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. :
New Original for Lenovo Thinkpad T420 SL410 SL410K T410 T510 E520 E50 W510 W520 L412 L420 L421 T520 51J0499 Laptop Battery 94WH - AliExpress
New Genuine 55+ Battery for Lenovo ThinkPad T410 T420 T510 T520 W510 W520 SL510 | eBay