Amplifier connected straight to battery with on/off switch would damage battery in any way?

So i did this on my ford fusion 2016
straight connected to the battery with the on/off switch but now the car does not want to start as before makes a little noise like when battery its dead but turn on after 3 to 4 seconds any suggestions? Ideas thanks

So much more going on than you may realize: How big is the amp? How much current does it draw? Can the alternator handle the extra load? Is the battery old? Is the battery weak?

Weak battery: Check the battery voltage after the car has been resting for a few hours. This is important so you can get a good idea of the health of the battery. If the resting voltage is 12.00 volts then it's time for a new battery. BUT WAIT! We still don't know if the alternator is charging the battery. Start the car. The battery voltage should go up into the 14 volt range (14.4 but not higher than that). If so then the alternator is working. Next turn on all the electrics including the amplifier. The battery voltage should not drop below 13.6 volts. If it does then either the alternator is not capable of pushing that much current out to the amp AND to the rest of the car. The battery will have to make up for the difference, leaving it drained. So at no time should the battery voltage drop below 13.6.

If it is dropping below 13.6 then you need to figure out why. Like I said, it could be the amp is just too much load for the alternator. But it could also be related to a slipping fan belt. If you are able to turn the alternator (WHILE THE ENGINE IS OFF - NOT RUNNING - SITTING QUIETLY) and you turn the alternator but the crank shaft does not turn then the belt is slipping. You may need a new belt, you may just need to tighten it.

If the battery is old - say 3 or more years then it may be time to replace it. Used to be batteries lasted 5 to 10 years in a car. Typically 5 years more so than anything else, but I've been seeing more and more batteries being killed off earlier. Just last night I replaced my neighbors battery. It had a resting voltage of 10.66 volts. Way too low to start the car. A few months ago I replaced my own battery. Its resting voltage was 12.05 and starting the motor was getting difficult. Both of these batteries (5 year guarantee) were only 3 years old. So if you replace the battery KEEP the receipt in the glove box. That way if your battery needs to be replaced before the warranty is up - you have proof of where and when you bought it.

Get the battery charged and tested. Put a fuse in your amp cicuit. Get a multimeter and check current draw in your amp when the engine is not running and your amp off. Over 50 milliamps and the amp is draining the battery. You can also check the total current draw with the engine running, AC on, headlights on and your ramp/stereo on. You could be maxing out your alternator.

HI you need a FUSE between the battery and the Amplifier so that it does not short circuit the battery as this could cause the battery to explode.