Why does my cell phone stay charged longer if I charge it in the car?

My wall charger broke and I they were having a sale on car chargers so I bought one of those. I used it several times and found my year old phone would stay charged for about three days. Then I found a wall charger from an old phone that fit and the phone only stays charged up maybe a day and a half like usual. Why is this? My husband says it’s because the car charges it harder and it’s bad for the phone. I think it charges it slower and that has something to do with it. Any ideas?