I'm building a circuit and I have no idea how electronics work. I want it to be roughly a 9 volt circuit. can i use a whole bunch of 1.5 volt rechargable batteries in series to get up to 9v or do i have to use a 9v battery?
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Both work, but I would use a 9v battery.
You can do either. If you crack open one of those blocky PP3 9V batteries you will actually find it contains six little 1.5V cells in series. You may find a set of six individual 1.5V cells gives you a longer life - a single PP3 9V battery might have a capacity of 400-550mAh (or as low as 120mAh for cheap rechargeables), while six 1.5V AA batteries may have a combined capacity of 2400-18000mAh. But on the downside, six AA batteries take up more space and cost more than a single PP3 battery.
Either way you will have 9 Volts. But the determinant as to which you use is the current needed by your project. Rule of thumb here: If your circuit draws less than 50 mA, then a 9V battery will do okay. If your circuit draws more than that, you are better going with the individual cells. You need to think of each battery as a battery with a series resistor in it (the resistance is a byproduct of how the battery is made). The 9V battery has a larger series resistance than 6 cells in series, so that when you load it heavily, the voltage will go way down.
