IF that is one of them onboard battery testers. with the led's. then it just plugs into an empty reciever slot. When you turn on Battery switch the led's should light up. I mouted mine so I could see it through the window on a slow flyby. If green, I keep on flying, If yellow, I land.
If red, oh oh! too late.

If it is a handheld Like a multi-meter? then it goes into your Charge Jack.
Hope this helps.
Smoky.