Hi I have same problem. I need a 24v to 12v ... I am using my truck battery 24v 90A each battery.. 180A total.. my load is an HID ballast 100W but power input is only 12v.. I need the power to be 12v but the current must still be at least 70A minimum.. the ballast will not work if current is below 70A.. so I'm looking at voltage divider using resistors.. a cement resistor for that.. but I don't know the value.. an I don't know the specifications of the resistor.. if you pls help me guys I'm no expert to this.


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A resistor is not the best solution for your project.
For what time period does your HID ballast require 70 amps ( minimum ) ?
70 amps seems way too high, for a 100 Watt HID bulb

A pair of 50 Watt Automotive HID Headlamps = 100 watts.
They require approx 13 amps total to start ( 15 - 30 seconds ) then drop to only 8 amps total.

 

Start with this simple proposition for ANY DC-DC conversion scheme:
Power Out will ALWAYS be less than Power In; sometimes it will be much less.
The last part is especially true when using resistors. The other reason you don't want do that is that you have to change the resistors if the load changes in ANY way.
What is an HID?

 

HID = High Intensity Discharge bulb
Headlights on many new cars are HID

Please post a URL to web page of actual HID Lamp.

 

Well something does not compute. 12V @ 70 A is a bit more than 100 Watts, more like 840 Watts. Fortunately the available input from a 24 V @ 90 battery is 2160 Watts so at least the proposition is feasible. I'm going out on a limb here, but if you can't buy what you need, your chances of successfully designing and building what you need are slim. The power levels are just too high for a first project, especially if it is built on a breadboard. This will require a carefully designed PCB with extra heavy copper traces.

 

I don't think these are "90 Amp" batteries.
More likely, they are "90 Amp-Hour" batteries.
It appears to be two (2) @ 90AH 24V Truck batteries connected in parallel as the power source.
24 Volt Truck batteries can deliver much more than 90 amps.

Just like many motors ... the "starting amps" for a HID bulb is much higher than the "running amps".
We still need the URL and many more details ...

A 70 amp 24v-to-12v Buck Converter is not cheap.
The real question is ... is 70 amps really needed?