When I worked for Applied Radiation (now Siemens Medical) we made an ion pump battery supply of 900 Volts. It used ten 90 volt batteries (each made of 60 AA cells) in a wooden box.
You may rest assured the 900 (plus when fresh) Volts was quite real.
It had to keep the ion pump running for up to a year for shipment, otherwise a mega buck X-ray cancer treatment machine wave guide, linear accelerator and target assembly had to 'come back to the shop'. Since it was sealed, if the vacuum degraded too much, the ion pump couldn't recover. About a thousand pounds worth, all totaled.
Biggest battery I ever worked on (voltage wise). Scary to solder together as well.....
Doug Owen