I have a Sangean ATS 818cs portable radio but no 6v lead input

One note is the power plug going into the radio appears to be atypical in polarity. The center appears to be negative ( - ) instead of positive ( + ).

Yes you can use any 6 volt DC for your radio but the most IMPORTANT thing to remember is to connect the terminals in the correct direcction. First of all you can solder a red wire from the positve terminal of the battery and a black wire from the negative terminal of the battery. This must be connected to the source of your 6 volt battery.You can buy a good DC jack socket female to be fitted into the vacant spot of your radio and use a male jack to supply the DC. Only be carefully to see the polarity. To avoid any reversing of polarity use can even connect a diode in the forwarrd bias towards the positve terminal. This will prevent any wrong connection of polarity to damage your set. Helpful? Good day