Only use original charger and battery. Messing with the battery and charger current levels can seriously damage the battery and the phone. Li-Ion batteries can even explode if improperly charged.
That is what I am trying to say. You can be assured of safety only with an original accessory. To answer your question about the amperage you could consult the manual or check the wall charger for the readings. I guess most car chargers are safe to use, but avoid the cheap ones.
The phone will only draw whatever current it needs. Otherwise everything in your house would need to be 13amp (UK) as that's what the sockets are rated at (or you'd need a different mains supply for every appliance). But yes, avoid the cheap ones, they're not able to maintain a constant voltage or current and could send your phone haywire.
