I started to mow, but it started to rain.

Electric mowers are great, but they don’t do well in the rain.

1. Electricity and water don’t mix.  My earliest experience with electric lawn mowers involved extension cords. It was also before Ground Fault Circuit Interruptors.   Although a little rain won’t hurt a battery powered mower, a lot of rain could risk a short circuit.  
2. Wet grass will clog a mower. The Black and Decker CM1936 will use a lot of battery power trying to mow wet grass.
3. Leaves don’t mulch well when they are wet.

All in all, the mowing can wait for another day.