When the self-charging device is turned off for the first time, it can be seen from Figure ① that 10m stops moving and the kinetic energy becomes 0, then only sliding friction does negative work F times 10m, which is equal to the original initial kinetic energy of 500J, and a functional relationship is generated according to the functional relationship; Namely: f× 10 = 500...①,

When the self-charging device is started for the second time, it can be seen from Figure ② that 6m stops moving and the kinetic energy becomes 0, so the negative work done by sliding friction F times 6m, which is equal to the initial kinetic energy minus the electric energy e charged into the battery according to the functional relationship, that is, f×6=500-E…

F=50(N) from ①, E = 500-50× 6 = 200 (j) into ②;

Therefore, the electric energy charged into the battery for the second time is 200 J;

Therefore, choose: d.