My bad for the NH-9WM and the NH-14WM being different sizes.
The problem with the D777's charging system is that it doesn't fully charge the NH-14WM batteries, so that the battery life between charges from the D777/NH-14WM combination is barely any longer than from the D777/NH-9WM combination. In other words, you are deliberately undercharging those NH-14WMs when you charge them inside the D-777. (BTW, I have read in another thread that the chargers built into PCDPs have rudimentary charging controls - fixed charging current with a timer - and the D777's built-in charger delivers just enough current/time combination to fully charge the NH-9WM's, with allowance for a very slight overcharge.)
OTOH, you don't
want to use the NH-9WM batteries inside the newer D-EJ825, D-EJ915, D-EJ925 or D-EJ01 PCDPs, because their charging systems will repeatedly - and severely - overcharge
those NH-9WMs. As a result, you must
use the NH-14WMs in those newer gumstick-battery-powered Sonys.