Battery drains even when plugged in while using Nav app on long road trips
It's probably your charger.

I tested my Nexus S with the following:


dumb charger (only had +/- lines connected), capable of 2Amps (2.000mA)
Inlined voltage meter to measure currents
Brightness maxed, set CPU to 1.2GHz, performance governor, benchmark running


The device adhered to the USB standard and never draw more than 500mA (0.5A), although the charger is capable of 4x more current. It lost 2% percent during &lt;5mins when the benchmark ran.

I know that apple encodes power settings into their wall chargers by setting voltages to D+/D- pins of the USB connector. I'm not aware of how to tell the phone to just draw more power and dismiss the USB's 500mA safety regulation.

If you have such a charger, give it a try.

EDIT:
I could fix my own Nexus S' charging behaviour by using a kernel with force AC fast charge option enabled (for example Trinity kernel for Nexus S / Android 4.0-ICS) and this Fastcharge widget.