Take your Lab599 TX-500 outdoors and enjoy the field operations. After all, that’s what it’s built for.
Your radio will work with different battery capacities and chemistry types, but for size, weight and durability I recommend Rechargeable 12V 6Ah Lithium Iron Phosphate (LiFePo4).
WARNING: nothing in this cable stops you from damaging your radio by connecting it to a wrong voltage battery or with wrong polarity. Red wire goes on the positive terminal of the 12V battery, black wire goes on the negative terminal. Pay attention and talk to a friendly elmer if you are not 100% confident!