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Many people listen to their stereo or use accessories (lights, radio, TV, computers, for those with RVs) 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. Most people opt for isolators that require no action on their part to keep the starting battery from being discharged. This type of isolator is slightly more complex to install than the most basic types of isolators. The most basic isolator is simply a heavy duty switch (generally rated for at least 100 amps of current) that is used to make/break the connection between the batteries. To use a battery disconnect switch like the one below to protect the starting battery from being discharged, you would simply connect it in series with the wire that connects the front and back batteries. In one position, the batteries are connected. In the other, they're not connected. As long as you don't mind having to manually flip the switch and can remember to do so, this is a good, simple solution. In this particular model, the 'key' is removable.
Most of those who use an isolator for marine applications have heard the Term Perko Switch. This is a term used to many of the switches of this type, no matter the manufacturer. This type of switch comes in several configurations. The one below can switch between two individual batteries, both batteries at once or can break the connection to the batteries completely. There are also options to break the connections to the field windings to the alternators but this one doesn't have that option. It's also available with a key to allow you to lock it. The locking version of this switch and the previous switch are good choices for people who have expensive, powerful amplifiers and speakers in vehicles which others are allowed to drive. When needed, the power can be cut to the amplifiers and locked out so that the driver cannot damage the speakers.
The back of this switch has 3 connections. Terminal 1, terminal 2 and a common terminal. The common terminal is the one that gets connected to the other terminals. In its most commonly used wiring configuration, the common wire feeds the starter for the boat's engine. Terminals 1 and 2 are connected to batteries. If this were used for car audio, the common would be connected to the main power wire that feeds the amplifiers. The engine starting battery would be connected to one of the other terminals. The rear/auxiliary battery would be connected to the remaining terminal. When in the 'ALL' position, all batteries would be connected together and also connected to the main power wire for the amplifiers. When in the '1' position, the amplifiers would be fed from the engine starting battery. In position 2, the amplifiers would be fed from the rear battery. In position 2, the rear battery would be disconnected from the main battery so it could not discharge it but the rear battery wouldn't be able to be recharged. This is the position that you'd use if you want to listen to your system without draining the starting battery. To allow the rear battery to recharge, you'd set it to 'ALL' again.
The switch above is a 'make before break' switch. This means that the contact that's currently engaged will be bridged to the next contact in the rotation before contact broken to the original contact. This is important when you don't want to lose power to the device being powered by the switch. For engine applications, a 'break before make' switch would mean that you may lose power to the computer or ignition system if the switch was changed from one position to another while the engine was running.
There are 2 main types of automatic battery isolators. The diode based type and the relay (solenoid) type.
Diode Based Isolator:
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As of now, from what I've read (I haven't taken one apart), there are 'diode-type' (semiconductor-based) isolators that either use FETs in parallel with the diodes or use FETs in place of the diodes. FETs are Field Effect Transistors and can pass current with less voltage drop than diodes.
Solenoid Based Isolators:
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.
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Ford Type Solenoid:
Continuous Duty Solenoids:
Many people only regard an isolator as a device to prevent draining the starting battery. There are other things to consider as well. Diode based isolators will not allow the rear batteries to help start the vehicle in case the front battery is accidentally drained. If there is enough voltage left in the batteries that switch on the solenoids, the rear battery can assist in starting the battery. With either the diode-type or solenoid type isolators, this can also be accomplished by jumping across the isolator to connect the batteries. Unless the fuses are rated for more than 200 amps, it's often best to let the batteries remain connected to equalize a bit so that all of the current doesn't have to flow through the fuses. Of course, this is for a vehicle in good operating condition, that starts easily but simply has a dead battery.
FET based isolators may not prevent the rear battery from draining if the front battery is drained. Typically, FETs will allow current to flow back through their terminals through the 'intrinsic diode'. Also, if high current is drawn though them in the reverse direction, they may quickly overheat and fail. If you're considering an FET based isolator, ask these questions if these characteristics could cause you problems.
In the case of any of the isolators, if you have a dead battery and a vehicle that doesn't start easily, the best option (if no help is readily available) may be to simply move the auxiliary battery to the engine starting location to start the vehicle.
Adding a second battery without using an isolator:
It was stated that the larger lugs are difficult to crimp. This is especially true if you use good quality connectors. For car audio, the leftmost type of terminal is often used. it has a gold plating but otherwise it's not a good choice where it's critical for the connection to be reliable. These don't even have a brazed seam. The opening in the front allows the wire to corrode easily (unless soldered properly). The middle terminal is the type you want to use but is a budget version. The metal around the hole for the stud/bolt is slightly thicker than the one on the left (0.053" vs 0.055) but still not as thick as on a really good terminal. The advantage it has is that the design makes it more rigid which is good if there is a chance that the wire may get pulled on, especially if it's pulled at a right-angle to the surface to which the terminal is bolted. The one on the right is the best of the bunch. The metal on the tab of this one is 0.083" thick. There are even better terminals that are thicker and more resistant to corrosion. They're generally sold as marine terminals. The marine terminals generally look like the two terminals on the right but instead of having bare copper, they're tinned. The tinning increases the resistance to corrosion.
When using a hammer-type crimper, you need to give it a really good whack. If you're using a standard framing hammer, you'll likely have to hit a good quality 4 gauge connector 4-5 times. If you're using a 4lb maul, you will likely still have to hit it at least twice. Of course, when doing this, you need to wear safely glasses.
When buying large lugs, you will find that there are quite a few aluminum lugs. These are typically used by utilities, not the general public. They are good for use on aluminum wire where the crimper is a specialized type that ensures that the crimp is perfect. You cannot solder the aluminum terminals so you need to use copper or tinned copper connectors.
For the best connection, you may also want to solder the terminal onto the wire. When doing this, using a paste flux (specifically designed for electronics, not plumbing connections), can make the solder flow better. Push the wire into the paste before crimping. This will get the flux between the strands instead of just on the outside of the bundle of strands. Radio Shack has paste flux in stock in most stores. When using a torch to solder and paste flux is used, be aware that the flux can catch fire but it's no more significant than a candle's flame. Don't panic. Remove the heat and 'gently' blow it out.
If you intend to solder the connector on the wire after crimping it, when you crimp the connector on the wire, leave a narrow opening between the insulation and the connector to allow you to feed solder into the connector.
The large connectors are generally soldered with a small butane or propane torch. I highly recommend using a small butane torch like the one below (~6" tall) instead of a larger propane torch.
Good quality cable with a neoprene insulation can take quite a bit of heat (but not direct exposure to the flame). Vinyl (PVC) insulation (much more common) cannot take much heat and may melt near the connector. If that happens, you may want to use heatshrink tubing to cover the area that has overheated. Heatshrink tubing is made to shrink to half its diameter when heated to about 120°F. A heat gun works better than a flame for shrinking. It can be done with a torch but great care is needed not to burn the tubing. Below, you can see the heatshrink tubing before shrinking and after it's been heated. When you buy heatshrink tubing, don't be surprised if it's expensive. $3/foot is about what you'll pay for good quality heatshrink tubing.