Battery to adapter?

Since it runs on 3.0V, it is better to use a 3V adapter. However, in some cases, obviously depending on the circuit, higher voltage (4.5V or even 6V) adapter can be used.

But, this is unlikely because the circuit is designed to run on two batteries, so, providing higher voltage than the actual requirement is not acceptable.

I will highly advice you to use 3.0V adapter with a current rating from 250mA to 500mA would be a good choice.

  An adapter, AC/DC adapter is a type of external power supply, often enclosed in a case similar to an AC plug. Other common names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, wall wart, power brick, and power adapter. Adapters for battery-powered equipment may be described as chargers or rechargers. AC adapters are used with electrical devices that require power but do not contain internal components to derive the required voltage and power from mains power. The internal circuitry of an external power supply is very similar to the design that would be used for a built-in or internal supply.

External power supplies are used both with equipment with no other source of power and with battery-powered equipment, where the supply, when plugged in, can sometimes charge the battery in addition to powering the equipment.

If you are tired of replacing batteries in your portable radio or in any other battery-powered device, using an AC power adapter is a good alternative. All you need to do is to determine the voltage(V) and current (mAh) of the device. Then, attach the appropriate adapter to the place where the batteries make contact inside the device.

Remove and count the batteries in the device you're adapting. Standard dry-cell round batteries such as AAA, AA, C or D are all 1.5 volts. Multiply 1.5 by the number of batteries. So, four batteries would equal 6 volts; six batteries would equal 9 volts and so on.

Find the current or amp (mAh) rating either in the specification sheet in the device's manual or on a sticker on the device itself. This value is the current (mAh) for which the adapter should be rated. Using an adapter with a higher mAh value than your device is OK because the device will only use what it needs from the adapter.

Purchase an AC adapter that matches the voltage and mAh you figured out in the steps above.

Cut off the low-voltage connector at the end of the adapter's wires. Strip about a half inch of insulation from the wire's ends and pull them apart about by 4 or 5 inches.