I'm trying to build a battery pack to power a small device. It needs to be able to provide around 11 amps at .7v into what is more or less a dead short (it's powering a small heating element). I plan to use either a pack of 6 NiMH AA cells or one or two lithium cells. Figure 6-9 volts input range. Runtime doesn't need to be exceptionally high but I'd like to get at least 30-45 minutes of continuous use at a minimum. I started looking into how to do this and I realized I have no real clue where to start.

I know I need a buck circuit. It needs to be regulated to keep the current between 10 and 12 amps. I've no clue how to go about creating either of those or what parts I need. I'm hoping someone here might be able to help me out a bit.

edit: if I could buy a premade module that will work or can be made to work, I would much prefer that over having to build the entire circuit from scratch.

edit 2: Could I use something like this?

 

From the data given, this sounds like a E-cig vaporizer.

 

From the data given, this sounds like a E-cig vaporizer.

Very astute.

I've decided that the part I linked above will probably do the job I need. I went ahead and bought one for a first attempt. Now I just need to get the rest of the parts I need.

One question, though. Does a buck module like that drop the voltage to keep the current in line or does it provide the voltage you set it at and just stops providing additional current if you hit the set limit? I was just thinking that if the latter is the case, I can keep the output voltage higher and improve the efficiency a bit.

 

If you really are serious about using the board you linked you will want a full set of appnotes from the vendor. But given the origin of the board you may be left wanting there. From the description I gather that the board has been configured for both constant current and constant voltage operation. Typically this means that for constant current operation you set the open circuit output for the minimum dropout Voltage (in this case 1V) above the required voltage at the load. Then you set the constant current with the output shorted to the desired level.

That said I think it fair to issue a warning. Judging from your questions it would appear that you are aware that this project is above your current skill set and I would concur. I am not saying that you are unable to acquire the skill but since this appears to be a biological application that you might try on yourself, it would be much more cost effective and safe to simply purchase a vaporizer.

Regarding your assumption concerning efficiency the output voltage will go to wherever the load takes it while drawing the fixed current as long as the voltage remains below the open circuit voltage plus the dropout. As such setting the open circuit voltage higher will not affect the operating voltage under load, or the efficiency. The efficiency in a buck SMPS is more a function of the components of the circuit and the operating frequency.

 

From the specs, it looks like its 0.8v to 28v output user set, and user set current limit upto 12amp. May need a cooling fan also.

 

That said I think it fair to issue a warning. Judging from your questions it would appear that you are aware that this project is above your current skill set and I would concur. I am not saying that you are unable to acquire the skill but since this appears to be a biological application that you might try on yourself, it would be much more cost effective and safe to simply purchase a vaporizer.

I have one. That's what the battery pack is for. It uses a direct connection to the battery so no extra electronics are needed beyond the power source.