Once the traction battery is charged to its allowed limit, the mechanical friction brakes take over. This by the way is a major reason that B mode exists - when on a long downhill, the traction battery will max out, and the mechanical brakes take over, which will then start heating, leading ultimately to brake failure (if given enough time). B mode lets the engine take up a good chunk of the load, relieving the stress on the brake pads.

--My hybrid came home!--
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Randy