Honestly, like everyone else has said, building a backup battery out of cells isn't the best place to jump into DIY with little knowledge about electricity.

What I would recommend is something like this.

https://ca.ecoflow.com/products/river-portable-power-station

You can charge it from solar or the grid and provides you with 5v, 12v and 120v and probably cost less than what it would take to buy the cells, let alone the equipment and parts you need to assemble said battery.

If you want to get into battery building, start with repairing cordless tool batteries. You only need 5-10 cells, not 260 plus. That is asking for a fire.