Richardson's 26-yard touchdown run proved to be the difference as the Browns' defense teamed up with the adverse weather to stymie the Chargers.
Richardson shook off a rib injury to pick up 122 yards on 24 carries while Brandon Weeden completed 11-of-27 passes for 129 yards as the Browns (2-6) won for the second time in three games.
The Chargers (3-4) couldn't unleash their offense in the rain as Philip Rivers connected on only 18-of-34 passes for 154 yards.
Ryan Mathews ran for 95 yards on 24 carries for San Diego, which dropped its third straight.
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