Just a quick technical question that I'm curious about. Is there a circuit that would let me take a couple of my old used 9V batteries and put them together but only draw 9V from them, whatever their combined voltage is? For example, let's say I have two old used 9V batteries that have dropped to about 6V each. Is there any way to put them together to make 12V, but draw only 9V from them? I'm wondering if there's a way to make some sort of external battery to use what's left in old batteries. Of course, this would only be practical if the 9V wasn't dependent on the overall voltage of the combined batteries, whether that was 9.1V or 18V or anything in between, if that makes sense. Is there any way to do this?