Sounds like you want a DeWalt 36 volt battery pack using the A123 batteries which you can really demand a lot of power from and they can charge fast and take a lot of abuse. You can get this on eBay for reasonable and it might save you a lot of trouble building one. I'm not certain about the current rating but I know with these A123 cells they can take heavy draw more than most batteries and survive. Some more knowledgeable people here may have better suggestions....