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#352776 - 28/09/04 03:28 AM

Batteries, Second Batteries and Isolators.

xTHANATOPSISx
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Since I’ve been seeing a lot of posts throughout my time here about batteries, I thought I would try to create a post to help everyone out with some good information about batteries. I want to make sure it’s clearly stated, the myths and misconceptions are removed, and the truth is laid out. So here goes!

AN INTRODUCTION TO AUTOMOTIVE BATTERIES

An automotive battery is an electricity storage device. It produces DC (direct current) current for operation of the car’s starter and electronic accessories. A battery is an electrochemical device, meaning that it uses a chemical reaction to store or release electrical energy. This is in the same manner as that of an alternator being an electromechanical device, because it uses a mechanical motion to create an electric current.

Batteries do not create an indefinite source of electricity. A battery can only store electricity. They must be recharged by an outside source. This is the purpose of the alternator. It charges the battery between starting and using accessories while the engine is running. When the car’s engine and accessories do not require the full output of the alternator, a portion of its output is taken by the battery to recharge it. Once the battery is fully charged to 12.6 volts, it requires less power from the alternator. This system of discharging and charging is called a cycle. If a battery is drained until it is nearly dead, the charging process completes what is called a deep cycle. Most automotive batteries are not capable of deep cycling and typically cannot handle this type of use more than once or twice. Repeated deep cycling of a battery will result in the battery’s inability to hold a charge for a long period of time, causing the battery to be dead by morning, or when you come out of a store or restaurant. Special batteries have been designed to handle this use, and are available from most audio store and parts stores. They are typically more expensive, and usually do not have the cranking power for starting. This is changing now, with batteries such as the Optima Yellow Top and Optima Blue Top. These batteries are designed for starting/deep cycling applications. Many other manufacturers produce a starting/deep cycle battery, but those two are the most popular brands.

WHAT BATTERY DO I NEED?

To understand what battery you need for your application, you will need to know a few things. First of all, will you be using this battery for starting or accessories or both? If you only need it for starting purposes, you will need to match the batteries Cranking Amps and Cold Cranking Amps with the specific needs for your car. You can do this by getting a battery that meets or exceeds the rating of your existing battery. If you want a second battery for accessories that will be run with the car off, you want to get a deep cycle battery with a significant Reserve Capacity. The battery’s Reserve Capacity tells you how long the battery can supply a 10 amp current before falling to 10.7 volts while at 80 degrees Fahrenheit. To do both starting and accessories, you will need a deep cycle battery with significant Cranking power and reserve time. You will need enough power to start your car, and some time to run your stereo or other accessories. You could still be stuck with a dead battery, but you may have a longer time to wait to start your car, and better battery life in this application. This is acceptable for short draws or low amperage applications. You won’t run a pair of 1500 watt amplifiers for very long at full volume and still start the car, but you could play the radio alone for quite some time, upwards of two hours for some batteries, and depending on the radio’s current draw and what other accessories may be on at the time.

I WANT TO PLAY MY SYSTEM WITH THE CAR OFF FOR A LONG TIME

If you want to run a large system with a high current draw while the car is off, you’ll need to add a second battery that is isolated from the main starting battery. By isolated, I mean only when the car is not running. This prevents two things. First, it prevents whatever you are running from draining your starting battery. If your starting battery is dead, you’ve done no good by adding a second battery. All you did was play your system longer before you car could no longer start. Second, you need to prevent the two batteries from draining each other. That’s right. Even two IDENTICAL brand, model and rating batteries will slowly drain each other dead over a period of time. This may be a week or overnight depending on the condition of the batteries, or how closely matched they are. This would be quite damaging to the batteries. You could totally drain your starting battery and your deep cycle, but now the starting battery will dissipate its charge more quickly and will drain your deep cycles faster with it. This draining of two batteries by each other is due to the slight electrical difference in each circuit. The internal resistance of each battery may be different, its capable charging voltage may be different, many other things can allow this. But what prevent these two scenarios from playing out?

A DUAL BATTERY ISOLATOR

A dual battery isolator is either an electronic, electrical or electromechanical device that separates the positive terminals of two batteries that share a charging system when the batteries are NOT being charged. The isolator is switched either by the car’s ignition switch, or by internal circuits in the isolator that detect the line voltage and can open or close the circuit. There are MANY different types of isolators. Just go to www.yahoo.com and search for “dual battery isolator” and look through the different companies you find. You’ll see every different types of isolators that are suitable for your needs. I won’t go into individual design of all the types but I will describe, briefly, the 3 major systems for preventing the draining of both batteries by the same current draw.

Diode isolators use a group of diodes to resist current flow in one direction. Since you’ll remember that your car battery stores DC, you’ll know that the power can flow only one way, but, this flow can be from one positive to another. This means that the power would flow out of one battery, and into the other. The diode isolator splits the positive wire from the alternator and runs the power through 2 groups of high amperage diodes to direct the charging voltage to each battery. When the charging system is not running (the engine is off), the diodes prevent power from flowing away from the positive terminal of either battery. There is no switching system or moving parts to this isolator, but it is typically very susceptible to power surges that can damage the diodes and can be limiting in the maximum amount of current available for charging and running accessories with the engine on.

A solenoid isolator operates much like the starter solenoid on your starter. It is similar to a relay, but is designed to carry a significantly higher amount of current, and for longer periods of time. This type of isolator is not as limiting in its current capability, but it has moving parts and requires that you connect a control wire to the car’s ignition circuits so the solenoid only closes when you are in the run position on the ignition switch. This unit can only be switched so many times and will eventually fail from continued use.

The third type of isolator is the electronic isolator. These units may use semi-conductors or internal relays like a remote starting system. They use internal electronic circuits to detect a charging voltage greater than 13 volts. When this voltage, higher than the voltage on a charged battery, is detected, the isolator will allow the circuit to close and the second battery to charge.

For all three types of isolator, you still use the battery your accessories are connected to for engine-off power. You batteries still charge from the alternator, and you accessories still receive power from the alternator when the car is running. This is an ideal configuration for your car’s electrical system. A second battery, paired with a proper isolator, a strong starting battery and high output alternator is a excellent foundation for any system. You’ll be able to reach maximum output while running your engine, and play your system for a long period of time with the engine off. Additionally, if you ever leave you headlights on, you can typically turn the key to run for a few moments and get the car started, or simply connect a jumper cable from the lead to the second battery and the starting batteries positive and get started that way.

I hope this helps people out in some way and I apologize for the length, but I feel its necessary to explain the purpose and operation of the electrical storage system. Thanks for reading, if you have other questions, please post them here so everyone can benefit from it. I will eventually try to do a tutorial on actually installing a second battery and isolator. If anyone else would like to do so as well, please feel free to post your install here. Good luck.

_________________________
Installer
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-Brakes
-Suspention & Steering
-Electrical & Electronic Systems
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