Most lithium small radio batteries are 3.6 volts. If AA or AAA batteries, multiply by 1.5 volts. 4 batteries = 1.5 * 4 = 6 volts MAXIMUM. Rechargeable batteries are 1.2 volts. Your charger would be a much higher voltage because it takes power to "push" the charge into a battery. You didn't say, so I am assuming you want to operate the radio with a wall charger. I've done this by soldering the wires to the battery contacts. You need a higher voltage than 3.6 volts, but milliamps is more important. You need enough milli amps to get the radio to transmit. Too much and it frys, literally. So, this is all on you to decide if the risk is worth it. I use a 5-6 volt power supply set on no more than 1 amp (800 ma is my favorite size) 1 amp = 1000 ma. Let us know if this helped!
Ive lost the charger