Battery life on backup car battery

[TheDingo]

I have an entertainment system in my car, nothing huge, but it still is enough to put a hurt on my cheap car battery. I've decided to rewire all non-essential power to a second car battery. This battery will be on a isolator so my primary has priority and I won't have to worry if I drain the second by playing around with the stereo when the car is off.
At the very peak, the system will only draw about 550w, so I don't need much in the way of battery power, so now I'm just looking for the smallest battery I can get that will last as long as possible. I've heard I don't want a deep cycle, as they are designed to be totally drained then filled to full and repeated. I need a battery that can survive being drained, but most use will be like a regular car battery. Any ideas? Should I get a small ATV battery and replace it every couple years? A regular car battery?
Thanx

 

[Colt45]

you are planning on hooking the second battery up to your alternator right?
otherwise you'll have to change it more than every couple years.. it will die in a few days..

 

[Evadman]

More like a few hours. Just how long are you looking for this to last?

And you do want a deep cycle. A regular car battery will kill itself if you totaly drain it a few times ( like 5 or less ) as when it is totaly drained the lead facing on the plates will flake off. A deep cycle can be killed by repededly only half charging. If you take 1/2 of then charge back up to 100, you will be fine. If you totaly kill it, then recharge it to 50%, then kill it then recharge to only 50% again, you can hurt the reserve ( amperage / wattage ) by sulfur buildup on the plates.

Definately go with a Deep Cycle. I have 2 Deep cycle batterys in my truck, both are charged by the alternator, and I never pull more than 30% or so from them before recharge. I have had them for over a year, no probs yet.

 

[Evadman]

Batterys are baised on reserve capacity or amp hours. If a battery is 1000 amp hours that means you can pull 1 amp for 1000 hours, or 1000 amps for 1 hour before the battry voltage hits 11.8v ( 11.8v is 80% discharged )

volts x amps = watts, so 12.6 x ? = 550 so 550/12.6 = 46.3 amps.

There is your magical number. assume a nice even 50 when you go looking for a battery. ( AKA 1000 A/H would be aprox 20 hours. most car batterys are less than 200-300 A/H's )

 

[TheDingo]

Thanx for the help Evanman.

And yes Colt 45, is will be on an isolator, on the alternator.

 

[Evadman]

Make sure the isolator can handle 100% of the output of your alternator or it could overheat. A 100 AMp should be ok for most vehics.