On The Road

Leading the charge on battery replacement

One of the maintenance activities handy people often still want to do on their own is change their car’s battery.

I have no problem with people trying do-it-yourself projects, but approximately a quarter of the time a new battery doesn’t actually fix the problem. In these circumstances, we usually end up discovering that a light was left on, a component is causing a draw or the car has some sort of parasitic draw.


The most important thing to know before changing a battery is the state of the electrical system. Check the physical state of the battery, the alternator and the battery cables.

To start, find out how old the battery is. Look for a sticker on the battery that should show the date of production. For example, the sticker might read “05/15.” That indicates the battery was manufactured in May 2015. A good life for an automobile battery is between four and five years.

If you have a digital voltmeter and an amp probe, connect it to the positive battery cable and check the alternator output at idle and at 2,500 rpm. The alternator should be at approximately 13.7 to 14.3 volts.

Also check the battery terminals. If the terminals (positive and negative) are clean, try to wiggle them with your hand. The terminal must not move. If the terminal is loose, it cannot deliver current or ground properly. If one is loose, find the correct tool and tighten it. If it won’t tighten, the terminal or battery cable may need to be replaced. It is critically important that you don’t leave tools lying on the battery. The wrench, for example, can easily arch if it touches the positive and negative leads at the same time.


There are many ways to test a battery. However, if you are not testing it under load, you will not get a correct reading. There are some new computer testers that work well, but they are expensive and mostly used in the professional field.

In most cases, a battery also has to be properly charged before testing, so if you are not exactly sure what happened to the battery, it might be better to have a professional check it.

Battery life also depends on how you use the battery. If you have a normal commute of 10 miles or more a day, the alternator will probably charge the battery. If you have a short commute (3 miles or less), the battery gets overworked and never has time to recharge. When starting a normal midsize car, you generally need approximately 60-200 amps. Once the car is running, the alternator kicks in and starts to deliver amperage to run the components on the car and charge the battery. If the drive is too short, the original draw from the starter has already put the battery at a deficit. Even though the alternator is helping the system, it may not make up the deficit. In the case of shorter commutes, a new battery can be dead in a year or so.

There are several things to look for when you suspect the battery might be dead or low. Make sure all the lights are turned off. When it comes to lights, the interior ones – such as dome, glove box, hatch and trunk lights – are often to blame for a dead battery Also make sure that all of the doors, trunk and hatches are closed all the way.


We had a 2010 MDX at the shop recently that had a golf tee stuck in the hatch latch. The hatch was closing but not all the way. The customer saw that all the doors were closed, but the dome light would not turn off. He then shut off the dome lights but because the body control module kept thinking the rear hatch was being opened, it kept several modules on and drawing current. These modules quickly ran the battery dead.

If you have a car older than 1995, it is safe to say you will most likely be able to change a battery without losing any computer adaptation, but you might lose a radio code. On most modern cars there are components that have to be readapted after a battery replacement (throttle angle, set idle speed, steering angle). If these adaptations are not performed, the car may not run properly, turn on dash lights or allow the traction-control system to work.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email This email address is being protected from spambots. You need JavaScript enabled to view it. or visit sunnyvaleforeigncar.com.

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