I have a JR 3810 and had troubles with a battery with a short that was discharging - almost bought down the plane so decided to replace it with a new 1 and put in a 4.8V micro battery checker...but where and how do I plug it in? can any one help please

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.

If your receiver is full, just plug a Y-harness into the battery slot in the receiver and plug in the battery and monitor to the other ends.

Hey thanks guys! it is the Led one and I not long figured out for myself how to get it working - had trouble plugging it in to the rx and thought I might have it wrong then it just slipped in - then it still wouldnt work till I discovered two little switches numbered 1 and 2 underneath it. and Now we have green.. where should I mount it in my super sportster so I can see it without having to take the wing off? cant see into the cockpit cus I painted that silver?

About your only option for mounting then would be on the left hand side of the plane. (Just below the canopy in your picture.)

That will keep it away from the exhaust residue as much as possible.

I prefer a clear canopy personally.

(That's Austin Powers in there.)
Dennis-