I was at a trade fair yesterday where I found myself looking at the "Guts" of a Prius. The batterys are really quite small, Alarmingly so infact. (Not much bigger than an attache case)
I asked the demonstrator what the electrical capacity was, he reckoned 30Kwhr!
(I understood that they were "Only" about 12Kwhr )
My responce at this stage was something along the lines of "Bloody hell, Its a bomb!" at which he looked a little shifty before agreeing with me.
Its all terribly clever though. The battery pack is made up of individual cells each of which is individualy monitored by the self diagnostics. Faulty cells can be thus identified and replaced individually. Long life is ensured by operating the battery withn a fairly narrow charge/discharge cycle.
It only cycles between 50% and 70% charge. Interestingly not "Fully" charging it gives extended life. Everyone knows that fully discharging a battery can damage it, But I must admit, the idea that fully charging it might also reduce lifetime is a new one for me. I guess its a LiOn thing, memory effect and all that.
Of course, In a Hybrid the battery is only there to "Average" out power demand. How Lithium batteries might actually perform in the real world of "Pure" Ev's where much wider charge/discharge cycles will be required remains to be seen