Question: With the winter cold upon us, I am thinking of buying one of the new batteries that have a switch that gives you a second battery if the first one goes dead. Is this a useful "gimmick"?--D.B.
Answer: The switchable batteries have been on the market for almost a year now, and the results are clearly mixed.
The basic idea behind these batteries is that if the battery goes dead, you can turn a switch and activate a reserve that can start your car. It's represented as similar to having a second battery, but it is really quite a bit less than that.
Whether the reserve can start your car depends on the reason the car would not start in the first place. In some instances, the reserve will come to the rescue and save you a service call from a tow truck, but in other cases you are not really any better off.
The main drawback to some of the switchable batteries is that they don't have any greater capacity in their combined main and reserve sections than a regular battery has in its single section.
For example, the Champion Switch, which was first on the market, delivers a total of 460 cold-cranking amps in its main section and 180 cold-cranking amps in its reserve section. (Cold-cranking amps are the number of amps a battery can deliver at 0 degrees over a 15-second period and maintaining a voltage above 7.2 volts.) While that's not inadequate, it is not any greater than most batteries. The company making the battery, however, says it has a type of plate technology that is superior, although the capacity seemingly should speak for itself.
In some other brands, switchable batteries have a main section of just 525 cold-cranking amps with a 90 amp backup for a total of 615-cold cranking amps. That's clearly less than the average battery of 630 amps and far less than high capacity batteries that go up to 900 amps.
The Sears Die Hard DualStart battery does better, offering 525 cold-cranking amps in its main section and 275 cold-cranking amps in reserve for a total of 800 cold-cranking amps.
So what are the merits of switchable batteries? They may be of little help in a cold weather situation in which your car's engine will not start and you repeatedly attempt to start the engine until you wear down the battery.
In this case, the switchable battery is not going to provide any greater engine cranking time than a non-switchable battery. It merely divides the time in which you attempt to start the engine between the main and reserve sections of the battery.
On the other hand, if you forget to turn off your headlights and you drain the battery, a switchable battery will allow you to start the car because only the main section's charge will drain. In that case, you would simply switch the battery to reserve, start the car and the engine's charging system would recharge the battery.
The usefulness of this feature is debatable, however. Most cars produced in the past decade have warning systems that alert the driver if he or she has failed to turn off the headlights when exiting the cars. And many of the newest cars have electronic systems that will shut off the headlights or other electrical devices if the driver forgets to do so.
So, switchable batteries can be helpful in some cases. If you are intent on buying one, it makes sense to find one with a large capacity. But you will have to consider that these batteries cost more than a standard battery.
Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, Times Mirror Square, Los Angeles, Calif. 90053.