If you're looking to replace your 9V battery with something with higher capacity and the ability to recharge, try this out. What we're going to do, is take a traditional USB Powerbank, boost the output the 9V, and use that as our battery. Use with discretion. Works great in most applications!
1. 9V Battery Snap Connector. - Do a Google search or eBay.
2. DC-DC Boost Module. - Again, eBay. Make sure it has the Micro-USB input. The economical model goes up to 28V, so try including that in your search term. Either the mt3608 or lm2577 will work great!
3. Micro-USB cable. - Shorter is better.
4. Voltmeter, soldering iron/gun with solder, flathead jewelers screwdriver, and electrical tape.
Step 1: Solder Your 9V Snap Connector.
IMPORTANT: Typically, you think of Red as Positive, but since this is output, we'll be reversing that.
Solder the Red Wire to the Negative Output Through-hole and the Black Wire to the Positive Output Through-hole.
The reason for this is that the connections on an actual battery are the reverse of the connections of the snap. So you want the bigger snap terminal to test negative, and the smaller terminal to test positive.
Step 2: Tune in the Voltage.
Start by hooking up the outputs of the circuit to your voltmeter, then plug in the circuit to your powerbank (make sure it's charged). Take your jewelers screwdriver and turn the small screwhead on the potentiometer. Usually, turning it to the right will raise voltage, but it might be left. Turn it at least three turns before deciding you're going the wrong way.
Tune it to 9V.
Step 3: Wrap It Up.
At this point, you can either keep your Micro-USB Cable connected, or not. Just make sure and disconnect power at the source before wrapping it up. I prefer to keep the cable connected and have the whole thing be one unit, but you might want to leave it off for flexibility. Either way, you'll want to add some strain relief to your unit by wrapping the wires around the circuit board a couple of times, then using the electrical tape to wrap the entire board so no connections are exposed.
Step 4: You're Done!
You're ready to plug this thing into your 9V device. Since the powerbank is rechargeable, you've got a rechargeable power source for your 9V.
Note: Check the Amperage consumed by your device. Max Power Output for this device is going to be between 300-1000mA. Don't overdrive the circuit. Good luck!