Extend listening time when engine is off

[smarcus3]

I have a slight problem with my new car stereo. Currently I have a 4 x 60 watt RMS and a 390 watt RMS amps powering my car door speakers and subs. Everything works fine, but it can kill the car battery in under 30 minutes if I have it running even at moderate volume when the engine is off.

 

I have thought about getting a deep cycle battery for the trunk that would give the system extra run time when the battery is off. However I don’t want to tax the legacy’s stock alternator which I believe is a 110 amp alternator. Doing some rough calculations I calculated that the system under PEAK load could pull around 80 amps and 52.5 under maximum sustained power.

 

I saw at Walmart that they sell a Marine Deep Cycle battery (MAXX) for ~$75 that offers 125 amp hours. What would y’all recommend to increase the ‘tailgating’ listening time to something more reasonable. Something like 2 hrs between having to idle the car to recharge the batteries would be great.

[FrozenNorthLGT]

A deep cycle battery wont necessarily extend your listening time. It will probably have a higher reserve capacity than your stock battery, but mostly deep cycle just refers to the ability of the battery to be drained and recharged (cycled) without damaging it. What do you have for a battery under the hood? Replacing that with a good deep cycle with a longer reserve than the stock battery may be all you need. Another thing to consider is idling the car will not effectively recharge the battery very quickly. Most alternators need to be ran at at higher speed than that to fully charge a battery in a decent time. At idle, they don't put out enough amps to do much more than maintain normal loads.

[smarcus3]

i have a duralast gold thats just under a year old under the hood.

[utc_pyro]

Eh, you wont over tax it just adding an extra battery. It will be additional load at idle, but you'll still be storing energy. Just put the deep cycle in the trunk and a battery isolator for the added capacity. It should last you a while, but you might want to look into class D amps as well (better efficiency).

 

For "tailgating", as FrozenNorthLGT said, standard idle wont be enough for a proper recharge. You'll need to reprogram the ECU for a "fast idle" for those times. Though a Legacy isn't quite what I'd use for tailgating... If this is true tailgating (listing outside the car), you'd probably be better off using some external speakers then blaring the ones in the car.

[smarcus3]

well it seems like its going to be more expensive to add a second battery than i thought. thanks for all the input. Since i don't listen to the radio too much when the car is off, i can just idle it when i do. I can buy alot of gasoline for 200 bucks.

 

Also, does anyone know the gallons/hour consumption rate for an idling engine?? i was reading around .2 - .3 gal/hr so about $1 an hour and less than 5 bucks for an entire tailgating time so i guess i will just idle the car if this data is correct.

[utc_pyro]

I pulled up one of my logs, and my bone stock legacy is pursing 4.3g/s air at idle with a 65f intake temperature. At the point, the front O2 sensor is telling me it's running at 13.78:1. Thus, .312 g/s of fule. 3600 seconds per hour, so 1.1232 kg/hour of fule. The spesific gravity of gas is around .739kg/L, so 1.52l/hour at idle. Putting that into Google gives .4 gallons per hour. So $1.20 per hour at idle. So 166.7 hours of tailgating to make up for the cost of the battery (ignoring wear an tear on the car).

 

Edit: One additional note: This is coming from the end of the logging session, so the engine is hot as hell.

[smarcus3]

thanks for the numbers. i assume one the engine is warm. idling it does very little.