JD 690B excavator help...please!

So i'm helping my landlord downsize after a nasty divorce, i've got most everything covered except this beast which is on the back 40.

I've worked around cat's on more than a couple of occasions, logging and firefighting i've driven them and helped work on them.

This beast has me baffled.

Hasn't been started in over a year, all the hydro lines look good-ish for an old piece of equipment. Was able to turn it over and get a sputter out of it using start assist before the batteries finally finished crapping out.

12v system or 24?
anything I should know about this beast?
Best way to sell a niche piece of equipment like this?

Did some research and the couple i came up with were going for 10-15k the one at 10k had a nuked motor.

Figure the batteries are under one of the plates that is bolted down but can't seem to figure out which one, i figure close to the cab because it makes sense but this thing is huge there are lots of places next to the cab.

Any help would be much appreciated, ill be the first one to admit that im way out of my element. But hell thats how we learn lol