4-cell battery replacement on X61s -- too many choices

[SteveDC]

My original 4-cell battery is shot on my 3-year-old X61s. I called Lenovo on the chance it might be covered under my extended warranty, and it is not.

I do want to keep the 4-cell and not get a larger battery. The support person said to call the parts department and request a model 42T4629 as a replacement.

I went to the Lenovo web site, and I see two 4-cell batteries for the X60 series:
-- ThinkPad Battery 22 model 40Y7001 ($129)
-- ThinkPad Battery 31 model 40Y6999 ($129)

That is three model numbers for the same thing. I don't have a three-sided coin.

I am tempted to try a $40 one from Ebay. I know that is fraught with peril, but I rarely use the battery. I mainly just want the yellow battery light to stop flashing, and would use it for short durations (e.g., 30 minutes or less) on occasion. I use an external battery pack when flying, etc.

Which would you pick?

[EOMtp]

Clarification:
-- model 42T4629 as a replacement. <--- FRU# for 4-cell Enhanced
-- ThinkPad Battery 22 model 40Y7001 ($129) <--- Part# for 4-cell Enhanced
-- ThinkPad Battery 31 model 40Y6999 ($129) <--- Part# for 4-cell Slim Line

It is regrettable that too many places conflate Part# with FRU#. Part# describes a specific item, for which there can be multiple suppliers and/or multiple "generations", each with its own FRU#, i.e., each Part# is likely to map to several FRU#s, each of which will be functionally interchangeable.

Back to the battery: I would get the 4-cell Enhanced (with the "spacer" you need for your X61s) -- it provides 50% more battery life when you need it, and has same physical profile and approx. same volume as the Slim Line. However, if lighter weight is more significant to you than longer battery life, then get the 4-cell Slim Line.

Assuming the the "trouble" is the battery and not the motherboard, any undamaged battery will stop the blinking charging light, so if that is all you need, then get the least expensive item off eBay.