separate battery in trunk

i was wondering if i could use a totally separate car battery in the trunk to power my neon lights and such that i would use whenever i have my trunk open.  would it be possible to use it with my amp and subs too? would it be on the whole time? thanx

I dont see why not. Thats usually what people do. They set up one battery to run the car itself, and the other battery to run all the goods.

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1990 Honda Civic HB:
Clarion DXZ545MP H.U.
2- 6.5" Power Acoustik interiors
Diamond Audio 600.1 amp
Diamond Audio 15" M6MKII
Pyramid PB881X 4 CH. Amp

Wrong. In fact, it will cut the work the alternator needs to do almost in 1/2. Alternators are voltage regulated, not current regulated.

Too many batteries can be a detriment, but 2 batteries on an 80-100 amp alt will be just fine. Be sure to use a proper isolator (not a constant duty solenoid) for extended battery and alternator life.

Here is my car, the correct isolator, and the second battery in the trunk (AC Delco Platinum Professional AGM battery):





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remember you must fuse the battery in both directions so that if any thing happens to the cable from front to rear your car will be protected.

A second battery would be on the whole time, as it will be a part of the cars electrical system.  By adding the trunk battery, you would be adding to the time that you can run your lights, system, etc. with the engine off, as there will be twice as much stored power.  If you're goal is to have the car parked with the trunk open for show reasons at the 7-eleven (j/k), then it is a reasonable idea. 

Until I read Seth's opinion above, I had read differing opinions on the alternator load of a 2nd battery from sources such as BCAE.com, where it reads:

Extra batteries:

Extra batteries are great if you want to listen to your system with the engine off. While the alternator is charging, the extra batteries will only draw current which could otherwise be going to your amplifiers. For proof, all you have to do is measure the voltage while the engine is running. It should be approximately 13.5-14.4 volts DC. Then turn the engine off and measure the battery voltage again. Now it'll be around 12-12.5 volts. Whenever the voltage at the battery is up around 14 volts, there is current flowing into the battery.

So when you are done hanging out for the evening and then start your car, the alternator will start charging two batteries instead of one.  You may need alternator and other electrical system upgrades  with the addition of the second battery. 

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

There are too many variables he does not take into account. A few quick thoughts...

The car will always need current to stay running, after a start the car will drag 30 amps to simply recharge the battery to it's desired voltage, as it is voltage regulated. Now let's add the additional load of running all our stereo on that drained starter battery. Not only are we replenishing the starter drained battery, but now we have additional reduced voltage that the alt needs to overcome, and what it delivers is being robbed by the stereo again. Keep the following in mind as you ponder the above statement...

In order to produce current, the alternator needs input voltage. If the input voltage drops, the alternator needs to work harder to produce the same current, generating additional heat in the alt case (hence, premature failure). By doubleing our reserve capacity, we essencially cut the voltage drop in half to the alt reducing it's work load.

Now, that is a constant duty solenoid install. Let's ponder a true isolator. A true isolator pulls it's input voltage off the main battery. The main battery sees minimal loads as we drive down the road. The requirements are typically small loads, rarely affecting total voltage of the main battery, and therefore not affecting the input voltage of the alt. By maintaining a steady solid input voltage, the alt needs to do less work to maintain the proper current to maintain system voltage. Keep in mind, the iso will detect which battery is lowest and send the proper charge to the batteries as they are needed, of course, biasing to the main alt as it is the input feed. So we have proper input voltage being maintained that reduces the work the alt needs to do, we have double the reserve capacity for those big bumps that will suck a single battery silly, and we still have the same load on the system overall!!!

While I am a firm believer in most of the info on BCAE, this is outside his realm of expertise. Being that I am AC Delco and ASE master certified including L1, with my specialty being computer, electrical, driveability and emissions diagnosis and repair I would say I can safely suggest this is within my realm of expertise.

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