Volt and amps are different things.
Your charger accept 15 volt as maximum. So 12 volt should be fine (however there's a minimum somewhere too).
The power supply should be able to deliver the amps you would like to charge with. Usually, it's good to have 0,5-1 amp more from the power supply, than you need for charging.
So if you want to charge with 7 amps, you need a 7,5-8 amp power supply.
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