What is the best way to test lipo batteries. I have a few new batteries and one of them seems to be defective. I use a Onyx 230 charger, and it shows that charging is complete. I plug the battery in and nothing works.

Start by checking the battery voltage with a voltmeter. Should be 4.2 volts per lipo cell. Check each cell with the balancing plug/taps, as well as the main plug.

I checked both batteries I have and they both checked out at 8.44 v total and 4.22 per cell (I use a Blinky balancer). One battery (Turnigy 5200 mah) works fine, but the other one (Yeah RAcing 5000 mah) doesn't.

Weird. Perhaps a partially loose connection somewhere, ‘cold’ solder joint maybe? Try measuring voltage when you turn the car on, wiggle the wires, try to find where the volts are stopping. I don’t know, different brands of batteries, double check the + -.