You need to keep the phone or tablet connected to charger while mining. Even if the charger is using full current, it will only use 10W/h (5V * 2 Ah) of electricity. That's about same amount of electricity one energy-saving light bulb uses.

You can't use always all cores of the phone for mining or the phone will hang. I've used all cores on my tablet and it still is responsible as it has higher priority for essential background tasks and doesn't have half of the cores designed for low-power mode. You should always use low or medium priority for mining, as high priority will eventually hang the device.