It's difficult to be sure, but I wouldn't rule out the possibility of a motherboard problem especially if you have tried batteries from different sources.
Not sure if it makes more sense to send the entire notebook back to ASUS (or somebody else?) for out-of-warranty service, but it's another option to consider just in case it turns out to be a motherboard problem (if you actually want to pay for a motherboard replacement, you may need to check availability though).
If you are moving the notebook around, try letting it sit on a desk without touching the unit as the battery drains just to confirm it's not related to moving the notebook which would indicate one type of motherboard issue, but would not expect the power offs would always occur at 50% battery charge in this case which may still be motherboard-related.
Good luck!