(Short Version)
Go here XHP70.2 P2 4000k Output test by Texas_Ace - it's over 9000!! lumens and still going strong!
Look at the graph for your target lumens. See how much current it needs and the Vf for your target. Find a driver that can supply both and see what input voltage it needs. Buy the best low voltage sag batteries you can buy with the amount you’ll need for input driver voltage and put it all together. If the driver uses a sense resistor then it maybe possible to increase output a little or reduce it.