Batteries may also be categorized by the number of volts that they provide. For instance, a 9-volt battery provides 9 volts of energy when in a circuit.
To know what voltage your battery is, check the battery specification sheet. This will tell you important information including voltage when fully charged and help you to know if the battery in question is right for your needs.
As an example, primary 'AA' batteries usually have a voltage range of between 1.2-1.5V. As a battery is used its voltage decreases, and you’ll see that battery-powered devices using primary cells will start to slow down and stop as the battery approaches the end of its life.
But this is not the case with all types of batteries. Li-ion secondary cells provide a constant discharge voltage rate, meaning that they will operate at full power until totally empty. This is why they’re used in devices like cell phones and laptops which require relatively high and stable power to operate in comparison to a simpler device such as an analog desk clock which will be fine with primary cells which eventually slow down and die.
As a buyer of batteries, you need to know the voltage required for your product’s circuit. After you know this, you will then know the battery voltage needed.