Default When using GPS and charging in car, phone won't charge?

When I use GPS and want to charge from car charger, phone drops battery percentage, little slower then without a charger, but still dropping. So I thought that the charger is not strong enough, and bought another one with 3.1A, little better but still dropping. Then I try with home charger and no dropping, the GPS is working and battery is charging!
Is that normal, that the car charger can not charge the battery when the phone is in heavy use? The home charger have only 1A, and car 3.1A.