Won't charge in car with USB charger

Trying to sort out a mystery. I bought a couple of high power car lighter to USB adapters from Monoprice. When I plug my iPod Classic into them, I get no charge indicator. I shorted the data pins together, thinking it wasn't giving max current, but still no luck.

The same adapters and cable will show charge on some other iPods I have, and the cable works fine with both my computer and an AC to USB power adapter.

Is there something strange about the Classic's charging detection?