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Ford has had the Hybrid vehicle line since 2005 and in that time frame I have only seen 1 battery pack go bad and it was under warranty. The Hybrids have a very good warranty to help prevent costly repairs like that. You could either have a 8 year 80k warranty or 15 year 150k warranty.
Again the price you saw of $8,000 is right around the cost of the battery to replace and I have gotten estimates from other experts of anywhere between $7,200 and 8,500 so $8,000 is pretty much in the middle.