There's a cross-platform way to check for battery status with SDL/SDL2/SDL3 with SDL_GetPowerInfo() in <SDL/SDL_power.h>. Then you can compare the status with SDL_PowerState enumerations, for instance:

SDL_PowerState systemPowerState = SDL_GetPowerInfo(NULL, NULL);
if (systemPowerState == SDL_POWERSTATE_NO_BATTERY)
    // check for mailbox
else
    // check for relaxed_fifo