What size of 12V battery to run a car stereo?

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Well the car forums I used to be on all the time had battery discussions periodically and there were a number of people that would claim to be using the Optima in a deep discharge fashion and saw little degradation in performance and capacity after years of use, unlike the standard flooded cell types.

I've never run across problems with an Optima, though I have had issues with flooded cell car batteries after just having my car sit idle for 3 months with a battery that was less than a year old. One nice thing about those spiral wound cells you can install the battery in horizontal if you want and it won't leak as the electrolyte is held in suspension in a glass fabric.
 

c_mitra said:
Most of the current will be coming from the buffer capacitors and we can have variable power output even with a const volt power supply and a low Ohm load.
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Since the negative feedback gives the amplifier an e3xtremely low output impedance then its output signal level does not change when an 8 ohm speaker is changed to 2 ohms if the amplifier has enough output current to drive 2 ohms. But the clipping level will probably drop a little.

Nobody listens when an amplifier is clipping badly. The loudest parts into 4 ohm speakers will be about 14W and the average output power with music or speech is about 14W/5= 2.8W per channel. Then with 4 channels and 50% efficiency the power from the battery is only 22.4W.
 


When car audio was still huge I use to install a lot of aux batteries for playing the stereo with the car off. Back then we used RV batteries or gel-cell batteries. They are suppose to hold up to many complete discharges. I think if you completely discharge a lead-acid battery it will never fully recover its charge capacity - even if it works ok in the car for a few more years.
 


Most pros will hook up a trickle charger if they have to have the doors open or car on for any more than a little while to avoid this. I've had this happen to me a few times working on cars - not from rocking out on the customers stereo but from having the ignition on for a few minutes to test some wires - most of the time its a battery that is a few years old and has been discharged before and was on its last legs - good enough to start the car a take a charge from the alternator but not enough to power the ignition for a few minutes.
 
Even if you leave a door open all day to work on something inside, that can be enough bleed off more amp hours than you might expect.
 

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