limp mode prius 2005

some weeks ago I left a door open for several hours with the key in the ignition whilst at work, on my way home the car became sluggish and the red triangle appeared if I want the triangle to disappear I unplug and replug the batter, but after 10-15 mins. the triangle is back, another 10 mins and it becomes sluggish again. Now the battery screen shows the battery going from almost full to empty and back again in 15-20 mins.and the sluggishness has nothing to do with the status of the battery full or empty. I have noticed that the visual of the interaction between the battery and the motor disappears as the car is about to be in limp mode. If I stop the car for a minute it will then be ok running wise but the red triangle is still apparent, but then sometime later back in limp mode. If anyone can save me from paying a small fortune to Toyota then I will be most grateful.