The Electronic Load Detector (ELD) detects any electical load on the engine from accessories like A/C controls, radiator fan, lights, heater motor, etc. The ECU uses the ELD's inputs to adjust alternator output and compensate for the additional electrical loads.

For a great in-depth article on the ELD and Honda Alternator dual mode charging system, please take a look at this study:

Honda's Dual Mode Charging System (.pdf)