Adding a second car battery is a great way to get lots of standby power when the vehicle is off. You can upgrade your car so that it has the same power setup as a recreational vehicle, with one battery for starting and running the engine and another for powering devices.
Adding a second battery doesn't really increase the number of devices you can power while the car is on—for that you need to upgrade your alternator [Hack #9] . What a second battery does do is more than double your power storage capacity, increasing the time for which your devices can run when the engine is off and ensuring that your primary battery always has the power to start your car.
To illustrate the information in this hack, I'm going to appeal to your intuitive understanding of batteries. If you look at the label on AAA, AA, C, and D batteries, you'll discover that they all supply 1.5 volts. But you would expect that a D is better than a AAA somehow, if only because it's bigger. What you may not know is that the main difference between the two types of batteries is the length of time each battery can put out 1.5V, and how many amps they can sustain. That's why devices that need to put out power for a long time, such as flashlights or boomboxes, use D batteries, while remote controls and pagers only need AA or AAA batteries.
You may or may not also know that you can wire a pair of batteries in parallel. The combination will output the same voltage, but for longer. For example, if you take 4 AA batteries and wire them in parallel (tops to tops and bottoms to bottoms, with wires daisy chained between them), they'll probably be able to put out more sustained power than a single D battery, but only at the same 1.5V that a single battery delivers.
What you wouldn't want to do is put a D battery in parallel with a AAA battery. After the AAA battery was discharged, the D battery would still be putting out voltage, and the AAA battery would be sucking it up—not the desired effect. The general rule is that you put batteries in parallel only when they are the same kind and age. Even then, if one of the batteries goes bad it can take the other ones down with it, so don't leave a dead battery mixed in with good ones.
If car batteries are connected in parallel, the more-charged one will always be trying to jump-start the less-charged one. So, although you could simply wire a second car battery in parallel with the first, there are better and safer ways to implement a two-battery system.
One excellent way to implement a two-battery system is with a battery isolator. These devices range from less than $50 to several hundred dollars, depending on the sophistication and features. Ideally, the job of a battery isolator is to combine the power of the two batteries for everything from powering devices to starting the car, while making sure that a fault in one battery can't kill the system. In essence, it becomes a redundant array of batteries—all the benefits of a backup battery, with none of the drawbacks. And depending on whether starting power or device power is more important to you, an isolator can make sure that devices deplete only the secondary battery, always leaving the primary battery charged to start the car.
In practice, the isolator uses a bit of the power itself ( diode isolators will drop alternator output by about 1 volt, have large heat sinks, and are somewhat inefficient). It's not that easy to keep two different batteries fully charged, united in power, but divided in risk. Isolators come in several kinds, from simple switches that flip from one battery to another, to complex microelectronic circuits that actively monitor the charge levels on both batteries, charge them appropriately using alternator power, and use both of them to power devices and start the vehicle.
Because of the large currents needed to charge a battery, any second battery setup will need to run thick (4-gauge or so) wires from the alternator/front battery back to the second battery, unless the second battery is also being mounted near the engine. Exactly how to do this will be in the installation instructions that come with the battery isolator.
If you want a very simple isolated system, you may be able to achieve it with a dual-output alternator. These are designed to charge two separate batteries independently. You can install your second battery to be charged by the secondary output of the alternator, and wire most of your devices to this second battery. When the engine is running, the vehicle will power the devices. When the engine is off, the second battery will power the devices, leaving the primary starting battery untouched and always charged and ready to start the car.
In "Upgrade Your Car Battery" [Hack #6] I discussed the two basic types of car batteries. In RVs, the usual approach is to have both kinds of batteries, each doing what they do best. A conventional, high-cranking-amps, lead-acid battery is connected to the ignition, lights, and basic car electronics. A second, big, deep-cycle battery (or several) is charged by the engine as well, but when the engine is off these batteries power devices such as refrigerators, TVs, computers, and so on.
There's never any risk of killing the starter battery by powering nonessential accessories, and if the deep-cycle battery goes dead, no harm is done—the engine can just charge it back up on the next drive, if it's long enough. However, keep in mind that the stock alternator isn't designed to charge batteries from a deep discharge on a regular basis, so you may want to upgrade it as well [Hack #9] .
This arrangement depends on a good battery isolation circuit, and the same system can be installed in any vehicle with room to put an extra battery.
An important consideration when installing and using a secondary battery is the potential for battery fumes (a.k.a. poison gas). These fumes are created when a battery is being recharged. While many batteries today are "sealed" lead-acid, it's difficult to completely seal a battery. RVs and work trucks have both their batteries vented to the outside, either in the engine compartment or on the sides of the vehicle. If you are installing a second battery in a hatchback, or even in a trunk that can allow gases into the passenger compartment, ensure that the battery you are installing is designed for this, or provide appropriate ventilation for it by building an enclosed battery box that vents to the outside of the vehicle.
Battery fumes are explosive and toxic, so a DIYer building a battery box should make sure that it vents to the outside. These gases are generated only when the battery is being charged. Consequently, deep-cycle batteries recovering from a deep discharge will produce more fumes then a starting battery recovering from an engine start.
Many battery makers recognize this dual-battery application and will tell you whether their batteries are safe near passenger compartments. As a practical note of experience, I've owned an Audi, a VW Bug, and a Mercedes, all of which had batteries under the back seat from the factory, so don't be paranoid—just vent appropriately.