What's important beyond the voltage is the amperage. Your power adapter indicates 650 mA output. A "AA" battery has an output of up to 2600 mA for an alkaline battery, but as low as 650 mA if you use a nickel cadmium battery. If you throw some nickel cadmium batteries in there and it works, you're fine with that adapter. You shouldn't be at any risk of damaging the scale using the adapter since the output is so low, but it also might not be enough power.