Whilst iPods are incredibly popular the Lithium rechargeable batteries locked inside only have a fairly limited number of charge/discharge cycles. Once the battery starts to fail the iPod performance falls off quickly.

Generation   mAh   V                              Cat Number
1st/2nd          1600   3.7                           SB-2570
3rd                    850    3.7                          SB-2572
4th                    900    3.7                          SB-2574
Mini                  500    3.7                          SB-2576