A5 has 2900mA battery and a charger with fast charging capability and j7 has 3000mA battery with normal charging.
Can I use that to fast charge the Galaxy j7?
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Galaxy J7 isn't rated for Fast Charge support either on Samsung's sites or other sites that host specs, and comes with a measly 5V 1.5A charger (for both the 2015 and 2016 versions). Adding the fact that it targets the lower end of the market, it's safe to guess it does not support Fast Charge. In comparison, A5 is at a higher end and supports Qualcomm Quick Charge 2.0.
You should still be able to use the charger, just not getting fast charge from it.