For the internal discharge, the RepCap is not the best register to use because it does a lot of processing to calculate a user reportable SOC. This register will accumulate all negative current noise and all negative voltage fuel gauge corrections but not any positive values unless there is significant charge current to indicate a real charger and not just noise. This behavior works well for applications where the battery is in standby mode for less than five day, but for very long standby times, it causes the RepCap/RepSOC to diverge from the other capacity/soc registers. If you are interested in knowing the battery discharge, then MixCap is the best register to use. The other register that could fit well in low discharge applications is VFRemCap.
It is not recommend to AvCap since it is still application driven, with capacity subtracted for load/temperature.