I have a 2 cm thickness (3/4 inch) stainless steel that I would like to drill some holes on it. What is the min. voltage cordless drill should I use? Are there other alternative beside drill to create holes on the stainless steel? (The holes diameter range from 0.5cm to 2cm [3/16" to 3/4"])
When drilling stainless go slow (do not over heat the metal) If the metal gets hot it will "work harden" and is hard to drill with a drill press. It may take a while but can be done with a good quality 1/2 drill motor and good drill bits. If you are purchasing a drill get one with at least 2 speeds and a 3 speed would be best used on the lowest speed. Cutting fluid or a light oil will help reduce heat and improve the cutting. Last you will want a high quality drill bit something with a titanium nitrite coating or carbide. I have a full set of carbide cutters for stainless but these are very expensive and if you drop them on cement they can shatter. If you only have 1 battery stop when the motor starts bogging down and recharge the battery. I have drilled 1-3/8" holes through stainless with my Dewalt but it did take 2 5AH batteries and part of a 4AH battery the stainless was only 3/4" thick.
It has nothing to do with voltage. You can use a 7.2v drill if you have enough batteries (or patience) on hand. Like others have said, a corded drill is the best bet, and a press is better for thick metal. Binding is a potential problem.
If you are drilling 2cm (0.78inch) holes in stainless... and it absolutely has to be battery operated. You do not have many safe options here.
There is only safe and portable drilling process I can think of, clamping a 12+mm mild steel plate to the stainless steel and drill it with a battery operated magnetic drill. Any of the common battery powered magnetic drills will work, the voltage does not matter.
Other common hole making options besides drilling 2cm stainless steel would be water jet cutting and plasma cutting.
If you have to use a hand drill I would suggest using a step drill bit as those are fairly stable for hand drilling.
There's no way in the world that you're going to drill a 2 cm diameter hole in 2 com thick stainless steel with a cordless drill. You should find someone with a drill press that you can use.