I found this maybe it may help Found here:
http://www.bcae1.com/battiso.htm
Battery Isolators:
Many people listen to their stereo without the engine running. This often leads to dead batteries and a vehicle that won't start. A battery isolator will let you completely discharge one battery without discharging the starting battery.
Different Types of Isolators:
There are 2 main types of battery isolators. The diode based type and the relay (solenoid) type.
Diode Based Isolator:
This type of isolator is basically two very high current diodes in a heat sink. The diode based isolator has the advantage of long life and trouble free operation but many of the diode based isolators cause a .4 to .6 volt loss which means that the batteries will not charge to as high a voltage as when they are connected directly to the alternator. The diagram below shows the connection for the diode based isolator. Some diode type isolators have a fourth terminal which controls the current flow through the device.
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Solenoid Based Isolators:
This type of isolator uses large high current relays to control the flow of current. The diagram below shows two solenoids. Solenoid A determines whether the second battery is connected to the rest of the charging system or not. Solenoid B connects the second battery to the distribution block. The second solenoid allows a complete disconnect from the amplifiers for safety. The diodes connected in parallel to the solenoid's coils are used to protect switch A and switch B from inductive kickback when the solenoid's coils are de-energized. The advantage of the solenoid based isolator is that there is virtually no voltage loss across the contacts. The main disadvantage is that the reliability isn't quite as good as the diode based isolator.
The coil of the solenoid will draw significantly more current than can safely be supplied by the remote/power antenna output of your head unit. You should NOT connect them directly to the head unit. You need to use a relay to buffer the remote/power antenna output of the head unit so that the head unit's switching transistor isn't damaged.
In the demo below, click on the head unit to switch it on and off. Click on either side of the toggle switch to change its position. The round green dots are voltage indicators. They are dark when there is no voltage and bright when voltage is present. The arrows indicate current flow through the respective part of the circuit. Switch A allows you to disconnect the main starting battery from the rear battery and amplifiers. This allows you to play the amplifier without fear of draining the starting battery. When switch B is off, the solenoid B is off (disconnected) and you can work on the amplifier's main power wiring without fear of shorting battery power to ground.
Click HERE to make this demo fill this window.
NOTE:
There are 2 types of solenoids. They look virtually identical but are designed for different uses.
Ford Type Solenoid:
If you're familiar with older Ford vehicles, you've no doubt seen the fender mounted solenoid. It's used to make/break the connection from the battery to the starter. This solenoid is designed to pass as much as 400 amps of current but only for a short period of time. The solenoid's magnetic coil has only 3-4 ohms of resistance. If used for continuous duty, the coil will overheat and fail. The low resistance provides higher the contact pressure that's needed to keep the contact resistance to a minimum.
Continuous Duty Solenoids:
For main power switching in car audio, you need a continuous duty solenoid. These solenoids will have approximately 15-30 ohms of resistance across the coil. As you can imagine, this will allow the coil to run much cooler than the starter solenoid mentioned above (although the solenoid will be quite warm after a while). The solenoid I've used is the White-Rogers 70-111224-5. It's available from Grainger or virtually any auto parts supplier. It's rated at 80 amps continuous and can take short periods of higher current flow. I've been using one on my test bench where I draw more than 100 amps through it for 20 minutes at a time and it hasn't failed yet.
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Adding a second battery without using an isolator:
Some people have good results when they add a second battery to their system. Especially if the second battery is close to their amplifiers. If you want to add a second battery, there are a few things that you should know.
If the battery is inside the passenger (in a hatchback vehicle or in a car with a back seat that folds down), you must use a sealed battery (i.e. Optima). If the battery is being used in the trunk of a vehicle you can use a standard battery but you will need put the battery in a battery box and you will need to provide proper ventillation. Proper ventillation would mean forced air (fan) ventilation from the battery box to the outside of the vehicle. When a standard lead acid battery charges, it produces flammable gases which must be removed from the vehicle to prevent a fire/explosion hazard.
The second battery must be fused. The fuse must be as close to the battery as possible (just as you have on the main battery). All of the power for the system must flow through through the fuses (at the batteries) to protect against electrical fires. The diagram below shows the proper fusing and wiring. The fuse at the main battery must be properly rated to protect wire 'A'. The fuse at the second battery must be properly rated to protect wire 'B'.
No matter what kind of battery you use, care must be taken to assure that nothing can come in contact with the battery's terminals. I would strongly recommend putting the battery into a plastic battery box with a cover that can be secured onto the box.
The ground for the second battery should be grounded to the floor pan of the vehicle. The floor pan is the main piece of sheet metal that forms the largest portion of most new vehicles. This will provide the best return path for the second battery.
