I need to design a circuit to charge a 400v capacitor with batteries?

The inverter from a car capacitor discharge system would do the job e.g. wbnoble.com/WN_articles/CD-ig….

If you use 12 volt batteries, you can use an inverter made for autos, which you can get at a low price. See if you can get one made for Europe, with a 240 VAC output. When you rectify and filter that, you will have 330 volts DC.

Two 6 volt lantern batteries in series will get you 12 volts with lots of current capacity.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.