If you seriously want to diy this then you need a bunch of those meanwell power supplies. You want to hook enough of them in series of the dc output for the cut off voltage of the prius pack 280 maybe a good ball park. You can use others as long as they are stackable.
Then you got to find the leads to tap and polarity to make sure you do not reverse charge it. There are plans online for makin grid or maintenance chargers. Mike at 99mpg car has some plans and you can find them for sale on ebay.