Decided it was time to replace my big, bulky, battery hungry W520, as it was starting to fail on me. So, I decided to get a smaller, but loved ThinkPad X230. :
Lenovo ThinkPad W520 Review - StorageReview.com
Lenovo IBM ThinkPad W520 Extended Life Replacement Battery
Battery for Lenovo ThinkPad W520 4284 | Lenovo Battery Shop
BTI 0A36303-BTI - Notebook battery (premium) (equivalent to: IBM 42T4710, IBM 42T4712, IBM 42T4714, IBM 57Y4186, IBM 42T4801, IBM 42T4802, IBM 42T4912, IBM 42T4803, IBM 42T4799, IBM 42T4853) | www.shi.com