Mine is an 03 and it had the original battery failing when I bought it last year. I went online (ebay) and bought a set of 38 2005 cells for $1000. (Very good deal.) I replaced my 2003 cells with the good 2005 cells because I wanted to move up to the next battery design. (There are three types of batteries in American Prii.)

After a few months I started getting battery check engine codes, so I took that battery out and carefully measured each cell voltage. Three cells were much lower, so I went online again and bought three good 2005 cells. I reassembled the battery with the bad cells replaced and ran it for a week. Then I ran it in reverse a while to drain the cells as much as possible, took them out, and measured again. I resorted the cell order to match each pair of cells as close as possible to the average pair voltage. Put it back in the car and I have never seen a battery error since.

So it is certainly possible to DIY, but you do have to be careful not to electrocute yourself! And old prii with 38 cells have REALLY heavy batteries!