The Moto X stock charger is either 850 mAH or 1150mAH and it will comfortably draw up to 1.5A. Most car chargers charge at .5A max (actual sustained charging is closer to 350-450mAH). When connected to low power sources, many modern devices will still draw a charge, they just won't necessarily always register it as actively being charged and in some cases, if the power source is too weak, it may not be actually charging. Additionally, there are two "modes" that charging occurs in and it basically boils down to whether or not the charger believes it is being charged by an AC outlet or a USB port or other auxiliary connection. In the former, the device will typically try to draw as much power as it can, while in the latter, even if the power supply is higher (2.0A for example), the device may still limit it to .5A. To ensure your device is charging as optimally as possible, it's recommended to use the stock charger (you can use it through a converter or power inverter to change your car's aux port to AC (with grounding!) or to use another charger drawing AC power at at least 5 volts with 1.0 (sometimes 1.1), 2.0 (sometimes 2.1) or 3.0 (sometimes 3.1 or 3.2) amps.