TX 2.5mm power plug like the one on the end of your wall charger to match your TX. Maybe you need to make a charge cord or battery test cord. Here is the plug so you can do it for yourself. Pliers holding part vertical so you can see tip. Plug taken apart below to show solder lugs and outer shell of plug. JR systems, use the plug center pole as negative. Futaba/Airtronics/Hitec/Ace chargers use the plug center pole as postive. Pack of six for $6.00!
Fits your Futaba/Robbe/Ace & Alpha 4 Chargers type 2.5

Whiles supplies last, discontinued by manufacturer