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I am creating a circuit with a 9v battery as its power source. For the sake of extended hours of usage, would it work to put another 9v battery in parallel, since placing it in series would make the supply 18v, which is too far away from 5v for the 7805 to deal with.
Bottom line, would it double capacity/lifespan, or would it go up in smoke?
You can parallel these batteries, but there will be some mismatch and they won't share the load equally.
A much better solution is to use a buck switcher instead of the 7805. As it is now, your power supply is only 56% efficient. You should be able to get that to about 90%. That efficiency boost alone may be enough to not need a second battery.
If you can use multiple 9 V batteries, you can use different batteries altogether. 9 V batteries have poor power density. With a small boost switcher, you should be able to run your device from two AA batteries for much longer than the single 9 V battery the way you are using it now.
If you are talking about PP3s, then 6 AAs in series would be a lot more economical.
I suggest to use 2x 14500 lithium ion cells in a 2xAA holder with a 9v-style snap. This gives 7.4 volts nominal. An added benefit is they are rechargeable and have much higher energy capacity.
A low dropout switching regulator module can also be added for even better efficiency (you can get ones small enough to fit on top of the battery pack).
Let me know if you want picture or part numbers.
