My keyboard is the 3-battery model. 2 of the batteries were stuck.

The 3rd battery is about 4" deep, and this keyboard had been sitting for several years. It was jammed deep.

I don't think there is any need to mess with WD-40 or vinegar etc.

1. For the first battery, I gently drilled in a skinny small-thread 3" drywall screw. I then pulled the stuck battery out by grabbing the head of the screw with some pliers. Worked like a charm.

2. For the second battery, it was down about 4" or more. I didn't have a drywall screw long enough. I did try drilling the screw in with a wire pull wrapped around it (picture-frame hanging cable and small-gauge wire). They both snapped; I suspect the screw head acted as a cleaver. I tried about a dozen times and the thing seemed impossible to get out.

In any case, now that there was a guide hole drilled in the battery, I used a flat router-bit (flat head with a sharp point) with a long throw arm that easily reached the battery. This entered the head of the battery and forced it to rotate within the keyboard tube; with the router bit jammed into it like a harpoon, popped out like magic.

Note the 3-battery model has got to be the most difficult situation. If you have only 2 batteries, drilling in a drywall screw and pulling it out with pliers easily got out the 2nd battery in a couple of minutes.