Milliamps Hour (mAh) is important because it's the easiest way to distinguish the strength or capacity of a battery. The higher the mAh, the longer the battery will last. Batteries with different mAh ratings are interchangeable. If your battery is rechargeable then the mAh rating is how long the battery will last per charge.
Milliamps Hour is 1/1000th of an Amp Hour, so a 1000mAh = 1.0Ah.
Think of a cars gas tank. Voltage is how much gas is being used, and mAh is the size of the gas tank. The bigger the gas tank (mAh) rating the longer the device will run. If your battery is rechargeable, then think of the gas tank as refillable (rechargeable). So, more mAh is fine and will help the battery last longer.
2. It's ok as long as you use a 9v battery, and the larger the battery the capacity, the longer you will use it.
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