24v to12v .. large amp problem

24v truck battery at 90A each.. 2 batteries.. that's the regular setup of my truck.. the load is a 2 HID ballast 100W each.. to do it.. I have to use a duals witch relay.. making a tapping to load each ballast to 1 battery 12v 90A and the other ballast 12v 90A.. the ballast only starts to work on a minimum of 70A.. this is good cause it's working.. but the battery drains fast.. my alternator cannot compensate.. so now I need a circuit that is 24v 180A to 12v with high amperage.. anything not less than 75A... I am thinking voltage divider circuit using a cement resistor.. but I don't know what specifacion of the resistor to retain high amperage.. and I don't know what value of the resistors to use.. can you pls help me