First, let me say I'm talking of the 2004 to 2009 model, and we call the battery the "traction battery" to distinguish it from the 12V lead acid one. The earlier model is only slightly different, but it is different.
Because the Prius battery is operated between -about- 40% charge to -about- 80% charge it lasts a lot longer than similar batteries would in other uses. The loss of capacity is mostly caused by loss of electrolyte (in the real world), and the above cycle limit is there to prevent that.
With entropy being what it is, the cells do deteriorate over time and use speeds this up. Eventually one (or more) cells will deteriorate enough that they will reach a capacity 40% lower than the average of the others. At this point they will completely discharge during use and reverse charge. This will destroy them. The battery electronics will eventually detect this and will complain (set codes, turn on lights, and otherwise bother the driver
). There are a lot of cells in series, in groups of 6. You can replace them in groups of 6, and Toyota has announced they will start doing this. They have to match the replacement module with the ones in the battery, so they will be keeping a lot of batteries in their rebuilding plant.
Toyota wants you to sell your "used up" battery back to them. There is a guaranteed price for it. I leave it to your imagination why. The batteries are not really toxic. They contain nickel and potassium hydroxide, with a little sodium hydroxide, and water.
How long before you have to replace the battery? There are taxis with over 350,000 km on them still running the original battery. Your mileage may vary.
How much will it cost to replace the traction battery? Toyota has just dropped the retail price, to $2000. It was in the $3000 to $4000 range before. That's for a new battery. A rebuilt would be much less.
